Seagrass Material Research Articles

δ15N and δ13C analysis of a Posidonia sinuosa seagrass bed

Potential food sources and dominant invertebrates and fishes were collected for the examination of variability in 13C/ 12C and 15N/ 14N to determine the sources of carbon available to consumers within a Western Australian Posidonia sinuosa-dominated seagrass bed. Autotrophs showed a wide distribution of δ 13C values, with P. sinuosa at −11.3 ± 0.8‰ and macroalgae ranging from −16.6 to −31.7‰. This variation allowed us to successfully identify macroalgae as the main contributor of carbon to the trophic structure, although no distinction could be made between epiphytic macroalgae on seagrass, or allochthonous macroalgal sources. The range in δ 15N ratios among potential food items at the trophic base was too small to make it useful as tracer of nitrogen flow pathways, but it consistently increased from macrophytes and detritus (4.1–6.8‰), to invertebrates (5.7–7.4‰) located near the middle of the food web, to fishes (8.3–11.9‰), with piscivorous species such as Leviprora inops generally having a higher 15N. δ 13C of seston (−12.8‰) and sedimentary organic matter (−8.7‰) indicate that seagrass material is the main contributor to these two carbon pools, and that very little of it contributes to animal biomass.

Carbon and nitrogen stable isotope analysis of an Amphibolis griffithii seagrass bed

Western Australia has a rich diversity of seagrasses, many of which are meadow-forming species with a high diversity of associated epiphytes. Potential food sources and dominant invertebrates and fishes were collected in a non-quantitative sampling programme designed to examine the variability in naturally occurring isotopes ( 13C/ 12C and 15N/ 14N) within an Amphibolis griffithii dominated seagrass bed in Western Australia. The aims of this study were to determine the isotopic composition of the organisms, and to determine the sources of carbon available to consumers using the variations in the ratio of 15N/ 14N and 13C/ 12C among organisms in the seagrass assemblage. Autotrophs showed a wide distribution of δ 13C values, with seagrass material significantly enriched in 13C relative to macroalgal sources by >10‰. This variation allowed us to successfully identify macroalgae as the main contributor of carbon to the trophic structure. δ 15N ratios did not vary to the degree that would make it useful as tracer, but it was applied to estimating the total number of trophic transfers of nitrogen. Analysis of δ 15N values suggested that four trophic positions were present, with fishes ( Acanthaluteres vittiger, Scobonichthys granulatus and Siphonognathus radiatus, Pelsartia humeralis, Pelates sexlineatus, Leviprora inops, Odax acroptilus and Notolabrus parilus) occupying the top two levels. δ 13C of seston (20–200 μm) and sedimentary organic matter indicate that seagrass material is the main contributor to these two carbon pools, and that very little of it is incorporated into the trophic structure.

Does transported seagrass provide an important trophic link in unvegetated, nearshore areas?

The contribution of detritus from seagrass and other primary producers to faunal production in unvegetated nearshore areas was examined primarily using stable isotopes. Fish, macroinvertebrates, meiofauna and primary producers (seagrasses, macroalgae, seston and benthic microalgae) were sampled from sites in south-western Australia. All samples were analysed for δ 13C and δ 15N values and fish gut contents were determined. δ 13C values for seagrasses in the region were high compared to other macrophytes, ranging from 49.9 to −8.2‰ compared to −19.8 to −12.6‰ for macroalgae. The δ 15N values ranged between 4.0 and 7.7‰ for the red, brown and green algae, and between 3.2 and 5.9‰ for seagrasses. Seston and benthic microalgae samples had a mean δ 13C value of −12.8 and −14.0‰, respectively, and their δ 15N values were comparable to the macroalgae. All invertebrate fauna had mean δ 13C values considerably lower than seagrasses. However, individual samples harpacticoid copepods and polychaetes had a value as high as −11.7‰. δ 15N values for consumers were higher than those of the primary producers, except for copepods and amphipods. The δ 13C values for fish had a relatively small range, between −16.6 and −13.1‰, and the δ 15N values of fish were elevated compared to the invertebrates and primary producers, ranging mostly between 10.0 and 12.6‰. Mixing model analysis based on δ 13C values indicated that seagrass ranked low as a likely carbon source for all invertebrates other than harpacticoid copepods at a single site and some samples of polychaetes. The δ 13C values for fish were similar to those of a combination of harpacticoid and calanoid copepods, amphipods and polychaetes. The consumption of harpacticoid copepods by some fish species indicates that Amphibolis and Posidonia species in south-western Australia can contribute to the food web of unvegetated nearshore areas as detritus, but brown algae is likely to make a greater contribution. At least for the time of year that was sampled, the flow of detrital seagrass material into the foodweb may be mediated by specific detrivores, in this case harpactacoid copepods, rather than by all detritivores.

