Microphytobenthic productivity in mangrove areas of different replanting regimes, Gazi Bay, Kenya.

Abstract

The principal primary producers in mangroves are the mangrove trees, although microphytobenthos contribute significantly to primary production. Recent studies have found microphytobenthos to be an important component of food webs in other intertidal ecosystems such as salt marshes and mudflats, and it has been suggested that their potential role in mangrove ecosystems deserves further study. The objective of this study was to estimate microphytobenthic production and the distribution of sediment chlorophyll a in areas of mangrove forests with different replanting regimes. The examined sites were a denuded area (D), a matrix plantation (MP) and an integrated plantation (IP), both 10 years old, and an old natural forest (N). All sites have previously been covered with natural mangrove forest. Conspicuous microalgal clods were also investigated (MA). The mapping of sediment chlorophyll a content was done within each site. The chlorophyll a content was also determined in the metabolism samples and used as an index for microalgal biomass. With the results from the metabolism and chlorophyll a measurements i.e. production, respiration and biomass, ratios like P/R and B/R could be calculated. Site D, site IP and site N were similar despite differences in both forest density and sediment type. The similarity in chlorophyll a between the mapping samples and the metabolism samples suggested that the metabolism samples were representative for all sites. MA had a respiration of 0.24 gCm-2, while the other sites had a respiration of between 0.06 and 0.1 gCm-2 Net production amounting to almost 2 gCm-2 was highest at MA and differed from all other sites. Site D (0.12 gCm-2 ) differed from both site IP (-0.04 gCm-2 ) and site N (-0.10 gCm-2 ), as did site MP (0.14 gCm-2 ). The gross production for MA was 2.20 gCm-2 and differed from all sites, also site D (0.19 gCm-2 ) and site MP (0.24 gCm-2 ) grouped together and differed from site IP (0.06 gCm-2 ) and site N ( 0.04 gCm-2 ). The turnover time varied between 8 and 21 days for the sites. The net production of 1.4 gC per g chlorophyll a showed that MA had higher production efficiency than all sites, which ranged between -0.1 and 0.3 g C per g chlorophyll a. There was a strong correlation between chlorophyll a and gross production, e.g. increased chlorophyll a resulted in increased gross production. For microphytobenthos, their part of the function as primary producers seemed to vary in proportion in mangrove areas of different replanting regimes. Being the only primary producer in denuded areas compared to being a small part of that function in dense natural forests. In conclusion, as a primary producer, microphytobenthic communities seem to vary in importance with the state of the mangrove ecosystem but yet they seem to be an important part of all mangrove ecosystems including the different replanting regimes.