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By Charles & Linda Raabe
Mactan Island, The Philippines
© 2010 All Rights Reserved

Photo by Charles Raabe
A typical Philippine mixed species seagrass bed

  On any tropical shoreline where there are shallow, flat stretches of soft sediment that is not subject to heavy wave action you will most likely find extensive beds of sea grass comprised of single or multiple species, often closely intermixed seeking to gain their share of sunlight and nutrients from the near mud like substrate. Seagrass communities are among the most productive communities in nature providing habitat for large populations of invertebrates and fish, and acting as some of the richest nursery and feeding grounds to be found anywhere on earth.




Seagrass - The primary producers

  With four families, twelve genera and about sixty species (Sullivan 1994) the seagrasses have been able to colonize all relatively warm locations providing a unique and very diverse habitat regardless of the species or mixture of species found. Although there are many seagrass beds comprised of single species found elsewhere in the world, here in the Philippines there is high diversity of speices (seven to nineteen according to various sources) and the grassbeds are most always of mixed species.
  Locally the most common species are the very large bladed slow growing and long lived (10 years) Enhalus acoroides,  the short wide bladed Thalassia hemprichii,  the short very thin bladed Syringodium isoetifolium and the short, paddle shaped Halophila ovalis.  Each of the four species plays a role in the formation of the grassbed's climax canopy.  With nearby open sandbeds, the Halophila acts as the pioneering species, being the first to establish itself in uncolonized sand acting to anchor the sand and preparing it for the Thalassia and Syringodium species to follow through rhizome growth. It is only when sufficient growth by the previous species has stabilized and enriched the sandbed through their leaf litter that the large Enhalus species establishes itself, which it appears to do more frequently through seed dispersal than by rhizome growth. I have only observed this large species being located in the central regions of the grassbeds indicating to me that it or its seeds were late arrivals onto the scene, giving the shorter lived, faster growing species time to prepare for its arrival while having spread far beyond their point of origin.

  Photo by Charles Raabe  The paddle shaped Halophila ovalis having pioneered open sand substrate allowing Thalassia hemprichii to follow.

  Photo by Charles Raabe  A young seagrass bed having been fully colonized by Thalassia hemprichii and Syringodium isoetifolium thus overgrowing and pushing out the pioneer Halophila ovalis.  The thick layer of leaf litter has yet to accumulate as found in mature beds.

  Photo by Charles Raabe  A mature seagrass bed containing multiple species of seagrass and having developed a thick layer of leaf litter. The fully developed canopy also provides yet another habitat utilized by many fish and invertebrate species, some being full time residents while others follow the tide in from the deeper reef to hunt for food within these very rich hunting grounds.  What seems most important for the associated species is the provision of shelter and food supply resulting from their extraordinarily high rate of primary production.

  The formation of coastal seagrass beds also help to provide the required conditions for the fringing coral reefs by slowing the flow of water and allowing sedimentation to occur before such particulates can become a hazard to the corals. Seagrasses also provide coastal zones with a number of other benefits including wave protection, oxygen production and protection against coastal erosion by anchoring the sediments in place and preventing their drift. The nursery habitat that is created and sustained by the seagrasses is an important contribution to the fisheries, greatly adding to the number of fish that reach adult size having been afforded the protection and food provided by the seagrass ecosystem.

  Seagrasses are monocotyledonous vascular flowering plants.  They are unique in that they are submerged in the seawater, possess a rhizome/root system with stems buried in a soft substrate, have vegetative and sexual reproduction and have flowers fertilized by water-borne pollen.  Seagrasses are the only true marine plants as all other "vegetation" found in the ocean are algae. While not a true grass, they are called grasses simply because their long, green leaves superficially look like the terrestrial grasses from which they evolved from.
 
 

Photo by Charles Raabe
Anatomy of a seagrass  ( Syringodium isoetifolium)


 Seagrass sediments - A place of complex nutrient dynamics and home to many species.

  The seagrasses are considered to be ecosystem engineers since they partially create their own environment simply by growing and living in most soft, sandy sediments. Once established by the pioneer species, their extensive root systems take up nutrients which are then transported to the leaves.Upon the leaves’ death and detachment, they will settle on the surface of the sediment and through decomposition not only return a portion of its sequestered nutrients to the sediment, but will also create the acidic conditions through decomposition that releases even more adsorbed nutrients from the sediment in which it grows. All of the nutrient exchange is also utilized by the multitudes of infauna that through their own actions within the sediment contribute to the distribution of nutrients and thus the growth of the seagrasses. 
 
  I feel it is noteworthy to point out that other studies done in locations outside of the Indo-Pacific region have come to different conclusions concerning the nutrient dynamics of seagrass sediments. This may be due to differing sediment compositions as well as the different seagrass genera found in those locations.  Not all seagrasses have the same requirements nor the same abilities in nutrient extraction/transportation.  Since this article is examining a Philippine (Indo-Pacific) seagrass habitat, I have tried to use only the reference material that pertains to these locations.  This should not pose a problem for the aquarium hobby as the majority of our aquarium systems are based on the Indo-Pacific regions and their calcium carbonate sediments.
  I also want to stress the fact that the nutrient dynamics involved in any seagrass ecosystem is extremely complex and not something I can or am willing to fully explore in a single hobby article.  I will however do my best to touch upon the most obvious of the actions involved as they do pertain to our keeping of marine aquaria.

