Biodiversity and trophic structure of nematode assemblages in seagrass systems: evidence for a coupling with changes in food availability

Nematode assemblage composition, trophic structure and biodiversity were followed over an annual cycle in a Posidonia oceanica bed of the NW Mediterranean to test the response to temporally changing food availability (sediment organic matter, bacterial and microphytobenthic biomass). The sediment-water interface of the seagrass meadow was characterised by high particulate organic matter concentrations. Also, seagrass sediments had high organic matter content, chloropigment concentrations and bacterial biomass. All organic matter components (i.e. carbohydrates, proteins and lipids) changed temporally, with higher concentrations in winter and spring and lower concentrations in summer; however, overall, large amounts of organic compounds were potentially available for (seagrass) benthic consumers throughout the year. Nematode assemblage in the P. oceanica bed maintained a high genus number (88 genera), and a trophic structure tightly coupled with the composition of the potential food sources. In agreement with the relevance of microphytobenthos, epigrowth-feeder nematodes (2A) represented the dominant trophic guild. The biomass of predator nematodes (2B) was significantly correlated with the biomass of other nematodes. Non-selective deposit feeders (1B) displayed a close relationship with the concentration of proteins, carbohydrates and biopolymeric carbon (i.e. labile organic detritus). Conversely to what was expected, epigrowth feeders were not correlated with chloropigment concentrations, but showed a significant relationship with the number of dividing bacteria, possibly suggesting a trophic plasticity of the 2A feeding guild. The coupling between temporal changes in food sources and temporal variability of the nematode trophic structure was highlighted by a CANOCO analysis, which allowed us to identify and associate, at each sampling time, nematode genera and their feeding habits, with specific environmental variables and food indicators. Furthermore, species diversity (H') and evenness (J) calculated on nematodes identified to the genus level displayed evident temporal changes, also reflected by the index of trophic diversity (ITD). Both structural and functional diversity were coupled with high concentration and highly heterogeneous composition of the food sources (including organic detritus, microphytobenthic algae and bacteria) potentially available to nematodes. These data suggest that temporal changes in quantity and quality of food sources do not only influence nematode dynamics and trophic composition, but also influence nematode structural and functional diversity.     

Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses

Loss of macroalgae habitats has been widespread on rocky marine coastlines of the eastern Korean peninsula, and efforts for restoration and creation of macroalgal beds have increasingly been made to mitigate these habitat losses. Deploying artificial reefs of concrete pyramids with kelps attached has been commonly used and applied in this study. As a part of an effort to evaluate structural and functional recovery of created and restored habitat, the macroalgal community and food web structure were studied about a year after the establishment of the artificial macroalgal bed, making comparisons with nearby natural counterparts and barren ground communities. Dominant species, total abundance, and community structure of macroalgal assemblage at the restored macroalgal bed recovered to the neighboring natural bed levels during the study period. The main primary producers (phytoplankton and macroalgae) were isotopically well separated. δ¹³C and δ¹⁵N values of consumers were very similar between restored and natural beds but varied greatly among functional feeding groups. The range of consumer δ¹³C was as wide as that of primary producers, indicating the trophic importance of both producers. There was a stepwise trophic enrichment in δ¹⁵N with increasing trophic level. A comparison of isotope signatures between primary producers and consumers showed that, while suspension feeders are highly dependent on pelagic sources, invertebrates of other feeding guilds and fishes mainly use macroalgae-derived organic matter as their ultimate nutritional sources in both macroalgal beds, emphasizing the high equivalency of trophic structure between both beds. Isotopic signatures of a few mollusks and sea urchins showed that they use different dietary items in macroalgal-barren grounds compared with macroalgal beds, probably reflecting their feeding plasticity according to the low macroalgal biomass. However, isotopic signatures of most of the consumers at the barren ground were consistent with those at the macroalgal beds, supporting the important trophic role of drifting algae. Our results revealed the recoveries of the macroalgal community and trophic structure at the restored habitat. Further studies on colonization of early settlers and the following succession progress are needed to better understand the process and recovery rate in the developing benthic community.     

