Accumulation of seagrass beach cast along the Kenyan coast: a quantitative assessment

Aquatic Botany 65 (1999) 221–238

Accumulation of seagrass beach cast along the Kenyan coast: a quantitative assessment Caroline A. Ochienga,1 , Paul L.A. Erftemeijerb,1,∗ b

a Kenya Marine and Fisheries Research Institute, P.O. Box 81651, Mombasa, Kenya KWS/Netherlands ‘Wetlands Conservation and Training Programme’, Kenya Wildlife Service, P.O. Box 82144, Mombasa, Kenya

Abstract 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 ∗

Corresponding author. Tel.: +66-74-429307; fax: +66-74-429307. Present address: Wetlands International – Thailand Programme, P.O. Box 21, Si Phuwanat, Hat Yai 90113, Thailand. 1

0304-3770/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 9 9 ) 0 0 0 4 2 - X

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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. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Beach cast; Decomposition; Kenya; Production; Seagrass; Seasonality; Tides; Wrack

1. Introduction The ecological role of the extensive multispecies meadows of seagrasses, found along many tropical coastlines, is not only restricted to the seagrass beds proper (Rice and Tenore, 1981). Depending on the degree of exposure to water movements and other factors, detached material from seagrasses and other macrophytes may form large patches in the surf zone and huge banks on adjacent beaches (Lenanton et al., 1982), generally referred to as beach cast, beach wrack, wrack bank or beach strand. Detached seagrass material may also be transported to submerged depressions on bare sediment throughout the intertidal and subtidal areas, or further offshore to the deep ocean floor where it may contribute to deep sea food chains (Josselyn et al., 1983). Such nearshore accumulations, while suspended, can provide nursery areas for fish (Lenanton et al., 1982), constitute important sites for nutrient regeneration and form an important link between plant biomass and higher consumers (Robertson and Hansen, 1982). The accumulations can also filter out wave effects thereby contributing to beach stability (McLachlan et al., 1985) and often support high densities of faunal populations (Griffiths and Stenton-Dozey, 1981; Griffiths et al., 1983). After fragmentation and partial decomposition of beach cast material, the resuspended detritus serves as sedentary food source for shallow water and benthic feeding fish (Chubb et al., 1981; Lenanton, 1982; Bach et al., 1986) and provides material for further decomposition thereby accelerating the release of dissolved constituents and fine particles into the water column. However, the environmental processes affecting the quantities and seasonality of deposition and the rates of fragmentation and decay of beach cast material may differ geographically, depending on locally prevailing climatological and hydrodynamic conditions. A major part of the coastline of Kenya is characterised by sandy beaches sheltered by fringing reefs. These beaches support a major tourism industry of significant importance to the Kenyan economy. At the same time, the fringing reefs and the seagrass meadows in the adjacent lagoons form important productive ecosystems supporting coastal fisheries of major importance to the livelihood of coastal fishing communities. To recreation and tourism sectors, accumulations of beach cast are often considered a nuisance because of the

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stench and non-aesthetic quality. Despite substantial accumulations of beach cast material on the beaches along the Kenyan coast and the economic importance of these beaches for tourism, there have been no previous detailed studies of this phenomenon in Kenya. Following on-going debates between Marine Park Management Authorities (KWS) and the tourism industry about sweeping or bulldozing the beaches of its detached macrophytes, the need for first-hand information about the beach cast phenomenon along this coast became apparent. The present study was made as part of an environmental impact assessment of proposed beach cleaning operations (Ochieng, 1996a). The aim of the present study was to give a quantitative description of the occurrence of seagrass beach cast along the Kenyan coast by analysing the distribution, composition, quantity, accumulation process, associated fauna, decomposition, of seagrass beach cast material along the beaches of the Mombasa Marine National Park and Reserve. The important contribution of seagrass beach cast material to beach stability, its functional relationship with the adjacent living seagrass beds and the role in nutrient recycling of the lagoon are discussed. 2. Materials and methods 2.1. Study area The Kenyan coast has a relatively narrow continental shelf and is characterised by sandy beaches, rocky cliffs, fringing coral reefs and mangroves (Schoorl and Visser, 1991). The monsoonal climate gives rise to two seasons: a rainy season dominated by the South-East (SE) Monsoon (March to October), and a dry season dominated by the North-East (NE) Monsoon (November to March). The physical oceanographic processes of the coastal waters along the Kenyan coast are described in more detail by McClanahan (1988) and McClanahan and Muthiga (1988). The Mombasa Marine National Park and Reserve, covering 10 and 200 km2 , respectively, has been protected since 1990 (McClanahan et al., 1994). A 9.5 km long stretch of sandy beaches, which are part of the protected area, is intercepted by some rocky protrusions (Fig. 1). Adjoining the beaches is an intertidal reef platform and a reef lagoon with a maximum depth of 6 m. The area is characterised by a semi-diurnal tide with a maximum amplitude of 4 m (Kenya Ports Authority, predicted tide tables). Seagrass beds form the most dominant bottom community in the lagoon covering approximately 60% of its surface area, according to the results of a recent manta tow survey (Muthiga, 1996). The quantitative assessment of the accumulation of seagrass beach cast was carried out at three representative beach sites located at Nyali, Reef and Bamburi beaches along the Mombasa Marine National Park and Reserve during 1995–1996. These sites were selected on the criterion that they were not cleaned or raked of the beach cast material by hotel workers, were representative of the entire beach stretch (see beach-wide survey) and were easily accessible via public beach access routes. 2.2. Composition of beach cast material Composition of the beach cast was determined by random sampling of the material at the three selected beach sites on various occasions (n = 13), using aluminium frames