  As with any plant, light and nutrients are the primary requirements for growth. With the surrounding sea water often having undetectable amounts of dissolved nutrients, the seagrasses derive the majority of their nutrients directly from the substrate by way of their root. Although the leaves can also uptake nutrients from the water their primary purpose appears to be conducting photosynthesis and storing nutrients transported by the roots. The nutrient concentrations within the water are usually so low that uptake by the leaves is considered insignificant relative to root uptake of nutrients from the sediment (Erftemeijer 1993).  As with any plant or algae that utilizes both phosphorous and nitrogen, they can be limited by not enough of one or the other. How much of one or the other is available is determined by numerous factors, most of which involve the geochemistry of the sediments that the seagrass finds itself growing along with the availability of organic matter that is broken down through decomposition, the primary source of both nitrogen and phosphorus regardless of the sediment's composition.

  Any good farmer knows that phosphorous and nitrogen within the soil is the key to a good crop in nutrient poor soils, hence the heavy use of fertilizers in farming operations.  This holds true for seagrass as well.  The ability of a substrate to provide the essential dissolved nutrients has been shown to be determined by the composition of the sediment (Short 1987) in of its composition, either terrigenous (land-based eroded rock) or calcium carbonate. The grain sizes also determine the nutrient dynamics involved.  It has been shown (Erftemeijer 1993) that Indo-Pacific, near-shore sediments comprised of terrigenous material has a significantly higher pore water concentration of nitrogen compounds than the calcium carbonate-based sediments while the reverse is true of phosphorous compounds.  This can be explained by the geochemistry found to occur within the various sediments and at varying depths within those sediments.

Prepared by Charles Raabe     Prepared by Charles Raabe
Data as given by Erftemeijer P.L. (1993), Sediment-Nutrient interactions in tropical seagrass beds

   The two sedimentary environments investigated by Erftemeijer showed considerable differences in sediment composition and nutrient availability. Total P and N were much higher in the terrigenous sediment in comparison to the nearly 100% calcium carbonate sediment  The difference was attributed to the terrigenous study area being near a river inlet causing an increase of organic matter from terrestrial sources. However, the exchangeable phosphate was considerably higher in the calcium carbonate sediment and was attributed to the much stronger adsorption affinity of the carbonate matrix to phosphate in comparison to the terrigenous sediment.
  Additionally, the apparently high levels of phosphate within the upper few centimeters of the carbonate sediment can be attributed to the carbon dioxide and acids produced as a result of aerobic decomposition of organic material and oxidation of reduced sulfur compounds.  These acids may cause the dissolution of calcium carbonate and the phosphate that had been adsorbed onto the calcium carbonate, resulting in a net enrichment of porewater phosphate.

  Within the upper few centimeters of calcium carbonate sediments,  bacterial fixation of N2 (Capone 1992) accounts for a large fraction of the NH4 produced within or released from the upper layers of the sediments, having a turn over rate of less than twenty four hours.  Capone has found that denitrification can be detected, even in apparently oxygen rich sediment, possibly accounting for the lowered nitrogen content in relation to phosphate content and thus limiting seagrass growth to being more dependant upon phosphate when growing in calcium carbonate sediment. Again, the reverse is true when the sediment is comprised of terrigenous materials.

  The relatively high availability of phosphate in porewaters from coarse-grained carbonate sediments in seagrass beds found within the study (Erftemeijer 1993) is in contrast to the general assumption that seagrass growth on carbonate sediments is phosphorus limited (Short 1987). But that study was working in fine-grained sedimentary environments (carbonate mud and silt) while another study (McGlathery 1992) found evidence of nitrogen limitation. Given the apparent discrepancies between nitrogen and phosphate limitations on seagrasses within the various studies done to date, Erftemeijer concludes that the grain size of the sediment is one of the primary factors determining the availability of phosphorus in a tropical carbonate sediment.  This is something to keep in mind when constructing a live deep sand bed for an aquarium.  

 Life within the sediment -  The Recyclers

  A few members of the sandbed infauna :

 Photo by Charles Raabe  Photo by Charles Raabe
 Foraminiferans and their remains are clearly the most abundant of the visible life forms found within the sediment. Not surprising given that Dr. Ron Shimek has sampled foraminiferans with a density of over 70,000 per square yard of ocean bottom.
Photo by Charles Raabe  Photo by Charles Raabe
 Polychaete worms and nematodes are also found in great abundance.  Most are microscopic containing both predator and prey species. By just their sheer numbers and relative mobility, they account for a great deal of the nutrient processing and recycling within the sediment and by their movements through the sediment help to turnover the sediment's layers.
Photo by Charles Raabe  Photo by Charles Raabe  Photo by Charles Raabe
            A barnacle cyprid                              Microscopic Gastropod                         A Gastropod veliger

   Two of the largest sediment dwellers that ingest sediment grains digesting any organics attached to the grains and any detritus that may have been pulled into the sediment.  Their movement also irrigates and disturbs the sediment.
 Photo by Linda Raabe  Photo by Charles Raabe
               Holothuridea sp.                                       Synaptid sp.

