Seagrasses and their epiphytes : Characterization of abundance and productivity in tropical seagrass beds

Abstract: Seagrass beds cover large intertidal and subtidal areas in coastal zones around the world and they are subjected to a wide variety of anthropogenic influences, such as nutrient enrichment due to sewage seepage. This study was undertaken to address specific questions focusing on whether near shore tropical seagrasses that receive a constant influx of groundwater nutrient inputs, would exhibit a higher productivity and to what extent epiphytic algae reflect the impacts of nutrient inputs. An additional aspect of study was to determine the prevalence of “acid zones” in tropical seagrasses. The productivity of the seagrasses Cymodocea rotundata, Thalassia hemprichii and Thalassodendron ciliatum was compared in two sites along the Kenyan coast; Nyali (a high nutrient site) and Vipingo (a low nutrient site). Of the three seagrasses T. hemprichii showed the most distinct differences with higher growth and biomass in the nutrient rich site whereas the growth of C. rotundata was similar in the two sites. A high epiphytic cover was found on the shoots of T. ciliatum found in the high nutrient site Nyali.Morphological and genetic characterization of bacterial and cyanobacterial epiphytes showed specific associations of nitrogen fixing cyanobacteria on the seagrass C. rotundata in the low nutrient site (Vipingo). At this site, shoots of C. rotundata had a higher C:N ratio compared to shoots in the high nutrient site (Nyali) indicating that the association with nitrogen fixing cyanobacteria is a strategy, for this species, to meet its nutrient needs. Bacterial epiphytes belonging to the group Cytophaga-Flavobacteria-Bacteroides (CFB) were found on T. ciliatum and T. hemprichii from the two sites. CFB bacteria are characteristic of waste water, particularly from livestock farming areas, thereby confirming seepage of groundwater from surrounding catchment areas. These prokaryotic associations were specific for the different seagrasses and it appears that the establishment of epiphytic associations may not be a random encounter but a specific association that meets specific needs.The seagrass T. ciliatum in the high nutrient site had an abundance of macroalgal epiphytes and the impact of the epiphytic coverage was assessed using Pulse Amplitude Modulated (PAM) fluorometry. The photosynthetic activity of seagrass parts that were covered by epiphytes was suppressed but the productivity of the whole shoot was not significantly reduced. In the nutrient rich site, epiphytes were found to contribute up to 45% of the total estimated gross productivity, during the SE monsoon season, while epiphytic contribution in the nutrient poor site, was 8%. Epiphytic abundance and contribution to productivity decreased during the NE monsoon. The photosynthetic activity of T. ciliatum shoots was similar in the two study sites with shoots in the nutrient rich site growing faster. T. ciliatum, in the low nutrient site, invested in the development of below ground root tissue which may indicate the development of a strategy to gain access to pore water nutrient pools.Carbon uptake strategies of eight tropical seagrasses were re-evaluated to determine how common the “acid zone” mechanism is among tropical seagrasses. Six of the eight species studied showed photosynthetic inorganic carbon (Ci) acquisition based on carbonic anhydrase catalysed HCO3- to CO2 conversions within an acidified diffusion boundary layer (“acid zone”). Cymodocea serrulata appeared to maintain its carbon uptake by extracellular carbonic anhydrase catalysed CO2 formation from HCO3- without the need for acidic zones, whereas, Halophila ovalis appeared to have a system in which H+ extrusion may be followed by HCO3--H+ co-transport into the cells. These findings indicate that competition for carbon, between the host seagrass species and epiphytes, could determine seagrass-epiphyte associations.