Eutrophication and trophic structure in response to the presence of the eelgrass <Emphasis Type="Italic">Zostera noltii</Emphasis>

In estuaries, eelgrass meadows contribute to fundamental ecosystem functions of estuaries, providing food to several predators and buffering the negative effects of eutrophication. We asked whether the presence of the eelgrass Zostera noltii decreased the nitrogen concentration in the overlying water, affected the sources of nitrogen sequestrated by primary producers and changed the benthic and pelagic food web structures. We also studied the importance of these food webs in providing food to fish. We compared bare sediment to sediment covered by a Z. noltii meadow, and examined nutrient concentrations in the water column and δ¹⁵N in primary producers as indicators of anthropogenic inputs of nutrients. We then measured both δ¹³C and δ¹⁵N in the tissues of plants and consumers to establish food web structures. There were no differences in the concentrations and sources of nitrogen between sites. Rather, δ¹⁵N values indicated anthropogenic inputs of N (e.g. sewage discharges, agriculture) in both sites. There were no major differences in the structure of the planktonic food web, which was in part sustained by particulate organic matter and supported most predator fish, and in the structure of the benthic food web. Nonetheless, there were differences in the sources of food for omnivore consumers and for the detritivore Scrobicularia plana. Overall, the benthic food web did not use food derived from the eelgrass or macroalgae deposited on the substratum. Suspension feeders used particulate and sediment organic matter, whereas the δ¹³C and δ¹⁵N values of the other consumers indicated a likely contribution of benthic microalgae. Furthermore, in both habitats we found large variability in the isotope signatures of benthic macrofauna consumers, which did not allow distinguishing clearly different trophic groups and indicated a high level of omnivory and a mixed diet opportunistically making use of the availability of food in the surroundings.     

Trophic structure of coastal Antarctic food webs associated with changes in sea ice and food supply

Predicting the dynamics of ecosystems requires an understanding of how trophic interactions respond to environmental change. In Antarctic marine ecosystems, food web dynamics are inextricably linked to sea ice conditions that affect the nature and magnitude of primary food sources available to higher trophic levels. Recent attention on the changing sea ice conditions in polar seas highlights the need to better understand how marine food webs respond to changes in such broad-scale environmental drivers. This study investigated the importance of sea ice and advected primary food sources to the structure of benthic food webs in coastal Antarctica. We compared the isotopic composition of several seafloor taxa (including primary producers and invertebrates with a variety of feeding modes) that are widely distributed in the Antarctic. We assessed shifts in the trophic role of numerically dominant benthic omnivores at five coastal Ross Sea locations. These locations vary in primary productivity and food availability, due to their different levels of sea ice cover, and proximity to polynyas and advected primary production. The delta15N signatures and isotope mixing model results for the bivalves Laternula elliptica and Adamussium colbecki and the urchin Sterechinus neumeyeri indicate a shift from consumption of a higher proportion of detritus at locations with more permanent sea ice in the south to more freshly produced algal material associated with proximity to ice-free water in the north and east. The detrital pathways utilized by many benthic species may act to dampen the impacts of large seasonal fluctuations in the availability of primary production. The limiting relationship between sea ice distribution and in situ primary productivity emphasizes the role of connectivity and spatial subsidies of organic matter in fueling the food web. Our results begin to provide a basis for predicting how benthic ecosystems will respond to changes in sea ice persistence and extent along environmental gradients in the high Antarctic.     