 

   Epiphytic Organisms - Important producers within seagrass habitats.

  Photo by Charles Raabe  The high productivity of seagrass beds is the product of not only the seagrasses but also a variety of epiphytic organisms that use the vast amount of surface area provided by the seagrass leaves on which to grow.  The most abundant of the epiphytic organisms are the microalgae, providing as much as 46% of the autotrophic production of seagrass beds. Since seagrasses are not known to produce any toxins or have any mechanisms to control the attachment and growth of epiphytes, epiphytes can be found on all exposed parts of the seagrass.
  Though the presence of epiphytes on the leaves of seagrasses is a natural phenomenon and contributes to the productivity of a seagrass ecosystem, eutrophication can cause abnormally high rates of epiphytic macroalgae and microalgae growth leading to the complete shading of the seagrasses and their subsequent loss. 




   Algae are not the only organisms that quickly take advantage of any substrate that affords them a position in the sunlight or water currents much to the detriment of the smothered individual leaf.  The sacrifice of individual leaves creates a much greater benefit to the seagrass meadow as a whole and is responsible in large part for the high rate of productivity which in turn fuels the complex ecosystem and nutrient food web that extends far beyond the confines of the seagrass meadows.  
  Epiphytic growth while seemingly detrimental to the seagrass may also benefit those plants that grow in areas that tidal movement exposes them to the air.  With a coating of epiphytic life forms, moisture retention is enhanced and may allow the seagrass to avoid desiccation (Bell 1997).

 Photo by Charles Raabe   As each leaf is covered in epiphytes, the ability of the leaf to perform photosynthesis is reduced and reaches a point where the leaf is of no use to the plant anymore. The leaf is cast off, along with any epiphytes unlucky enough to have settled on what seemed a permanent home.  The cast off leaf now further enriched with other life becomes part of the leaf litter mat and is acted upon by bacterial and fungi creating the detrital matter that so many other organisms find of use.  Having lost a leaf, the plant then pulls even more nutrients out of the sediment to create a new leaf to regain its photosynthesis capacity and makes sediment bound nutrients available once again. In turn, yet another new surface area arises for the epiphytes to colonize, and so the circle begins again.  With individual leaf life spans having been estimated to be anywhere from 3 to 10 days, there is a vast amount of organic material that a seagrass meadow is producing in a single week.


   Benthic microalgae (microphytobenthos) while very important in other shallow ecosystems do not contribute to the biomass and productivity in any significant amount within a mature seagrass bed. The lack of benthic microalgal activity is attributed to the sediment being shaded by the seagrass leaves, its leaf litter and the thick layer of detritus that blocks the sunlight and prevents photosynthesis from occurring.   In a developing seagrass bed the benthic microalgae would play a larger role in nitrogen fixation within the sediment since it is unlikely that a sun blocking layer of leaf litter and detritus would accumulate for quite some time. This microalgal layer may account for the added nutrient enrichment that the pioneering seagrass species need to gain new territory. 

 
   Epiphytic & Off Shore Drift MacroAlgae - Damaging intruders or contributors?

 
Photo by Charles Raabe   A tropical seagrass meadow will also likely contain macroalgae species (Bell 1997) that have either grown as epiphytes on any of the available surfaces or having been carried into the area by water currents and snagged on the seagrass blades.  In mature seagrass meadows, the unstable leaf litter does not present many substrates on which to attach other than the seagrass leaves or the larger exposed rock fragments. 
  I have noted marked seasonal variations in the abundance of macroalgae within the seagrass meadows.  During the monsoon season there is an obvious increase in the amount of macroalgae present due to frequent storms that create enough force that detach epiphytic or benthic macroalgae and drive them into the seagrass areas.  During the relatively dry season, storms are rare allowing the epiphytic macroalgae to remain where they have attached or settled. 
  The storm driven macroalgae that finds itself stranded within the seagrass meadows at the end of the monsoon season is most often left undisturbed during the dry season allowing the algae to stabilize and grow only to be torn away at the start of the next monsoon season. The macroalgae, having grown larger, now presents more surface area to the water currents.  I believe this and the lack of a stable substrate ensures that the macroalgae do not dominate or destroy the seagrasses and the result is that they are mostly transitory.
  During the relatively brief stay within the seagrass meadow, the macroalgae will continue to remove nutrients as they would anywhere else that they can grow. They take from the local nutrient pool only to transport the nutrients elsewhere when the season changes and the macroalgae is set adrift once again. Eventually the macroalgae's luck will run out and they will be washed up onshore, snagged on the coral reef eaten by herbivores or sink into the abyss. Either way the macroalgae has transported a fraction of the seagrasses productivity elsewhere.  Being seasonal and dependant upon the severity of the monsoonal storms, how much nutrient transportation takes place can be highly variable from year to year. 
  I have not observed any detrimental affects of any significance by the epiphytic or drift macroalgae as they are transitory in nature. Any damage done is restricted to small localized areas and is temporary (i.e. the macroalgae can shade/smother an individual seagrass plant and cause its demise). If the macroalgae is an epiphyte upon the seagrass leaves, the loss of the seagrass can also mean the loss of the macroalgae as it is dropped into the leaf litter.  If the macroalgae is adrift, the loss of the seagrass leaves will most likely allow the macroalgae to drop down onto the leaf litter and find itself becoming shaded and smothered, as well as possibly being consumed by the local herbivores.
  This all points to transient macroalgae having their nutrients either transported into the seagrass ecosystem by drift, or having their nutrients and any additionally gained nutrients through growth being transported out of the seagrass ecosystem or simply being recycled back into the seagrass ecosystem through herbivorous action and decay.