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Fig. 1. Map of the study area showing the location of the three sampling sites.

(25 × 25 cm). Samples were rinsed and sorted according to species and plant parts (i.e. seaweed, seagrass leaves, roots, rhizomes and stems) before drying in an oven to constant weight. An additional series of samples (n = 38 at Nyali beach; n = 20 at Bamburi and Reef) was collected to distinguish dark ‘old’ seagrass material from green freshly-detached seagrass material in order to estimate the proportion of beach cast material that newly joins the accumulated pool on the beach during each tidal cycle. 2.3. Rapid visual assessment technique The quantities of beach cast material were determined by using a rapid visual assessment technique – a modification of the technique by Mellors (1991) for estimating living seagrass biomass. Metal frames (25 × 25 cm) were thrown randomly on the beach and the amount

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of beach cast material within the frames was ranked on a linear scale of 1–5. Each rank represented the amount of beach cast in percentage cover, whereby 1 = 0–25%, 2 = 25–50%, 3 = 50–75%, 4 = 75–100% (thin layer), and 5 = >100% (thick layer). Photographs of these ranks were taken and agreed upon by the observers to be used as a source of reference. Try-out experiments established that 50 frame-throws on a beach section covered with beach cast were more than sufficient to reduce variation from the average below the 5% level. Replicate samples (n = 40) from each rank were rinsed and dried at 70◦ C to constant dry weights. The linear regression (Y = 0.59264; X = 0.47) relating ranks to the amounts of beach cast material m−2 (log-transformed) was significant (r2 = 0.92). 2.4. Survey and monitoring of beach cast accumulation The distribution of beach cast material along the Mombasa Marine National Park and Reserve was determined on 17 September 1995 by a survey of the amount of detached seagrass on the entire beach, using the rapid visual assessment technique. The 9.5 km stretch of beach was divided into sections of 1–1.5 km long, each covered by one observer. The metal frame was thrown 50 times at random within each 50 m beach interval and the scores were noted. The width of the beach at each beach interval was also recorded. Total amounts of beach cast material were calculated using the linear regression equation and expressed in kg dry weight m−2 and in kg dry weight per given beach section (250 m). The process of beach cast accumulation was monitored over a one year period between September 1995 and August 1996 at three selected beach sites at Nyali, Bamburi and Reef, to study (seasonal) dynamics of the phenomenon. During regular visits (usually 4–7 days apart) the quantity of beach cast material within a permanent plot of a 10 m long stretch of beach (i.e. between the low water line and the uppermost part of the beach) at each of the three study sites was assessed using the rapid visual technique. Attempts to compare the results of the rapid visual technique with the results obtained by raking together all the material on the beach within the plots were unsuccessful due to many practical problems involving the accuracy of the latter method (huge quantities of raked material with a high variation in water and sand contents). The data were pooled together according to the tidal cycle (spring or neap) to determine the tidal influence on accumulation. 2.5. Exposure to water motion Exposure of the living seagrass plants to water movements was assessed in March 1996 in the adjacent lagoon by measuring the weight loss of fixed blocks of plaster of Paris over a 24-h period, according to the method described by Jokiel and Morissey (1993). The diffusion factor (DF) was calculated as net weight loss (in g) 24 h−1 relative to controls in stagnant water, and used as a relative measure of exposure to water motion. 2.6. Fauna associated with beach cast material The density and spatial distribution of beach fauna associated with the beach cast was determined by sampling, for which three points along a cross-section of the beach were