Carbon sources supporting benthic mineralization in mangrove and adjacent seagrass sediments (Gazi Bay, Kenya)

Abstract. The origin of carbon substrates used by in situ sedimentary bacterial communities was investigated in an intertidal mangrove ecosystem and in adjacent seagrass beds in Gazi bay (Kenya) by δ13C analysis of bacteria-specific PLFA (phospholipid fatty acids) and bulk organic carbon. Export of mangrove-derived organic matter to the adjacent seagrass-covered bay was evident from sedimentary total organic carbon (TOC) and δ13CTOC data. PLFA δ13C data indicate that the substrate used by bacterial communities varied strongly and that exported mangrove carbon was a significant source for bacteria in the adjacent seagrass beds. Within the intertidal mangrove forest, bacterial PLFA at the surface layer (0-1cm) typically showed more enriched δ13C values than deeper (up to 10cm) sediment layers, suggesting a contribution from microphytobenthos and/or inwelled seagrass material. Under the simplifying assumption that seagrasses and mangroves are the dominant potential end-members, the estimated contribution of mangrove-derived carbon to benthic mineralization in the seagrass beds (16-74%) corresponds fairly well to the estimated contribution of mangrove C to the sedimentary organic matter pool (21-71%) across different seagrass sites. Based on the results of this study and a compilation of literature data, we suggest that trapping of allochtonous C is a common feature in seagrass beds and often represents a significant source of C for sediment bacteria - both in cases where seagrass C dominates the sediment TOC pool and in cases where external inputs are significant. Hence, it is likely that data on community respiration rates systematically overestimate the role of in situ mineralization as a fate of seagrass production.

Relative impact of a seagrass bed and its adjacent epilithic algal community in consumer diets

The aim of this work was to identify and compare, using nitrogen and carbon stable isotope data, the food sources supporting consumer communities in a Mediterranean seagrass bed (Gulf of Calvi, Corsica) with those in an adjacent epilithic alga-dominated community. Isotopic data for consumers are not significantly different in the two communities. Particulate matter and algal material (seagrass epiflora and dominant epilithic macroalgae) appear to be the main food sources in both communities. Generally, the δ13C of animals suggests that the seagrass Posidoniaoceanica (L.) Delile represents only a minor component of their diet or of the diet of their prey, but the occurrence of a mixed diet is not excluded. P. oceanica dominates the diet of only of few species, among which holothurians appear as key components in the cycling of seagrass material.

Limited coupling of macrophyte production and bacterial carbon cycling in the sediments of Zostera spp. meadows

