Accumulation of seagrass beach cast along the Kenyan coast: a quantitative assessment
Accumulation of seagrass beach cast material was monitored along the beaches of the Mombasa Marine National Park and Reserve, Kenya between September 1995 and August 1996. Weekly surveys using a rapid visual assessment technique revealed an average total of 93,000 kg dry weight of beach cast material along a 9.5 km stretch of beaches in this area. An average of 88 ± 18% of the beach cast dry weight consisted of seagrass material (88% leaves) while the remainder was composed of the seaweeds Sargassum sp. and Ulva sp. The seagrass Thalassodendron ciliatum (Forsskal) den Hartog constituted the major part (76%) of the seagrass tissue on the beach, followed by Syringodium isoetifolium (Ascherson) Dandy (15%). An average of 19.7 ± 24.7% (n = 90; SE = 0.27) of the beach cast consisted of freshly-detached (green) seagrass material. The beach cast material was part of a pool of detached macrophytes in the intertidal zone washed back and forth between the beach and the adjacent reef lagoon with the ebb and flood tides. An average net diffusion factor (DF) of 29.1 ± 3.5 g 24 h-1 was measured in the lagoon using blocks of plaster of Paris, indicating a relatively high degree of exposure to waves and currents. Significantly (p = 0.006) larger amounts of beach cast were recorded during spring tide periods compared to neap tide periods. Weekly monitoring at three beach sites (Nyali, Bamburi, Reef) revealed that accumulation of beach cast was markedly seasonal with largest amounts observed during the South-East (SE) monsoon (March to October) and minimal amounts during the North-East (NE) monsoon (November to March). Extrapolation of the monitoring results indicated that the total amount of beach cast along the entire beach (9.5 km) varied between a minimum of 14,700 kg dry weight (or 31 g m-2) during the NE monsoon to a maximum of 1.2 million kg dry weight (or 2.5 kg m-2) during the SE monsoon. Decomposition of the beach cast material was measured by litter bag experiments. T. ciliatum leaves in litter bags lying on the beach surface showed a decomposition rate (k) of 0.017 day-1 ash-free dry weight (AFDW). The material in the litterbags took 42 days to lose 50% of its initial ash-free dry weight. Burial of litterbags under the sand did not result in a significant reduction of the decomposition rate. Large numbers of amphipods, isopods, nematodes and oligochaetes were associated with the beach cast material. Most dominant were amphipods which had an average density of 23,182 ± 10,697 animals m-2. A positive correlation (r = 0.4) was found between faunal density and amount of beach cast material. Above-ground biomass and primary production of seagrass meadows in the adjacent lagoon were 760 ± 96 g dry weight m-2 and 8.2 ± 2.8 g dry weight m-2 day-1, respectively. The total net production by the seagrass beds covering 60% of the 20 km2 lagoon was estimated to be 36 million kg dry weight year-1 (or 14.7 million kg C year-1). The turn-over of the beach cast material was in the order of 73 times per year, implying that approximately 6.8 million kg dry weight of seagrass material is being casted on the beach annually. This indicates that approximately 19% of the total seagrass productivity in the lagoon passes through the beach, where exposure to wind and sun, fragmentation, leaching and decomposition contribute to efficient recycling of nutrients.