  Near-Shore Ulva spp. - Now you see it, now you don't.

 
Ulva spp. are another drift macroalgae that can also affect the seagrass community. Where the drift macroalgae mentioned previously originate from further offshore in relation to the seagrasses locale, the Ulva sp. originate near the shoreline prior to the seagrass meadow.  
   Following the seasonal cycle of the tropics, of which there are only two, a wet monsoon season and a dry season both of equal duration, limits the impact that these algae may have on the seagrass to a few months of the year when heavy rains wash the land and create eutrophic conditions near shore.
  I have often wondered at how such a loosely attached and often free floating algae could seem to completely disappear for many months only to make a rapid reappearance seemingly out of no where, hence the title of this section.  The answer lies within its life cycle and within the local seasonal variations.

  Ulva follows a reproductive pattern called alternation of generations, in which it takes two generations to complete its life cycle, one that reproduces sexually and one that reproduces asexually. Although mature members of both generations look the same to the naked eye, microscopic chromosomal differences distinguish one from the other. The first generation, which has two complete sets of chromosomes (2n), the second generation has only one set of chromosomes (n). The first generation, called the sporophyte, undergoes asexual reproduction to form spores, tiny reproductive cells that develop into mature individuals called gametophytes. Gametophytes produce gametes, male and female reproductive cells that fuse together during fertilization to produce a zygote, an organism with two complete sets of chromosomes that matures into a sporophyte, thus completing the life cycle.
  Ulva spp.are relatively simple when compared to more advanced algae and vascular plants. They do not differentiate into tissue layers or show much specialization among cells, making them a colony of like cells forming new cells perpendicular to the surface which gives this species their distinctive thin "tissue paper" appearance.  Since each cell, including the reproductive cells, are capable of photosynthesis, the uptake of nutrients and adjustments to light intensity is very rapid.
   Due to their high nitrogen requirements and their limited ability to store nitrogen, Ulva distribution is nitrogen limited and explains their seemingly quick appearance and subsequent disappearance after heavy rainfalls. Such occurrences make Ulva spp. good bioindicators of local water quality pertaining to its level of nitrogen enrichment. In such areas of constant enrichment, Ulva spp. become a permanent feature and can easily smother all benthic organisms found below it.   
  It is worth noting that there are no local Ulva populations here that could account for the sudden growth observed during periods of eutrophication after heavy rainfalls. This leads me to conclude that their zygotes are being carried in from distant locations that do contain permanent populations due to year round nitrogen enrichment as found near sewage discharges or river inlets.
  Once nutrient conditions favor local growth, these algae can make a very sudden appearance near the shoreline.  As they grow they will begin to settle on top of the bare sand and rubble substrate nearshore to the seagrass meadow and remains near shore. I at first attributed the shallow water propensity of Ulva to one of light intensity but have since learned that its appearance in near shore shallow water is simply a matter of available nitrogen through terrestrial runoff and only remains near shore due to a lack of sufficient wave disturbance.  A study of U. lactuca (Hansen 1992) has shown that this macroalgae can compensate for reduced light intensity by its ability to rapidly increase its cellular chlorophyll content and has its highest rate of growth with a mere 10% of available ambient light.  These facts do not bode well for the marine habitats as the only limiting factor of U. lactuca appears to be the availability of nitrogen, making any relatively shallow habitat vulnerable to being smothered by its rapid growth should nitrogen eutrophication occur over extended periods of time.
   With their appearance and having reached a sufficient size to present enough surface area to be uplifted by storm driven waves the algae is then carried offshore by tidal action where they become entangled with the seagrass leaves.  I have not noted any detrimental affects to the seagrasses other than the localized loss of single leaves due to shading by these macroalgae. Upon the loss of the leaves, the attached Ulva are either dropped down into the leaf litter where local grazers consume it or if left uneaten, will in a matter of days die as any available dissolved nitrogen is quickly consumed. Upon decomposition its limited nitrogen content does not appear to have any long term consequences as I have observed this local seasonal phenomenon since 2004 with no apparent loss of seagrass cover to date.  
  

   Bacteria / Fungus  -  The workhorses of all environments.

