Accumulation of seagrass beach cast along the Kenyan coast: a quantitativeassessment

Accumulation of seagrass beach cast material was monitored along the beache s of the Mombasa Marine National Park and Reserve, Kenya between September 1995 and August 1996. Weekly surveys using a rapid visual assessment techni que revealed an average total of 93,000 kg dry weight of beach cast materia l 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) wh ile the remainder was composed of the seaweeds Sargassum sp. and Ulva sp, T he seagrass Thalassodendron ciliatum (Forsskal) den Hartog constituted the major part (76%) of the seagrass tissue on the beach, followed by Syringodi um 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) seag rass material. The beach cast material was part of a pool of detached macro phytes in the intertidal zone washed back and forth between the beach and t he adjacent reef lagoon with the ebb and flood tides, An average net diffus ion 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 exposur e 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 th at accumulation of beach cast was markedly seasonal with largest amounts ob served during the South-East (SE) monsoon (March to October) and minimal am ounts during the North-East (NE) monsoon (November to March). Extrapolation of the monitoring results indicated that the total amount of beach cast al ong the entire beach (9.5 km) varied between a minimum of 14,700 kg dry wei ght (or 31 g m(-2)) during the NE monsoon to a maximum of 1.2 million kg dr y weight (or 2.5 kg m(-2)) during the SE monsoon. Decomposition of the beac h cast material was measured by litter bag experiments. T. ciliatum leaves in litter bags lying on the beach surface showed a decomposition rate (k) o f 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 litt erbags under the sand did not result in a significant reduction of the deco mposition rate. Large numbers of amphipods, isopods, nematodes and oligocha etes were associated with the beach cast material. Most dominant were amphi pods which had an average density of 23,182 +/- 10,697 animals m(-2) A posi tive correlation (r = 0.4) was found between faunal density and amount of b each 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 km(2) lagoon was estimated to b e 36 million kg dry weight year(-1) (or 14.7 million kg C year(-1)). The tu rn-over of the beach cast material was in the order of 73 times per year, i mplying that approximately 6.8 million kg dry weight of seagrass material i s 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. (C) 1999 Elsevier Science B .V. All rights reserved.