Using biochemical and isotopic tracers to characterise organic matter sources and their incorporation into estuarine food webs (Rufiji delta,Tanzania)

Fatty acid biomarkers and stable isotope signatures were used to identify the sources of particulate and sedimentary organic matter and its input into the food web through the dominant consumer within the mangrove-dominated Rufiji estuary, Tanzania. Specific fatty acids were used to identify the preferred basal sources of dominant fauna (i.e. filter feeder bivalves, snails, crabs, shrimps, and three fish species), and their presence in the water and sediment samples in the estuary. Both fatty acid and stable isotope results revealed that food web in the Rufiji estuary depended on a variety of carbon sources (mangroves, allochthonous terrestrial inputs, macroalgae, and phytoplankton), contributing to a different degree into the diets of primary consumers and members of near-shore fish, but none of them were obligatory for the survival of these species. The δ¹⁵N values of major primary producers and consumers/predators revealed a trend for δ¹⁵N enrichment with increasing trophic level. The ratio of docosahexaenoic acid to eicosapentaenoic acid (DHA:EPA) decreased from pelagic to benthic feeding fish. This indicated that fish with different feeding modes derived their fatty acids from different primary sources of nutrition, and suggested that the DHA:EPA ratio may be a useful indicator of feeding mode.     

Trophic relationships among invertebrates at the Kairei hydrothermal vent field (Central Indian Ridge)

Exploration of hydrothermal vent systems in locations remote from well-studied sites allows ecologists to determine the degree of site-specific variation in trophic relationships among communities. A preliminary outline of the trophic structure of the Kairei hydrothermal vent community on the Central Indian Ridge (25°19.23′S; 70°02.42′E) is provided here, based on analysis of collections from an April 2001 expedition. Invertebrate biomass at Kairei is dominated by organic carbon with a δ¹³C isotopic value of about –13‰, due to the abundance of primary consumers (shrimp: Rimicaris aff. exoculata) and secondary consumers (anemones: Marianactis n. sp.) with this isotopic composition. Filamentous thiotrophic episymbionts on shrimp have been interpreted to be the major diet items of the shrimp and hence are the dominant primary producers within the community. Free-living autotrophic microorganisms are implicated as the dietary base for other invertebrate species. Four trophic groups are identified within the Kairei invertebrates based on carbon- and nitrogen-isotope ratios, but these groups do not always define discrete trophic levels. Ontogenetic shifts in diet are documented for R. aff. exoculata and brachyuran crabs (Austinograea n. sp.). Diets of symbiont-bearing mussels (Bathymodiolus aff. brevior) and two species of gastropods are isotopically constant throughout the range of sizes analyzed. There is a consistent but unexplained pattern of increasing nitrogen isotopic composition with increasing carbon isotopic composition in vent communities from geographically disjunct oceanic regions. Given the assumptions associated with interpretations of isotopic data, there remains a missing pool of carbon (presumably unsampled bacterial biomass) that contributes to the maintenance of the ¹³C- and ¹⁵N-enriched primary consumers in these ecosystems.     

Size-related trophodynamic changes in three target fish species recovering from intensive trawling

The consequences for food-web structure of protecting marine communities from trawling have been little explored. We investigated the Gulf of Castellammare, northwest Sicily, where a trawl ban initiated in 1989 has led to great increases in the biomass of some target species. Using stable isotopes of carbon and nitrogen, our objectives were to see if mean trophic level and omnivory had increased after the ban in three of the most common fishery-target species, namely, the anglerfish Lophius budegassa, Mediterranean hake Merluccius merluccius, and red mullet Mullus barbatus. We compared size data from before and after the fishery closure, but we also compared recently derived data from outside and inside the closed area. In all three species '¹³C and '¹⁵N were found to differ markedly depending on fish size. '¹³C was found to decrease in L. budegassa and Merluccius merluccius but increase in Mullus barbatus with increasing fish length. '¹⁵N increased in all three species, and this was thought to reflect feeding at increasingly higher trophic levels during the animals' lives. Mean length, and therefore '¹⁵N-derived trophic level, increased after the trawl ban only in the anglerfish L. budegassa. Based on '¹³C and '¹⁵N data, none of the species became more or less omnivorous after the ban. After 9 years of no trawling, increases in numerical abundance were not accompanied by substantial size-related trophodynamic shifts in any of the three species of fish studied.     