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chosen, i.e. along the high water mark (loose sand with high amounts of beach cast), the shore-ward side (usually wet sand with lower amounts of beach cast), and beach sections without beach cast (bare sand). A corer with a surface area of 625 cm2 was used to collect the beach fauna samples randomly at each of the chosen points. The corer was placed quickly over the substrate to retain both the beach cast and the fauna. The beach cast material, the fauna and the sand (to a depth of ca. 10 cm) were collected in labelled plastic bags and stored. In the laboratory, samples were preserved in sea water formalin and coloured with rose-bengal to ease sorting. Plant parts and animals were separated from the samples through sieving. Animals retained by the sieve (mesh-size: 0.5 mm) were sorted, counted and identified with the aid of a stereo-microscope. Seagrass material was weighed wet, dried at 70◦ C in the oven, and re-weighed. Results were expressed in numbers per g dry weight of seagrass material and numbers per m2 . 2.7. Decomposition of beach cast material Litterbag experiments were carried out to determine the decomposition rate of seagrass beach cast material and the effect of burial (as recommended under the EIA). Leaves and stems of Thalassodendron ciliatum were freshly harvested from meadows in the adjacent lagoon (at a depth of 1–2 m) and transported on ice. At the laboratory, epiphytic material was carefully wiped off the leaves and stems, which were then dried on filter paper at room temperature (28–30◦ C) for 2 h. Accurate amounts of 7 g of this material was sewed in synthetic litter bags (15 × 20 cm) of 1 mm mesh size. A separate set of samples was used to determine constant dry weight (at 70◦ C) and ash-free dry weight (AFDW) by ignition at 540◦ C in a muffle furnace for 24 h. The average AFDW value of these samples was used as the initial weight against which weight loss (due to decomposition) would be measured. A total of 100 litterbags were deployed at mid-tidal level on the beach surface (50 with leaves and 50 with stems), and another 50 (with leaves only) were secured with a continuous nylon rope and buried at a depth of 50 cm. Five litter bags were retrieved from both buried and exposed series after 4, 8, 16, 24, 40, and 56 days. They were dried and their AFDW determined. The remaining AFDW at time t was expressed as a percentage of the initial value (t = 0). The data were fitted according to a single first-order exponential decay function (Wieder and Lang, 1982), using non-linear least square regression: Xt = X0 e−kt In this equation, Xt is the remaining AFDW of the material in the litterbags at time t, X0 the initial AFDW, t the time (days of field exposure) and k the first-order decomposition rate. 2.8. Biomass and production of the adjacent living seagrass beds The above-ground biomass of seagrass beds within the lagoon, dominated by T. ciliatum, was estimated by harvesting all shoots within metal frames (25 × 25 cm) thrown randomly in the meadow. Seagrass material was sorted into stems and leaves in the laboratory. Each component was dried at 70◦ C for 24 h and weighed. Shoot density was determined by counting all shoots within frames. Leaf production was measured by the leaf marking

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technique according to Kirkman and Reid (1979). An average of 60 shoots in random plots within the meadow were punched at a known reference level. Five days later the shoots were punched again at the same reference level, harvested and carried to the laboratory. New growth (i.e. the part between the first and the second holes) was measured, cut out, dried and weighed to determine the relative growth rate. Seagrass production was calculated from the relative growth rate in the plots and the general biomass data from the meadow. 2.9. Data analysis The comparison of beach cast quantities between spring and neap tides was statistically tested with a two-factor ANOVA with replication. The single first-order exponential model used to describe the decomposition kinetics of the beach cast material allowed for a comparison of data between litterbags on the beach surface and those buried under the sand. The relationship between faunal densities and amounts of beach cast material was studied by simple linear correlation analysis (in which ‘r’ represents the correlation coefficient).