  Other than the much larger fish and animal grazers, most other animals can not directly consume seagrass due to its fibrous composition. The bacteria and fungi are the dominant consumers of seagrass primary production once such production has been added to the leaf litter and begins decomposition.  By their actions upon the cast off leaves they break down the fibrous material making it available to the majority of animals that otherwise would not be able to utilize seagrass production.  Bacteria not only use organic matter supplied by the seagrasses, but also any organics that have been recycled from animals and previous bacterial activities.  While the bacteria and fungus first make the cast off seagrass blades available to most other animals through decomposition, they also process the waste from the animals that benefited from their originally breaking down the seagrass production. They also utilize the byproducts of their own decomposition which results in a net gain of nutrients available to the seagrasses, and the bacteria and fungus themselves are food for many other animals in the form of detritus.  The nutrient net gain is further enhanced by the geochemistry that occurs within the sediment as briefly discussed above concerning nutrient availability per sediment composition and grain size.
  The microbial mats found on the surfaces of both the sediment and the leaves of the seagrasses are composed primarily of cyanobacteria that have a dual role related to productivity by fixing carbon dioxide and atmospheric nitrogen which often limits primary production in many other ecosystems (Hamisi 2004).  The cyanobacteria found in such mats also provide food to the heterotrophs. The inorganic nitrogen released by the heterotrophs utilizing the cyanobacteria, supports continued primary production by seagrasses in another cycle.  

Photo by Charles Raabe   As the seagrass leaves are decomposed they release both particulate and dissolved carbon and organic matter, which the bacteria and fungus assimilate and transform into detritus (also known as marine snow), a nutritionally important food source for detritivores. With a wide range of animals that consume detritus in all habitats throughout the oceans, it is of no surprise that given the massive production found within seagrass meadows the diversity of detritivores is equally as massive.

Examples of some common detritivores both below and above the sediment.
  Photo by Charles Raabe Photo by Charles Raabe Photo by Charles Raabe 
                Nematode sp.                                       Cirratulid sp.                                        Polychaete sp.

  Photo by Charles Raabe
               Foraminiferan sp.                                     Synaptid sp.                                     Holothuridea sp.

 Photo by Charles Raabe Photo by Charles Raabe Photo by Charles Raabe 
                  Copepod sp.                                       Amphipod sp.                                       Isopod sp.    

  As each animal consumes and then digests the detritus, a fraction of the digested food, mostly the amino acids and protein fragments will be used by the organism to build or repair tissues. Some of this will eventually be recycled and eliminated from the organism's body as ammonium in urine. The rest of the digested foods, primarily the carbohydrates and most of the lipids will be utilized in cellular respiration, oxidized to produce energy. Eventually they get eliminated from the organism as carbon dioxide and water (Shimek, 2002).


The Grazers -  Of seagrasses and epiphytic algae

 
In a previous study (Thayer 1984) done on the effects of large herbivores feeding upon seagrass productivity, the large herbivores were found capable of exerting an influence on the seagrass nutrient web and the stimulation of seagrass growth. Fish, sea turtles, sea urchins and dugongs that graze directly upon the seagrasses, representing at least 10% of their diet, can significantly alter the nutrient and detrital pathways by exporting the nutrients out of the seagrass meadows by swimming away and defecating elsewhere. Their grazing can also have both a stimulatory and negative impact on plant production affecting community structure and function.
  I have observed that the once much more abundant local large herbivores no longer have a significant effect on the seagrass community, the numbers of such grazers have been greatly reduced or eliminated by human activities within the relatively shallow areas that lack any enforcement of management regulations. With the uncontrolled harvesting, the local seagrass nutrient web has lost an important nutrient export link through the elimination of their primary herbivores. This is a pandemic problem.
  In areas where grazing by large herbivores still occurs, the seagrasses have a below ground reserve of available nutrients which allows the seagrasses to recover rapidly to levels that equal or exceed those in nearby ungrazed beds. In areas of intense grazing, these reserves have a stabilizing influence by allowing the seagrass to persist as their rhizomes and roots are largely left intact and able to quickly produce more leaves (Valentine 1999). Such grazing contributes much more to the transportation and disturbance of seagrass nutrients elsewhere than is found to occur locally in this study area. 

The removal of any and all herbivores
The local removal of herbivores and detritivores

 With the loss or significant reduction of all local large herbivores due to human predation, the only remaining major herbivore with any significant population is the inedible (to humans) Diadema sea urchin.  During my translocation study of this species I was able to determine that adult sea urchins restricted themselves to the immediate area surrounding their shelter, only venturing out during darkness to graze within a meter or two of their daytime shelter.  Such self-restriction limits their impact on seagrass to only those sea urchins that have found suitable shelter on the edges of the seagrass meadows or in the deeper depressions within the seagrass meadows that contain a suitable rocky substrate in which to gain shelter from.  Those depressions that do contain sea urchins graze most macroalgae from the hard substrate as well as the seagrasses that extend into the depression.  This constant clearing of all algae and plant growth creates suitable conditions for the settlement and growth of a number of coral species, that in their growth provide more substantial shelter for the sea urchins. Is this the birth of a shallow inshore reef?
  During the first two months after the end of the monsoon season, large numbers of Diadema setosum gather together and roam across the seagrass meadows grazing upon the epiphytic macroalgae species clearing their path of such growth while  releasing some of the macroalgae nutrients back into the seagrass ecosystem as waste and detritus.  Such congregations are what I believe to be the sea urchin's strategy to ensure a mate is always nearby while also having an abundant and readily available food source to gain or regain the energy and nutrients expended by sperm/egg production. 