Two approaches were used to study coupling between seagrass production and bacter- ial processes in the rhizosphere. In the first approach, stable carbon-isotope ratios of bacteria, sedi- ment organic matter and plants were determined to infer sources of organic carbon used by bacteria in the sediments of 4 European Zostera marina and 2 Z. noltii meadows. Bacterial isotope ratios were derived from bacteria-specific polar lipid-derived fatty acids (PLFA), mainly methyl-branched i15:0 and a15:0. Bacterial δ 13 C ratios in the sediment from both vegetated and unvegetated sites were mostly similar and did not resemble Z. marina ratios, suggesting that seagrass material was of limited importance as a bacterial carbon source. Bacterial ratios were in most cases similar to benthic macro- algae and did correlate well with ratios of diatom biomarkers. Sediment organic matter inside the meadows had δ 13 C ratios similar to those of nearby unvegetated sites, and ratios were clearly differ- ent from the material produced by seagrasses, indicating that little seagrass material accumulated. Results from the 2 Z. noltii sites were less conclusive, as there was no clear difference in δ 13 C ratios between the potential source materials. In addition, bacterial δ 13 C ratios were highly variable at one Z. noltii site. In the second approach, cultured Z. marina was labeled with 13 C-bicarbonate to study the short-term transfer of label from plants to bacteria in the rhizosphere. However, no label was detected in bacterial PLFA after 20 h of incubation. In conclusion, a close coupling between macro- phyte production and bacterial carbon cycling could not be detected in the sediment of Z. marina meadows, and benthic production by algae was probably the main carbon source for bacterial growth.

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 km 2 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.

The production and trophic ecology of shallow-water fish assemblages in southern Australia III. General relationships between sediments, seagrasses, invertebrates and fishes

Fishes and benthic invertebrates were sampled in seagrass and unvegetated habitats at 14 localities across southern Australia in order to determine any consistent relationships between animal production, fish consumption and environmental parameters over a large spatial scale. Most of the features identified in a related study at Western Port were confirmed over the extended geographic range; however, an exception was that the production of fishes in the smallest size-classes (i.e. <1 g wet weight) was not consistently greater in seagrass habitat than unvegetated habitat. The more important characteristics of seagrass and unvegetated habitat types identified in the extended study were (1) more fish species were associated with seagrass habitat than unvegetated habitat, (2) the majority of small-fish species in both habitat types fed on crustaceans, with relatively few species capable of utilising algae and virtually no species utilising seagrass, (3) the few species that did ingest algae often occurred in high abundance, (4) the size of prey eaten by fishes increased consistently with fish size, with prey length averaging ≈6% of fish length, (5) abundant large fish species generally consumed smaller prey than rarer large fish species at the same body size, (6) greater benthic invertebrate and demersal fish production occurred in seagrass habitat than in unvegetated habitat, (7) most of the production of crustaceans >1-mm sieve size was ingested by fish predators while only a small proportion of non-crustacean benthic production was consumed. Fish production was highly correlated with crustacean production and seagrass biomass, and was negatively correlated with wave exposure (measured as fetch) across the range of sites. The estimated production of crustaceans was highly correlated with the biomass of seagrass material and also with the proportion of particles <63 μm in the sediments. The overall relationships between macrofaunal production ( M; mg · m −2 · day −1), macrocrustacean production ( C; mg · m −2 · day −1), demersal fish production ( D; mg · m −2 · day −1), fetch ( F; km), seagrass biomass ( L; g · m −2), proportion of particles <63 μm ( S;%) and temperature ( T; °C) were: ln M = 1.41 + 0.088 ln ( L + 1) + 0.38 ln S + 0.89 ln T, ln C = −0.93 + 0.27 ln ( L + 1) + 0.22 ln S + 0.89 ln T, ln D = −1.23 − 0.62 ln F + 0.36 ln ( L + 1) + 1.04 ln T. These regression equations can be used as models to predict the production of macrofauna, crustaceans and small fishes at unexamined sites. When predictions were compared with estimates of annual production at the eight sites previously examined in Western Port, most predictions lay between 50 and 200% of measured values. Additional work in Australia and overseas should allow these models to be refined.

Stomach content analysis of a Dugong (Dugong dugon) from South Sulawesi, Indonesia

A quantitative analysis of the stomach contents of an individual female dugong (Dugong dugon) caught by fishermen in February 1991 in the coastal waters of South Sulawesi, Indonesia, is presented. Of the total dry weight of digesta (about 3.4 kg), 98.9% consisted of seagrass material. Rhizome and root material of smaller pioneering genera (i.e. Halophila, Halodule, Cymodocea) accounted for 71.5% of the total dry weight. Enhalus acoroides dominated among leaf material (about 50%), but its contribution to the total biomass of the stomach contents was only 13.5%. Rhizome material of Enhalus and Thalassia was absent. The amount of sediment in the stomach was negligible. The results are compared with those from similar studies, and the implications for possible feeding selectivity are discussed.