3. Results 3.1. Composition of beach cast material An average of 88 ± 18% of the beach cast consisted of seagrass material while the remainder was composed of the seaweeds Sargassum sp. and Ulva sp. (Table 1). Approximately 88% of the seagrass material consisted of leaves. The seagrass T. ciliatum, which also dominates among the living seagrass meadows in the adjacent lagoon, constituted the major part (76 ± 29%) of the seagrass dry weight on the beach, followed by Syringodium isoetifolium (15 ± 20%). The composition of the beach cast varied somewhat between the three beach locations sampled: while at Reef and Nyali beaches the composition of beach cast material was dominated by T. ciliatum throughout the seasons, other species such as Thalassia hemprichii (Ehrenberg) Ascherson , S. isoetifolium and Cymodocea spp. collectively conTable 1 Composition of beach cast at three sites along the Mombasa Marine Park and Reserve, Kenya. Data are presented as average percentage (±SD) of total dry weight of beach cast material Nyali (n = 20)

Reef (n = 6)

Bamburi (n = 8)

Overall (n = 34)

Seagrass material: Thalassodendron ciliatum Syringodium isoetifolium Other seagrass species Seagrasses (total)

78.6 ± 21.7 6.8 ± 13.2 4.1 ± 6.4 89.5 ± 16.6

89.9 ± 8.6 8.2 ± 8.9 0 98.1 ± 1.7

32.7 ± 24.6 33.4 ± 21.4 10.9 ± 19.1 76.9 ± 22.7

69.8 ± 29.6 13.3 ± 18.7 5.0 ± 11.1 88.1 ± 18.2

Seaweed material: Ulva spp. Sargassum sp. Seaweeds (total)

4.1 ± 5.8 6.4 ± 17.1 10.5 ± 16.5

0.7 ± 0.5 1.2 ± 1.7 1.9 ± 1.7

0.1 ± 0.2 23.0 ± 22.5 23.0 ± 22.7

2.6 ± 4.8 9.4 ± 18.8 11.9 ± 18.2

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tributed an almost equal share to the beach cast material at Bamburi beach, except during the peak accumulation periods, when T. ciliatum dominated. The beach cast material was observed to be part of a pool of detached macrophytes in the intertidal zone. With the ebb and flood tides, material from this pool is washed back and forth between the beach and the surfzone of the lagoon. On the beach, the material dries in the sun and changes in colour, and after partial fragmentation and decomposition ultimately washes back into the water column either as detritus or as dissolved nutrients. Meanwhile, new seagrass material is detached from the meadow in the lagoon, becomes part of the pool and is added to the beach cast. An average of 19.7 ± 24.7% (n = 90; SE = 0.27) of the dry weight of beach cast material consisted of green (newly-detached) seagrass material. 3.2. Distribution of beach cast accumulation along the study area The survey on 17 September 1995 using the rapid visual assessment technique, revealed an estimated total of 32,400 kg dry weight of beach cast along the 9.5 km of beach. The distribution of beach cast material over the length of the beaches in the study area was not uniform (Fig. 2). Accumulation was highest (up to 8.3 tonnes dry weight per stretch of 250 m) at beach sections immediately adjacent to the rocky protrusions of Ras Iwetine (Reef) and Ras Mkuungombe (Nyali). Beach cast material was fairly evenly distributed

Fig. 2. Distribution of beach cast material (in g dry weight m−2 ) on the beaches along the coastline of the Mombasa Marine Park and Reserve (17 September 1995). Y-axis represents distance in km along the park’s shore from south to north starting at Nyali Beach. (SD < 5% of average).

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Fig. 3. Seasonality of beach cast accumulation at the three study sites (Reef, Nyali and Bamburi beaches) showing the quantity of beach cast material (in g dry weight m−2 ) measured with the rapid visual technique in permanent beach plots. (SD < 5% of average).