Incorporating temporally dynamic baselines in isotopic mixing models

Stable isotopes (particularly C and N) are widely used to make inferences regarding food web structure and the phenology of consumer diet shifts, applications that require accurate isotopic characterization of trophic resources to avoid biased inferences of feeding relationships. For example, most isotope mixing models require that endmembers be adequately represented by a single probability distribution; yet, there is mounting evidence that the isotopic composition of aquatic organisms often used as mixing model endmembers can change over periods of weeks to months. A review of the literature indicated that the delta13C values of five aquatic primary consumer taxa, commonly used as proxies of carbon production sources (i.e., trophic baselines), express seasonally dynamic cycles characterized by an oscillation between summer maxima and winter minima. Based on these results, we built a dynamic baseline mixing model that allows a growing consumer to track temporal gradients in the isotopic baselines of a food web. Simulations showed that the ability of a consumer to maintain or approach isotopic equilibrium with its diet over a realistic growth season was strongly affected by both the rate of change of the isotopic baseline and equilibration rate of the consumer. In an empirical application, mixing models of varying complexity were used to estimate the relative contribution of benthic vs. pelagic carbon sources to nine species of juvenile fish in a fluvial lake of the St. Lawrence River system (Québec, Canada). Estimates of p (proportion of carbon derived from benthic sources) derived from a static mixing model indicated broad interspecific variation in trophic niche, ranging from complete benthivory to > 95% reliance on pelagic food webs. Output from the more realistic dynamic baseline mixing model increased estimated benthivory by an average of 36% among species. Taken together, our results demonstrate that failing to identify dynamic baselines when present, and (or) matching consumers with baseline taxa that possess substantially different equilibration rates can seriously bias interpretation of stable isotope data. Additionally, by providing a formalized framework that allows both resources and consumers to shift their isotopic value through time, our model demonstrates a feasible approach for incorporating temporally dynamic isotope conditions in trophic studies of higher consumers.     

Oxygen and carbon stable isotopic profiles of the fan mussel, Pinna nobilis, and reconstruction of sea surface temperatures in the Mediterranean

Stable oxygen and carbon isotope profiles ('¹⁸O_skeletal and '¹³C_skeletal), taken along the direction of growth from the umbo to the shell margin in shells of the pinnid Pinna nobilis, were used to reconstruct sea surface temperatures (SST) in the south-east Mediterranean and ontogenetic records of metabolic CO₂ incorporation. Comparison of the seasonal cycle of SST, predicted from the '¹⁸O_skeletal record of a small (young) rapidly growing pinnid and temperature measured with a continuous in situ recorder showed that P. nobilis calcifies under isotopic equilibrium with surrounding seawater, thus indicating that P. nobilis shells can be used as a reliable predictor of SST. A 10-year SST record for the south-east Mediterranean was reconstructed from the shell profiles of four pinnid shells of different sizes and ages collected in 1995 and 1996. Reliable resolution of the seasonal SST could only be achieved during the first 4 years of shell growth. As the pinnids grew older, the temperature record was poorly resolved because the shell growth had diminished with age, resulting in time-averaging of the record. The amplitude of the generated seasonal temperature cycle compared favourably (DŽ°C) with a long-term temperature record from northern Mediterranean waters. Clear seasonal cycles in '¹³C_skeletal were observed with an amplitude of ~1.0‰, similar to the calculated seasonal changes in '¹³C of seawater (0.6‰) overlying seagrass meadows. An ontogenetic trend towards less positive '¹³C_skeletal values was too large to be attributed to any decrease in '¹³C in seawater resulting from the invasion of anthropogenic CO₂. It is suggested that the temporal changes of '¹³C_skeletal are due to incorporation of respiratory CO₂ into the extrapallial fluid and reflect changes in the metabolic activity of the pinnid rather than changes in the isotopic composition of dissolved inorganic carbon within the surrounding seawater.     