Photo by Charles Raabe  
The type of shelter required by Diadema setosum and their impact on nearby seagrasses

     Astralium okamotoi is the most abundant of the gastropods within the local seagrass meadows, not selective in its feeding, leaving only the encrusting species behind.  Other commonly found snails include the Euplica sp., Trochoidea sp. and the Cerithidae sp.

 
Photo by Charles Raabe   Phanerophthalmus smaragdinus is one of many herbivorous slugs, possibly a detritivore as I only find them amongst the leaf litter where they can avoid predation.

  Photo by Charles Raabe   The only large gastropod found, feeding upon the epiphytic and drift macroalgae that it can reach as it is restricted to the floor of the meadow due to its size. Its movement on and in the leaf, detritus litter and sediment helps to distribute nutrients through disturbance.  Human collection for food has greatly reduced their numbers.

 Photo by Linda Raabe   Salarias fasciatus also known as the lawnmower blenny is the most numerous of the herbivorous fish with small juveniles found amongst the leaf litter making forays up to the seagrass blades to forage the epiphyte algae growth.  During periods of high tide, schools of both adult and juvenile rabbitfish species enter the seagrass meadows to graze upon drift Ulva spp. and seagrass epiphyte growth.

  The herbivores shown above are only a sample of the most commonly found species, there are of course far to many others for me to include.


  The Larger Predators -  Some are transitory, others are full time residents.

Both fish and invertebrate species find the seagrass meadows to be rich hunting grounds. Many fish species, especially larger predators, are transient residents as the seagrass beds become too shallow for them during low tides.

 Photo by Charles Raabe  Invertebrate predators
such as this Archaster sp. (sand sifting starfish) are permanent residents of the seagrass beds as they consume the infauna of the sediment.  Other large invertebrate predators include most other starfish species, hermit crabs, the swimming crabs and many other crustaceans.  

 
 Fish Predators such as this pipefish are also abundant given the high productivity of the seagrass ecosystem.  As shown above, fish such as this pipefish species are clearly full time residents, evident by their coloration and markings allowing them to blend in with the seagrass. File fish species also take the same colorations and markings while the flamboyantly colored fish species make themselves obvious as to their having come into the seagrass meadows from the coral reefs and are thus transitory opportunists.

 Photo by Charles Raabe  Schools of both juveniles and subadult Plotosus lineatus (striped sea catfish) are a common sight as they leap frog over each other sifting detritus and sediment infauna.


  Local Seagrass Distribution - The various hues of green in the below photograph are not entirely due to seagrass growth. The healthy seagrass meadows are found between the shoreline out to the 2 meter depth range, beyond that depth the frondose macroalgae dominate with an outer band of kelp growth prior to the coral reef.  The seagrasses found at depth prior to the coral reefs are at their toleration limits having shorter and fewer leaves with individual plants widely spaced in comparison to those plants found in the shallows.
  As shown below the vast flat expanse greatly reduces the wind driven waves and slows the effect of tidal flows providing a shallow, sheltered and near calm environment critical to the formation of composting leaf and detritus mats that are responsible in large part for the high productivity of seagrass meadows.


Seagrass distribution of Mactan Island, The Philippines


  Disturbances - Weather, water movement and fauna

 
As mentioned throughout this article, the feeding activities and movements through the seagrass, detritus and sediment as well as climatic and tidal events cause disturbances. These disturbances of the nutrients are yet another important factor in the seagrass nutrient web.  During periods of storm activity the larger than normal wind driven waves can uplift and suspend the leaf and detritus litter, moving large amounts of organic matter either towards shore or far out to sea with the tides and making the nutrients available to a large number of other animals outside of the seagrass habitat.  After periods of unusually strong winds and high waves, the shoreline can accumulate large mounds of wind and wave driven leaf litter that decomposes on shore releasing nutrients that wash back into the ocean through rainfall runoff spurring the growth of shoreline filamentous algae and thereby transporting the seagrass productivity elsewhere.  
  Such transportation also occurs when the prevailing tide and winds carry the leaf litter and detritus out to sea and deposits it onto the coral reef.  Carried far enough, the organic material can find its way to the deep ocean and drift downwards thousands of feet, being consumed and broken down further by pelagic plankton and fish and microbial action in the deep benthos, only to be carried back to the surface again in areas of ocean upwelling. Upwellings then fuel the production of plankton, contributing once again to the recycling of nutrients made available to the coral reef inhabitants.  
 

 Complex.  The only single word that best describes the diversity and nutrient webs that the seagrass meadows provide.  Doing the research for this article has made me much more aware of what used to be a little thought of habitat, giving me a greater appreciation and a sense of gratitude that the seagrass meadows are where they are.  Without such meadows, the coral reefs that we tend to focus on would be less for it.  






Photo by Mollie Dible
Mollie Dible's beautiful Seagrass Refugium

  Having explored a natural seagrass meadow we now turn our attention to the keeping of seagrass within an aquarium. Given the conditions and the nutrient dynamics discussed pertaining to natural meadows we now have a greater understanding of their needs and what they can bring to a reef aquarium system.  When used as a refugium plumbed into a reef display aquarium the nutrient processing and production of an aquarium reef system is enhanced and negates the need to use equipment and other products that attempt to do what nature does best, if allowed to. 