over the width of the beach throughout the study area, particularly during the SE monsoon season. The average width of the beaches along the Mombasa Marine National Park and Reserve was 46 ± 18 m (n = 138). During the NE monsoon (and some neap tide periods) most material usually accumulated around the MHWS line, especially of the species S. isoetifolium, compared to the other sections of the beach. 3.3. Seasonality and tidal influence of beach cast accumulation The phenomenon of beach cast accumulations was found to be clearly seasonal. During the SE monsoon (March to October), seagrass beach cast accumulation reached its peak, measuring up to 2000 g dry weight m−2 , whereas during the NE monsoon (November to March), beach cast accumulation was minimal (Fig. 3). Extrapolation of the results of a year long monitoring at the three beach sites indicated that the total amount of seagrass beach cast along the entire Mombasa Marine Park and Reserve (approximately 9.5 km beach stretch) varied between a minimum of 14,700 kg dry weight during the NE monsoon to a maximum of 1.2 million kg dry weight during the SE monsoon periods, with an overall average of 93,000 kg dry weight (excluding sand). Tides had a marked influence on the accumulation of seagrass on the beach. Significantly (ANOVA; n = 26; F1,2 = 8.0122; p = 0.006) larger amounts of beach cast material were recorded during the spring tide periods compared to the neap tide periods (Fig. 4). This

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Fig. 4. The influence of tides on the accumulation of beach cast material at two of the study sites (A: Bamburi and B: Reef), showing fluctuations in the quantities of beach cast material measured in permanent beach plots (in g dry weight m−2 ). Peaks coincide with spring tides, depressions with neap tides.

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Table 2 Average densities of fauna (in N m−2 ±SD) associated with seagrass beach cast material and bare sand at Nyali beach, Mombasa, Kenya n=4 seagrass (g dry weight m−2 ) Faunal groups

High water mark 395 ± 186 (N m−2 )

Low water mark 173 ± 41 (N m−2 )

Bare sand 0 (N m−2 )

Amphipods Isopods Nematodes Oligochaetes Bivalves Gastropods Coleoptera Other insects Mites Ants Unidentified larvae

23,182 ± 10,697 1207 ± 1161 1584 ± 2731 1140 ± 1110 967 ± 884 560 ± 362 474 ± 390 76 ± 86 51 ± 52 25 ± 33 0

1823 ± 620 3106 ± 1985 31 ± 42 5±8 1115 ± 649 92 ± 4 0 15 ± 17 0 10 ± 17 0

25 ± 2 97 ± 121 20 ± 25 0 0 31 ± 23 0 20 ± 20 0 5±8 5±8

Total

29,266 ± 11,204

6195 ± 2179

203 ± 129

phenomenon was consistent throughout the year at each of the three study sites. Although the quantity and composition of beach cast material showed variations between sites, the trends that indicate the influence of tides and seasonality were similar at all three sites. Experiments to measure exposure to water movements in the lagoon adjacent to Nyali, Reef and Bamburi beaches showed average net DF of 32.45 ± 0.65, 23.78 ± 1.02 and 30.08 ± 1.66 g 24 h−1 , respectively. The average net DF for the lagoon was 29.1 ± 3.5 g 24 h−1 , indicating a relatively high degree of exposure to waves and currents. 3.4. Fauna associated with beach cast material The fauna associated with the beach cast was composed largely of amphipods, isopods, nematodes and oligochaetes, the majority of which were found to inhabit the beach side where the biomass of beach cast material was highest (Table 2). Most dominant were amphipods which had an average density of 23,182 ± 10,697 animals m−2 . Isopods and young bivalves were more abundant at the wetter seaward side at areas covered in substantial amounts of seagrass beach cast material. A positive correlation (r = 0.4) was found between faunal density and amount of beach cast material. Faunal densities ranged from 35–75 animals per g dry weight of seagrass beach cast. Significantly larger numbers of invertebrate animals were found in parts of the sandy beach covered with beach cast material compared to bare sand (ANOVA, p < 0.05). 3.5. Biomass and production of adjacent living seagrass beds The average above-ground seagrass biomass was 760 ± 96 g dry weight m−2 , with an average leaf biomass of 569.9 ± 92.2 g dry weight m−2 . Since it was estimated that seagrasses cover 60% of the substrate in the 20 km2 lagoon, an overall total standing stock of 9.1 million