Origin and Distribution of Suspended Organic Matter As Inferred From Carbon Isotope Composition in A Mediterranean Semi-Enclosed Marine System

Abstract

The origin and distribution of suspended organic matter, the trophic features and the stable carbon isotopic composition of particulate organic carbon (POC) were studied monthly in a Western Mediterranean semi-enclosed basin. Sampling stations were selected as a function of wind-exposure and the degree of vegetation cover and then compared with an adjacent unvegetated site. the predominant vegetation was seagrass (Posidonia oceanica and Cymodocea nodosa) and Caulerpa prolifera. Water samples were analyzed for total suspended matter (inorganic and organic fractions), photosynthetic pigments (chlorophyll-a and phaeopigments), dissolved organic carbon, particulate organic carbon and their isotopic composition. Temperature and salinity were also measured at the same sampling sites within range of Mediterranean limits. the suspended organic matter concentration was 1.77 ± 1.55 mg l^?1; the chlorophyll-a concentration was low (0.35 ± 0.24 μg l^?1); the disolved organic carbon concentration was 2,140 ± 2,010 μg l^?1; the particulate organic carbon concentration was 212 ± 106 μg l^?1 and the isotopic composition was 18.77 ± 2.51%_°. There were significant temporal differences except for phaeopigments, POC and its POC isotopic composition, and there were no spatial differences other than for δ¹³C. This picture highlighted a general seasonal trend and trophical features similar to adjacent sea.

Spatial differences in δ¹³C showed that the source of suspended organic matter was different between stations as that between sources and wind-hydrodynamic constraints. In     

Large-scale oceanic distribution and population structure of Calanus finmarchicus, in relation to physical environment, food and predators

The investigation was carried out from 62°N to 73°N and from 14°E to 11°W in the Norwegian Sea during 19 June-12 July 1997. Regional differences in the phase of the seasonal development of the plankton community were evident, most pronounced across the Arctic front. In the Coastal and eastern Atlantic domains, post-bloom conditions prevailed, characterised by low chlorophyll a (chl a) levels and a phytoplankton assemblage dominated by coccolithophorids and small flagellates. During the study period, egg production rates of Calanus finmarchicus were low (<10 eggs female^-1 day^-1), older copepodite stages dominated, and the seasonal descent to deeper waters had started. In the Arctic domain, bloom conditions were evident by high chl a levels and a high abundance of large diatoms. Egg production rates were higher (a maximum of 29 eggs female^-1 day^-1), but the dominance of stages CI-CIII indicated that considerable spawning had already occurred prior to the spring bloom. The seasonal descent had barely started. Both invertebrate and fish predators were most abundant in the Coastal and eastern Atlantic domains, with abundance strongly decreasing north-westwards. No tight relationship between total abundance of invertebrate or fish predators and that of C. finmarchicus was apparent. However, a weak, but significant, relationship between abundance of young stages of chaetognaths and Euchaeta spp. versus young stages of C. finmarchicus was found, indicating that these invertebrate predators develop parallel to the development of the new cohort of C. finmarchicus. In early summer, C. finmarchicus had reached overwintering stages, and had started to accumulate in deeper waters in areas with the highest abundance of horizontally migratory planktivorous fish.     

Gut characteristics and assimilation efficiencies in two species of herbivorous damselfishes (Pomacentridae: Stegastes dorsopunicans and S. planifrons)