   Sediments -  The seagrass refugium's sediment is vital to the health and long term survival of the seagrass just as it is in natural meadows.  How you construct the sandbed is going to determine its functionality in providing a nutrient rich environment for the seagrasses rhizomes and root structures.  Seagrasses are plants that depend upon their roots for the uptake of nutrients, roots that require extremely fine grain sizes, it will be imperative that a calcium carbonate substrate with grain sizes ranging from 0.2 - 1.02mm be used with a depth of no less than six inches, deeper if possible.  Incorporating a live mud into the sediment during the sand beds construction will ensure a suitable number of infauna are introduced.  Just as in nature the infauna are critical to the functionality of any sandbed to process nutrients and prevent the formation of sand clumps by their movement through the sediment (Shimek 2001).

  Patience -  With the refugium plumbed inline with the rest of the aquarium system now comes the most difficult part of it all. The wait. It will take at least six months to one year for the sediments to have become enriched enough to support the requirements of the seagrasses (Calfo 2005).  To rush the introduction of the plants will most likely ensure their loss.  Having been uprooted, transported and their root systems likely damaged, the plants will need everything to be in their favor to get off to a good start.

 Indo-Pacific Plant Selection -  With the construction of the sandbed having been completed you now have at least six months to determine which species are available to you and study their husbandry.  My experience with any of the seagrasses is limited to the indo-pacific species yet through my study and recent attempts at keeping indo-pacific seagrasses it appears they all share some basic needs and care requirements, evident by their all being found within the same seagrass meadow.  

 Photo by Charles Raabe   Halophila ovalis appear to be the most tolerant of less than ideal handling and capable of surviving being shipped with bare roots and wrapped in moist paper towels for a number of days (Borneman 2008).  Being a pioneer species may account for this hardiness as they are frequently the first species to grow into uncolonized soft substrates. This apparent ability to go where no plant has gone before would in my opinion make them the best candidate for establishing a seagrass habitat with the later introduction of other seagrass species. Their very short growth and relatively low lighting needs in comparison to other seagrasses make them ideal for placement in coral reef aquariums as there is no danger of this seagrass shading or becoming abrasive to the corals and will tolerate the lowered light intensity found at the aquarium's sediment level.

 Photo provided by Molly Dible   Thalassia hemprichii  -  While not as common as T. testudinum (shown in the photo), the two species share very similar morphology and husbandry requirements. Given such similar morphology, I doubt many hobbyists can distinguish between the two other than by knowing where they were collected. With T. testudinum being an Atlantic species it is most likely that those hobbyists in the United States will use this species as their first seagrass keeping attempt(s) as it would be the most readily available of the species.  While not impossible to maintain, this species does appear to be sensitive to uprooting and the subsequent exposure to air.
 
 Photo by Charles Raabe   Syringodium isoetifolium  -  Second only to the Halophila sp. in its ability to colonize.  I have found S. isoetifolium to be hardy and fast growing.  It transplants much easier than the Thalassia sp. with a high rate of survival.  This species would make a good addition to either a newly established or mature seagrass aquarium, able to colonize rapidly while making a suitable companion species with established Thalassia spp. Normally not growing as tall as the Thalassia, it is not affected by partial shading and with their very thin, tubular leaves they pose no risk of shading the wider bladed Thalassia spp. either.  These traits between the two genera may explain their combination being the dominant structures in natural seagrass meadows here in the Philippines.

 Photo by Charles Raabe   Enhalus acoroides  -  With leaves averaging a length of 130cm and 3cm wide with root systems that can extend well beyond 30cm deep, this species is not a realistic choice for home aquarium systems.  For scale, the floor tiles shown in the photo are each a square foot with the plant being so long that I had to stand on a chair to get the entire plant into the frame. However, if one were to set up a suitable aquarium for this species it would make for a very unique display.  The affect of such a planted aquarium would be reminiscent of a kelp forest.  

Lighting the Seagrass Aquarium -   Given the environment the seagrasses are found in as discussed previously it should be obvious that they require high light intensity to thrive. Estimates from various studies (Kenworthy 1996) done on the minimum lighting requirements of seagrass species have shown that their lower toleration limit to be in a range from 24% to 37% of the light just beneath the water surface which equates to photosynthetically active radiation (PAR) levels of between 200 to 600 PAR at wavelengths between 400nm and 700nm.  Levels below these minimums result in the seagrasses producing shorter and fewer leaves, obvious indicators that more light intensity is needed over the aquarium. Extended periods of light levels below their minimum requirement places a stress on the seagrasses that some species are unable to recover from.  Halophila spp. are the exception and have much lower lighting requirements.
  Since most hobbyists have their own preferences in the lighting systems that they use, I am not going to suggest any particular type of lighting and instead stress again the importance of  light intensity to the seagrasses.  How that intensity is provided is irrelevant unless it has an impact on animals being kept within the same system due to heating of the water.