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kg dry weight was calculated for the entire lagoon, or nearly 100 times the annual average amount of seagrass beach cast in this area. Leaf marking measurements of T. ciliatum in the lagoon (n = 224) showed an average shoot growth of 13.4 ± 1.4 mm shoot−1 day−1 and a relative growth rate of 0.0142 ± 0.0029 g g−1 dry weight day−1 . Seagrass production was 8.24 ± 2.85 g dry weight m−2 day−1 . Taking the total estimate of 60% seagrass cover in the 20 km2 lagoon, the total seagrass productivity of the entire lagoon was therefore estimated to be in the order of 36 million kg dry weight year−1 . As elaborated above, 19.7 ± 24.7% of the total amount of beach cast material consisted of green (newly-detached) seagrass leaf material. Sub-samples of green leaves taken to the lab turned dark (nearly black) within 6 h when exposed to air in the sun. Assuming therefore that all freshly detached (green) material turns dark within one day while exposed on the beach, this would indicate an estimated turn-over of the beach cast material of 73 times per year. By extrapolation of this turn-over rate, it is estimated that 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. 3.6. Decomposition of beach cast material Detached T. ciliatum leaves in litter bags deployed on the beach surface showed a decomposition rate (k) of 0.017 day−1 AFDW (R2 = 0.797; 95% ci: 0.013–0.020; Fig. 5). For litterbags that were buried under the sand the decomposition rate was 0.016 day−1 AFDW (R2 = 0.824; ci: 0.013–0.018; Fig. 5). The decomposition rate (k) of litterbags on the surface was higher than of those buried under the sand, but this difference was not significant. Decomposition of stems (not shown in the figure) did not differ significantly from leaves. From the exponential curve fitting it was calculated that it takes approximately 42 days for the detached leaves on the beach to lose 50% of their initial AFDW. By extrapolation from the average amount of seagrass beach cast along the Mombasa Marine Park and Reserve, it was then calculated that a total of 404,107 kg dry weight of material is decomposed on the beach per year, representing roughly 1% of the total annual seagrass productivity in the adjacent lagoon.

4. Discussion Large volumes of seagrass material periodically wash ashore on the beaches along the Kenyan coast. Similar accumulations of seagrass and other marine macrophyte material on beaches have been reported elsewhere, such as in Australia (Lenanton et al., 1982; Robertson and Hansen, 1982), South Africa (Griffiths and Stenton-Dozey, 1981; Griffiths et al., 1983), Mauritania (Hemminga and Nieuwenhuize, 1990; Hemminga and Nieuwenhuize, 1991), and the Netherlands (Pellikaan, 1984). The present study made a year-round quantitative assessment of this phenomenon along the Mombasa Marine Park and Reserve in Kenya, and found an overall average of 93,000 kg dry weight of beach cast material distributed over 9.5 km stretch of beach, dominated by above-ground material of the seagrass T. ciliatum. The accumulation of beach cast is a result of the interaction between dense nearshore seagrass meadows and physical factors such as winds, currents, waves

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Fig. 5. Proportion of remaining AFDW of Thalassodendron ciliatum leaves in litterbags deployed on the beach surface or buried 50 cm below the sand at Nyali beach as a function of time. X0 and k describing decrease of AFDW (n = 5) on the beach surface were 0.93 (ci: 0.80–0.98) and 0.017 (ci: 0.013–0.020), respectively; R2 = 0.797. X0 and k describing decrease of AFDW samples buried (n = 5) were 0.97 (ci: 0.93–1.10) and 0.016 (ci: 0.013–0.018), respectively; R2 = 0.824. (SD < 10% of average in all deployment situations).

and tides that determine their exposure to water motion. Fringed by a relatively narrow continental shelf, the Kenyan coastline is in very close proximity with the open ocean. Prevailing winds, waves and surface currents are predominantly onshore (i.e. towards the beach). Combined with a relatively large tidal range of 4 m (McClanahan, 1988), these factors result in a relatively high level of exposure of the nearshore seagrass meadows to the sea action, responsible for the large accumulations of seagrass beach cast material. This is supported by an average net DF of 29.1 ± 3.5 g 24 h−1 found in the seagrass meadows in the