The morphological and physiological mechanisms by which marine herbivores assimilate energy and nutrients from primary producers and transfer them to higher trophic levels of reef ecosystems are poorly understood. Two wide-ranging Caribbean fishes, the dusky damselfish, Stegastes dorsopunicans, and the threespot damselfish, S. planifrons, defend territories on patch reefs in the Archipelago de San Blas, Republic of Panama. We examined how relative intestine length and retention time influence digestion and absorption of energy and nutrients in these fishes. The dusky damselfish has a relative intestine length (RIL=intestine length/standard length) of 1.2 and a Zihler index {ZI=intestine length (mm)/10[mass(g)^1/3]} of 3.4. These values are significantly lower (P_RIL=P_ZI<0.0001) than those for the threespot damselfish (3.0 and 8.2, respectively). Both RIL and ZI for both species fall well below previously published values for other herbivorous pomacentrids, and may reflect their primary food resource at San Blas (diatoms). Energy-rich diatoms may be easier to digest than refractory macroalgae characteristic of diets of many herbivorous fishes (RIL range: 2-20). Despite differences in RIL and ZI between these two species, gut retention time is the same (P>0.05) for both dusky (6.6 h) and threespot damselfish (6.5 h). Thus, food travels the length of the threespot damselfish intestine ~2.5 times faster than it does in the dusky damselfish intestine. Levels of protein, carbohydrate, and lipid are significantly (0.003<P<0.030) higher in the feces of dusky damselfish than in the feces of threespot damselfish, when both species were fed a natural diet of benthic diatoms collected from damselfish territories. This indicates threespot damselfish have a greater nutrient-specific and total assimilation efficiency than do dusky damselfish. Furthermore, when fed an artificial pellet diet, protein absorption efficiency differed significantly (P=0.014) between species; threespot damselfish absorbed 98.3% of dietary protein, whereas dusky damselfish absorbed 96.4% of dietary protein.     

Stable isotopic compositions of hydrothermal vent organisms

Stable isotopic analyses were used to study trophic relationships in two communities of deep-sea hydrothermal vent organism in the Pacific Ocean. The community at Hanging Gardens on the East Pacific Rise (21°N), sampled in 1985, is dominated by two species of vestimentiferan tubeworms; communities at Alice Springs and Snail Pits on the Marianas Back Arc Spreading Center (western Pacific), sampled in 1987, are dominated by gastropod mollusks, barnacles, and anemones. In both locations, carbon and nitrogen isotopic values of vent invertebrates are significantly different from those of non-vent invertebrates collected at 11°N on the East Pacific Rise and elsewhere in the deep-sea. These distinct isotopic compositions reflect local sources of organic carbon and nitrogen used by vent consumers. Many vent invertebrates lacking chemoautotrophic endosymbionts have ¹³C-enriched values of-11 to-16%. compared to values of-17 to-22%. normally observed in deep-sea fauna. This suggests that a ¹³C-enriched food source is trophically important in both vent communities. Free-living bacteria colonizing surfaces and suspended in the water column may constitute this food resource. Nitrogen isotopic analyses show that the food web of the East Pacific Rise community has more trophic levels than the Marianas vent community.     

Influence of nutrients in the feeding ecology of seagrass (Posidonia oceanica L.) consumers: a stable isotopes approach

Nitrogen inputs to coastal environments can considerably alter the abundance of primary producers. However, how herbivores modify their trophic signatures and adjust to changes in food resource conditions remains controversial. Here, we assess the effect of nutrient availability on the diet shifts of the two main Mediterranean herbivores, the Sparid fish Sarpa salpa L. and the sea urchin Paracentrotus lividus (Lmk.) that feed mostly on the seagrass Posidonia oceanica L. (Delile), epiphytes and benthic macroalgae. To do this, we (1) investigate the patterns of isotopic composition (δ¹³C and δ¹⁵N signatures) of the two herbivores and their potential food sources in three areas of contrasting nutrient conditions and, (2) we assess the diet shift along this nutrient gradient by estimating the isotopic nutrient enrichment (i.e., the contribution of δ¹³C and δ¹⁵N signatures in consumers’ tissues relative to potential food sources). Food web signatures of δ¹³C were similar among the three study sites, and no patterns of δ¹³C shift were observed in their diets. In contrast, there was a consistent increase in N contents and δ¹⁵N along the nutrient gradient for all primary producers and their consumers. The rate of δ¹⁵N enrichment was also clearly distinctive between the two herbivores: in P. lividus it increased by 61% along the nutrient gradient, while in S. salpa it remained constant. Our results suggest that sea urchins behave as facultative omnivores and feed on vegetable or mixed diets depending on the trophic status of the system. It is unclear, however, if this modification is behavioral or the consequence of mere changes in the availability of food items, as animal epiphytes (e.g., hydrozoans, bryozoans and ascidians) can also became more abundant on seagrass leaves under increased nutrient conditions. In contrast, adult fish appear to feed on vegetal material independent of nutrient availability in the ecosystem.     