Water Flow & Temperature -  Tropical seagrasses have a high thermal tolerance averaging 90 degrees fahrenheit and live close to their thermal limits in the shallow, protected environments they are found in.  An aquarium that maintains the average Indo-Pacific coral reef water temperature of 82F is well within the temperature range that tropical seagrasses are adapted for, those aquariums that maintain lower than normal coral reef temperatures may encounter slower seagrass growth and their failure to thrive. Temperature is second only to light intensity pertaining to tropical seagrasses primary needs.
  Water flow through the seagrass aquarium should not include the use of powerheads or other high flow pumps and instead simply allow the overflow volume from the rest of the system to provide bulk water movement that will not suspend the leaf litter and detritus and encourage diversity of fauna and infauna.

 Water Parameters -  Being a plant, the seagrasses utilize the same common nutrients as terrestrial plants.   Nitrogen, phosphorus, iron and carbon dioxide, most of which is absorbed by the roots from the sediment with the exception of carbon. Phosphorus will not normally be a limiting factor in an aquarium system due to the input the system receives through the addition of food.  Nitrogen and iron is more likely to be utilized first by algae and bacteria and may become limiting to the seagrasses.  These limitations can be overcome by the use of plant food sticks or tablets (Borneman 2008) pushed down into the sediment near the plant's roots.  It is unlikely that nutrients provided in this manner will have any affect on the system unless the sediment is disturbed allowing the release of the nutrients into the circulating water.
 
Planting the Aquarium -  Now that the long wait is over with the sediment having been given time to sequester nutrients and the plant species selected for planting, its time to break out the gardening tools and... actually you will need nothing more than your gloved hand to accomplish the delicate task of placing your seagrasses into the sediment.

 Photo provided by David Smith   The roots of seagrasses are fragile with any damage done being the biggest factor in losing purchased plants.  This is unavoidable when purchasing seagrasses from commercial sources as the roots are usually stripped of any sediment to lessen the shipping costs involved with heavy sediments (Calfo 2005).  A good reason to start out with the hardier species that are known to have a relatively high survival rate when transported in such a manner.  If seagrasses are being shared or purchased from a local established seagrass aquarium then you have the opportunity to collect individual plants with less damage or disturbance to the roots by gently moving the sediments to expose the rhizome and cutting the rhizome with scissors in six inch lengths.  Once cut, gently lift the plant so as to keep as much of the root attached sediments intact and place the plant in a suitable container while being held under water (Borneman 2008).
  Having collected seagrasses from the local meadows, I have found that it is much easier to properly plant such lengths of the seaweed by simply making a trench in the aquarium's sandbed and gently place the rhizome and its roots into the trench and cover with sediment.  Do not force or push the rhizomes into the sediment as it will only break the rhizome and cause further damage.  Again, having access to the local seagrass meadows means that I can take extra measures to tip the plants survival rate in my favor.  A shallow, wide tupperware container and a spatula allows me to lift entire sod sections containing the rhizomes, roots and all of its surrounding sediment gently into a tupperware container (all done underwater) for easy transport back to my aquarium.  Digging a suitably sized pit into the sandbed I can then lower the tupperware onto the sediment and gently slide the entire sod section into the pit and cover with a centimeter or two of sand.
  Once planted, it is not uncommon for the seagrasses to drop all of their leaves due to the shock of having been disturbed.  With their relatively fast rate of leaf production, new leaves should begin to emerge within a week or two at most.  You can help ensure the plant has additional nutrients to replace its lost leaves and to recover more quickly by following a tip from Eric Borneman who has had increased success with newly planted seagrasses by purchasing freshwater plant food tablets that are broken in half and pushed down into the sediment close to the plant's root structures.


    As with any available lighted surface, microalgae will grow upon the seagrasses leaves and shorten the leaves usefulness to the plant by blocking the available light.  With Astralium spp. being the most commonly found snail consuming the microalgae on seagrass blades in natural meadows and being the most commonly sold species, they would make the best choice for keeping your seagrasses clean of epiphytic microalgae. A good stocking number to start out with would be one snail per plant, increasing their numbers if you find that the snails are unable to keep up with microalgae growth.  I feel I should point out that these species are most often sold to the reef aquarium hobby not because they are suitable for our rocky coral displays, they are not, but simply because they are found in great numbers in the seagrass meadows and with the meadows being nearshore and easily accessed they are collected by simply wading through the meadow and picking them off the seagrasses without any need for scuba gear as is required to collect the snail species that are found in coral reef areas. 

Conclusion :
 

   With what I have learned and observed of a tropical seagrass meadow it became obvious that a suitably sized refugium containing a live, deep sand bed constructed with calcium carbonate sediment of the correct grain sizes and stocked with seagrasses in a specific species sequence, will provide a diverse and functional habitat allowing a reef aquarium system an enhanced capability.  

Related Reading :

 
A Philippine Fringing Reef & The Reef Aquarium Part One

 
An Online Philippine Reef Tour

  The Reef Aquarium Clean Up Crew

Acknowledgments :  I would like to thank my wife Linda for her loving support and understanding of my interests in all things marine. A special thank you goes out to Eric Borneman for his generosity in providing assistance with this article and in helping me to make sense of tropical reefs. To Dr. Ron Shimek and Leslie Harris, thank you for the many identifications made as well as teaching me a great deal about marine biology and zoology.

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