A typical Philippine mixed species seagrass bed

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 species (seven to nineteen species according to multiple 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.

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

   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.

   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.
 
 

  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.


   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 :

   
 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.
 
 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.
   
            A barnacle cyprid                              Microscopic Gastropod                         A Gastropod veliger


 

   Epiphytic Organisms - Important producers within seagrass habitats.

    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. 


   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?

    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.


   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.  

  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.
    
                Nematode sp.                                       Cirratulid sp.                                        Polychaete sp.                                   Terebellidae sp.

 
            Cephalochordata sp.                              Foraminiferan sp.                                     Synaptid sp.                                     Holothuridea sp.

 
                  Copepod sp.                                       Amphipod sp.                                       Isopod sp.                                        Ostracode 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. 


 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. 


     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.

    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.

    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.

    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.

   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.

   Schools of both juvenile 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.



  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.  





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

  A Philippine Fringing Reef & The Reef Aquarium Part Three

  A Philippine Fringing Reef & The Reef Aquarium Part Four

  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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