Isotopic composition, growth rates and biological behaviour of Chamelea gallina and Callista chione from the Bay of Trieste (Italy)

Shells of the bivalve molluscs Chamelea gallina L. and Callista chione L. from the Bay of Trieste (northern Adriatic) were analysed for their oxygen and carbon isotopic composition. The main purpose of this study was to obtain detailed information on the biological behaviour and, particularly, on the growth rates and the settlement time of these two species. Isotopic measurements were carried out on "spot" samples collected from the umbo to the ventral margin of each shell. The results obtained from nine specimens of C. gallina (plus some 20 specimens studied only for the isotopic composition of their initial umbo and for the last section of their ventral edge) and from two specimens of C. chione allow the following conclusions: (1) the calcium carbonate of C. gallina shells is not deposited under isotopic equilibrium conditions with the environmental variables (temperature and oxygen isotopic composition of seawater); (2) this species usually initiates precipitation of carbonate during the springtime, although some late summer-early autumn generation also occurs; (3) the rate of calcium carbonate deposition decreases with increasing age, and the overall accretion rate is a rather slow one; (4) there is a negative correlation between the carbon isotopic composition and the size of each shell; (5) the mean growth rate of the shells can be easily evaluated according to the oxygen isotopic composition of the spot measurements; (6) the specimens of C. chione do not precipitate the calcium carbonate of their shells in isotopic equilibrium with environmental conditions; (7) their accretion rate is rather fast during the first years of their life and progressively and considerably reduced after about 5 years; (8) the carbon isotopic composition of these shells does not show the marked evolution towards more negative values shown by C. gallina, suggesting that neither respiratory CO₂ nor carbon species derived from the oxidation of organic matter are systematically used for the precipitation of shell carbonate; and (9) a marked preference for the warmer half of the year is shown by the oldest specimen of C. chione (12 years).     

In situ feeding rates and grazing impact of zooplankton in a South African temporarily open estuary

Recent studies in temporarily open estuaries of South Africa have shown that phytoplankton biomass is at times low, when compared to the high standing stock of the grazers. In situ grazing rates of the dominant zooplankton species were estimated at the Mpenjati Estuary once during the winter closed phase, in August 1999, and once during the summer open phase, in February 2000. The study aimed at determining what proportion of the energetic demands of the dominant grazers of the estuary is met by the available phytoplankton. Results show that the gut of all species exhibited higher pigment concentrations during the night than during the day, both in winter and summer. Gut pigment contents ranged from 0.27 to 5.38 ng pigm individual^-1 in the mysid Gastrosaccus brevifissura, from 0.16 to 1.63 ng pigm individual^-1 in the copepod Pseudodiaptomus hessei, from 0.12 to 0.45 ng pigm individual^-1 in the copepod Acartia natalensis, and from 0.8 to 5.44 ng pigm individual^-1 in the caridean Palaemon sp. [where pigm is the sum of chlorophyll-a (chl-a) and phaeopigments]. During the winter closed phase, gut evacuation rates for G. brevifissura, P. hessei, and A. natalensis were 0.62, 0.42, and 0.46 h^-1, respectively. In summer, gut evacuation rates were 0.68, 0.48, and 0.46 h^-1 for G. brevifissura, P. hessei, and Palaemon sp., respectively. The rate of gut pigment destruction for G. brevifissura was 99.6% of the total ingested, one of the highest values ever recorded for any crustacean. A gut pigment destruction of 79.0% was measured for Palaemon sp., 95.7% for P. hessei, and 93.8% for A. natalensis. During winter the total grazing impact of the dominant zooplankton species ranged from 5.05 to 22.7 mg chl-a m^-2 day^-1 and accounted for 34-69% of the available chl-a in the water column. During summer, the grazing impact ranged between 0.45 and 0.65 mg chl-a m^-2 day^-1, accounting for 17-41% of the available chl-a in the water column. This shows that the dominant zooplankton species of the Mpenjati have a very high grazing impact on algal cells. At times this may exceed 100% of the available phytoplankton production, suggesting that the zooplankton community may often resort to other food sources to meet all its energetic demands.     