The role of epiphytic periphyton and macroinvertebrate grazers in the trophic flux of a tropical seagrass community

Biomass and production of epiphytic periphyton, and the abundance, distribution and grazing rate of epifauna were measured in tropical seagrass beds in the Philippines. Periphyton comprised mainly detritus, diatoms and filamentous algae ( Polysiphonia sp. 1, Centroceras clavulatum (C. Agardh) Montagne, Ceramium gracillimum Harvey and Cladophora sp.). Mean biomass of periphyton was 0.16 mg ash-free dry weight (AFDW) cm −2 frond of Enhalus acoroides (L.f.) Royle and 0.24 mg AFDW cm −2 frond of Cymodocea serrulata (R. Br.) Aschers. and Magnus. Total periphyton biomass per unit area (m 2) of seagrass bed varied between habitats because of differing densities of seagrass, and ranged from 598 to 1061 mg AFDW or (24–646 mg C). Maximum (midday, summer) in situ rates of photosynthesis and respiration by epiphytes colonising artificial seagrass material averaged 11.6 μg O 2 cm −2 h −1 and 2.0 μg O 2 cm −2 h −1, respectively. Daily net productivity was 14 μg C cm −2 frond. Productivity of epiphytes per area of seagrass bed varied with site (36–77 mg C m −2 day −1). Relative to biomass, these data show that epiphytes are highly productive, with turnover times of 6–8 days, compared with known values of 30–100 + days for tropical seagrass fronds. The epifaunal grazer community was dominated by a few species of gastropod molluscs (especially Strombus mutabilis Swainson and Cerithium tenellum (Sowerby)). Within habitats, numbers of grazers on particular seagrass species were directly related to their available surface. Three groups of grazers were identified: those occurring on fronds day and night (e.g. S. mutabilis); those foraging over sediment during the day and fronds at night (e.g. Cerithium tenellum); those mainly confined to sediments (e.g. Strombus urceus L.). All epifaunal grazers exhibited upward movement into the seagrass canopy at night. Grazing was non-selective, removing the periphyton, except for the unutilised encrusting coralline algae, in proportion to abundance. Epifaunal grazers consumed between 20 and 62% of periphyton net production and, as in temperate systems, must therefore play a major role in the trophic flux of this tropical seagrass community.

Lipid components and utilization in consumers of a seagrass community: An indication of carbon source

Abstract 1. 1. The lipid components of three animals, the rock crab Nectocarcinus integrifons , the rock flathead Platycephalus laevigatus and the southern garfish Hyporhamphus melanochir , feeding in the seagrass beds at Corner Inlet, Victoria, Australia have been examined in detail in order to provide further information on seagrass community structure. 2. 2. Biological marker compounds detected within animal gut content material were used to recognize dietary sources and then utilized by community members. 3. 3. Both H. melanochir and N. integrifons have been shown to ingest and to varying degrees incorporate seagrass lipid material, thus further confirming the importance of seagrass carbon in the Corner Inlet environment. 4. 4. The southern sea garfish H. melanochir is observed to remove C 18 PUFAs (polyunsaturated fatty acids) from ingested seagrass material. 5. 5. Seagrass sterols are altered during incorporation into the lipids of this fish. 6. 6. Lipid-rich digestive juices play a role in the digestive processes of all three animals. 7. 7. Components tentatively identified as (NMI) (non-methylene interrupted) fatty acids have been detected in the lipids of the garfish H. melanochir and the crab N. integrifons . 8. 8. The fecal material of all three animals represent possible sources of these lipids (NMI acids) in Corner Inlet sediments. 9. 9. Based on lipid compositional data, N. integrifons feeds on Posidonia australis detritus and associated epiphyte material. 10. 10. The removal of both plant and epibiota cellular lipids along the digestive tract of the crab was observed, although structural components such as long chain mono- and α,ω-dicarboxylic acids, which have been previously recognized as seagrass marker lipids are not directly absorbed.