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nearshore lagoon during March (shortly before the SE monsoon), which is relatively high compared to values reported in literature for other seagrass and reef environments (Jokiel and Morissey, 1993). Due to the lack of previous quantitative data, it remains unclear whether the accumulations on the Kenyan beaches are increasing over the years or not. Anecdotal reports (incl. photographs) of huge heaps of beach cast material at Watamu from as early as 1969 (Anonymous, 1969), however, do not appear indicative of a recent increase. Any increase in the amount of beach cast would, however, most likely be the result of changes in the factors that influence the coastal hydrodynamics, such as by degradation of the fringing coral reefs, rather than by eutrophication or pollution of the lagoon, as sometimes suggested. The irregular spatial distribution of the material found during the beach-wide rapid assessment survey of the beaches in the study area indicates the impact of coastal morphology (e.g. rocky protrusions) on hydrodynamics and thus on the accumulation process, although localised beach cleaning by hotel operators might have further contributed to the patchy distribution. The beach cast material in the study area appears to originate largely from the seagrass meadows in the adjacent lagoon, which can be substantiated by the similarity in species composition between the beach cast and the adjacent meadows, which was not uniform throughout the study area. Even more obvious was the example of the beach stretch known as Jomo Kenyatta Public Beach, where the near-absence of beach cast coincided with the virtual absence of nearby seagrass meadows due to excessive recreation, watersports activities and reef-walking in the adjacent intertidal area. It is also noted that the above-ground parts of some seagrass species, such as S. isoetifolium (which has relatively large lacunar spaces), appear to be more easily torn loose by wave action than some other more sturdy species. T. ciliatum is often reported in literature as the dominant species in tropical coastal waters that are characterised by a relatively high degree of exposure to water movements which seems to explain the dominance of this species in the study area. The accumulation of beach cast in the study area showed a marked seasonality, with highest amounts being encountered during the SE monsoon period when downwelling, wind speed, wind run/force, wave heights and water column mixing are usually greatest (McClanahan, 1988). Quantities were rather insignificant during the much calmer NE monsoon. Seasonality of deposition of beach cast material has also been reported for kelp wrack in South Africa (Griffiths et al., 1983) and detached macrophytes in Australia (Lenanton et al., 1982; Robertson and Hansen, 1982), where deposition was highest during seasons characterised by frequent storms and heavy swells. It is therefore clear that hydrodynamics play a major role in the process of detachment and deposition. The contribution of some seaweed species, (Sargassum sp. and Ulva spp.) in the beach cast material became prominent only during the beginning of the SE monsoon (March to May), when rains coincided with short periods of macroalgal blooming. Tides had a marked influence on the quantities of detached seagrass material accumulating on the beach. The amounts of beach cast were always significantly higher during spring tides compared to neap tides (2-factor ANOVA; n = 26; F1,2 = 8.0122; p = 0.006). Spring tides are usually associated with stronger winds, currents and wave action compared to neap tides. In addition, parts of the seagrass meadow may be exposed to air during extreme spring low tides, leading to desiccation (especially during day-time) which further

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contributes to the detachment of seagrass plants (Erftemeijer and Herman, 1994; Stapel et al., 1997). Formation of detritus from seagrass material is of fundamental importance in supporting associated food-chains and nutrient recycling in coastal marine environments (Harrison and Mann, 1975). In the present study, the rate of decomposition of the beach cast material (k = 0.017; or 50% loss of AFDW in 42 days) as measured by in situ litterbag experiments, was within the range of decomposition rates reported in literature for leaf material of other tropical seagrass species (Harrison, 1989; Hemminga and Nieuwenhuize, 1991; Stapel, 1997). Although the decomposition of beached seagrass material is less studied than submerged decomposition within meadows, rates are known to be faster in drier material and in detached seagrass material compared to wet fresh material (Barnes, 1980; Newell et al., 1986). The high temperature at the beaches in Mombasa hardly would allow newly deposited material to stay fresh for more than a few hours after exposure. The decomposition rate of seagrass material in litterbags buried under the sand appeared slower than material lying on the beach surface, but this difference was not significant. Occasional strong waves during the experiment led to unintended burial of litterbags on the surface which might have affected our data. A reduction in decomposition rate due to burial might be attributed to reduced exposure to the impacts of wave action at high tides, reduced drying by exposure to sunlight at low tides, reduced oxygen availability, and reduced grazing/fragmentation by fauna. The process of washing ashore of detached seagrass material subjects the material to a period of drying as well as exposure to maximum oxygen availability (usually less in submerged areas), vigorous fragmentation by wave action back and forth to and from the lagoon, and consumption and fragmentation by large numbers of associated small animals, especially amphipods. Drying accelerates the release of cell contents by leaching, which contributes substantially to the total loss of the plant’s organic matter. The beaching of seagrass material therefore may accelerate the release of dissolved nutrients, inorganic materials and fine particulates (after desiccation, fragmentation and leaching) back into the water column, thereby contributing significantly to inshore marine nutrient pools and in sustaining the productivity of the lagoon ecosystem (Welsh et al., 1979; McLachlan, 1979, 1980; McLachlan et al., 1981). The exposure of detached seagrass material on the beach thus appears to enhance the rate of decay of the material, yielding it more rapidly into the detrital foodweb than do decaying leaves in submerged areas, as previously reported by Newell et al. (1986). Primary production of the seagrass beds in the lagoon in the study area was estimated to be 8.2 g dry weight m−2 day−1 . This is within the range of values reported by others for T. ciliatum in Gazi Bay, Kenya (Hemminga et al., 1995; Ochieng, 1996a, b) and in Eastern Indonesia (Brouns, 1985). The total net annual production by the seagrass beds covering 60% of the 20 km2 lagoon was estimated to be in the order of 36 million kg dry weight year−1 (or 14.7 million kg C year−1 ). The total annual deposition of seagrass beach cast in the study area (along 9.5 km of beach) was estimated to be in the order of 6.8 million kg dry weight, indicating that approximately 19% of the total seagrass productivity in the lagoon passes through the beach. This figure compares well with the results of a similar study in Western Australia, where an estimated 25% of the annual primary production of nearshore seagrass meadows was found passing through the surf zone and sandy beaches (Robertson and Hansen, 1982).

