Figure 15. Thalassia blades showing tips encrusted with calcareous epiphytic algae. 

 Several of the larger blades show the effects of grazing on the leaf tips. 



Seagrass leaves are more heavily epi- 

 phytized at their tips than their bases 

 for various reasons. For the snail algae, 

 being on the leaves has the advantage of 

 raising them higher in the photic zone. 

 The shading effect produced by epiphytic 

 organisms on seagrass leaves decreases 

 photosynthesis by 31% (Sand-Jensen 1975). 

 In addition, the upper leaf surface exper- 

 iences much greater water notion than the 

 lower surface. This not only provides a 

 much greater volume of water to be swept 

 by suspension-feeding animals, but also 

 reduces the gradients for photosynthetic 

 organisns. Studies have shown that there 

 is transfer of nutrients from seagrasses 

 to epiphytes. Harlin (1975) described the 

 uptake of PO4 translocated up the leaves 

 of Zostera and Phyllospadix . Epiphytic 

 blue-green algae have the capacity to fix 

 molecular nitrogen, and Coering and Parker 

 (1972) showed that soluble nitrate fixed 

 in this manner was utilized by seagrasses. 



Epiphytes also contribute to the pri- 

 mary production of the seagrass ecosystem. 



In some areas there are few epiphytes and 

 little contribution, but in places the 

 amount of production is high. Jones (1968) 

 estimated that in northern Biscayne Pay 

 epiphytes contributed from 255^ to 33% of 

 the community metabolism. Epiphytes con- 

 tributed 18% of productivity of Zostera 

 meadows in North Carolina (Penhale 1977). 

 The trophic structure of these leaf com- 

 munities can be quite complex and will be 

 discussed later. Much of the epiphytic 

 material, both plant and animal, ultimate- 

 ly becomes part of the litter and detritus 

 as the leaf senesces and detaches. 



5.2 INVERTEBRATES 



Composition 



The invertebrate fauna of seagrass 

 beds is exceedingly rich and can only be 

 characterized in broad terms unless one is 

 dealing with a specific, defined area. 

 This is because the fauna of the grass 

 beds is diverse, with many hundreds of 



45 