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The estimated turn-over of beach cast material of 73 times per year does not imply that all this material (6.8 million kg dry weight per year) is completely decomposed on the beach. The major proportion of the material stays only temporarily on the beach before being washed back into the lagoon. While on the beach, however, the material is exposed to sun, wind, consumption and fragmentation by beach fauna, partial decomposition and leaching. The combined forces of waves and sand in the surf zone further contribute to fragmentation of the material. The decomposition figures derived from the litterbag experiments indicate a turn-over by decomposition of 4.3 times per year. Compared with the turn-over of 73, this means that only about 6% of the beach cast material is actually decomposed on the beach. A predominance of amphipods (and to a lesser extent isopods and nematodes) in the invertebrate benthic fauna associated with beach cast, as found in the present study, is in accordance with previous studies on stranded kelp in South Africa (Griffiths and Stenton-Dozey, 1981; Griffiths et al., 1983) and intertidal shoreline deposits of Thalassia testudinum Banks ex Konig in Florida (Newell et al., 1986). Consumption by amphipods accounted for 74% of the weight loss of the deposits in the South African study (Griffiths and Stenton-Dozey, 1981), where consumers in the sandy beach systems derived up to 90% of their energy demand from the beach cast material. Deposit feeders play a major role in fragmenting the beached plant material, thereby enhancing the process of microbial decay and nutrient recycling into the nearshore ecosystem. The fauna associated with the stranded macrophyte material are an important food source for shorebirds and beach crabs at low waters, and – especially the amphipods – for (juvenile) fishes at high water (Lenanton, 1982; Lenanton et al., 1982). The removal of stranded detached macrophytes, as being considered by marine park management authorities in Kenya and as indeed implemented along many beaches of key importance to tourism or recreation elsewhere in the world may not come without environmental consequences. The various studies on this topic clearly point to the important role of beach cast accumulations in nutrient regeneration processes, beach stability, and as nursery sites and feeding grounds for fishes, crabs and shorebirds. Incidental removal of massive amounts of seagrass beach cast stranded after severe storms, such as recently in Florida after Hurricane George (Naples Daily News, 9 October 1998) may at times be desirable and acceptable. However, at beaches where seagrass constantly washes ashore in significant quantities throughout the year, regular removal involving use of machines, manual raking or burial of beach cast in trenches dug in the sand, could have substantial impacts on the beach stability by enhancing beach erosion. While lying on the beach, the beach cast material keeps the sand wet and compacted reducing its susceptibility to wind erosion. The beach cast also attenuates wave energy in the surf zone and augments the filtration process. When collecting beach cast material for disposal elsewhere, the collected material typically contains significant amounts of sand (estimated 84% of dry weight in this study). Calculations in the study area have shown that beach cast removal from the beaches along the Mombasa Marine Park and Reserve would result in erosion of 492,000 kg of beach sand (equivalent to 182 m3 ) per removal event. In addition, raking, digging and burial activities cause a breaking of the hard pan of the beach surface and loosening of the sand, thereby further enhancing its susceptibility to erosion. It was also noted that beach cast accumulation on higher sections of the beach contributes to the formation of dunes, known to play a vital role in maintaining stability of shorelines.

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