Food web analysis in intertidal <Emphasis Type="Italic">Zostera marina</Emphasis> and <Emphasis Type="Italic">Zostera noltii</Emphasis> communities in winter and summer

The food web of two intertidal seagrass (Zostera marina and Zostera noltii) beds that may be influenced by the seasonal variation in food source abundance was studied in winter and in summer with δ¹³C and δ¹⁵N analysis. In spite of high relative variation of abundance of main primary producers at the two sites, the food web did not vary between winter and summer. The δ¹³C range of primary producers was wide. Zostera leaves, the most ¹³C-enriched source, were not consumed directly by grazers. Deposit and filter feeders have a similar δ¹³C and could use a mix of suspended and sedimented organic particulate matter, largely composed of detritus from macroalgae to seagrass. This trophic pathway allows the local incorporation of the high biomass produced by seagrasses. The wide δ¹⁵N range of predators was linked either to a large variety from omnivore to carnivore predators or to the also wide ranges of δ¹⁵N of primary consumers.     

Energy budgets and feeding rates of Coryphaenoides acrolepis and C. armatus

This study develops energy budgets and estimates feeding rates for two macrourid fishes, Coryphaenoides acrolepis, dominant in the bathyal eastern North Pacific, and the abyssal cosmopolitan species, Coryphaenoides armatus. Daily energy expenditure by C. acrolepis was nearly twice that of C. armatus. C. acrolepis allocated nearly equal amounts of energy to metabolism and growth. Once sexual maturity was reached reproduction became the dominant energetic cost. Either these costs are necessary to retain adequate numbers of eggs and larvae on the continental slopes, or this fish does not reproduce on an annual basis and the calculated costs are an overestimate. C. armatus allocated relatively more energy to metabolism than growth. It may be semelparous, and this strategy would be of great energetic savings in its food-poor but stable environment. Individual daily ration for C. acrolepis decreased from 0.31% to 0.07% of body weight (BW) and for C. armatus from 0.12% to 0.02% BW with increasing fish length. These rates are substantially lower than those for fishes living in cold waters on the continental shelves. The population feeding rates for C. acrolepis ranged from 0.8 to 15 kg km^-2 day^-1 and for C. armatus from 5 to 2,800 g km^-2 day^-1. The scavenging behaviour of C. acrolepis was used to investigate the role of carrion as a food supply to the deep-sea benthos. It was estimated that the carrion eaten by C. acrolepis is equivalent to 0.04 mg C m^-2 day^-1 or only 0.2-0.4% of the average small particulate flux. Carrion consumption is important for scavengers like C. acrolepis, but it is not an important component of the carbon flux into the deep-sea benthic environment.     

A comparison of catches of fishes and invertebrates by two light trap designs, in tropical NW Australia

Light traps were deployed in two sampling programs. In the first, small and large traps were released to drift with the current at stations along a cross-shelf transect on the NW Shelf off the coast of Western Australia. In the second program, pairs of small and large traps were deployed on moorings 150 m off the coastline. The composition and size-frequency distributions of catches of fishes in small and large traps were similar for both modes of deployment. In drifting traps, nearly 78% of this catch was composed of reef fishes, and these were collected in significantly greater numbers by the small design than by large traps (9.51 vs. 5.84 individuals h^-1, respectively). Nine taxa (amphipods, mysids, crab megalopae, copepods, cumaceans, isopods, caridean shrimps, polychaetes and the euphausiid, Pseudeuphausia latifrons) accounted for 99% of the total catch of invertebrates by drifting traps. Of these, catches of amphipods, copepods, cumaceans and P. latifrons were greater in large traps than in small traps (3,134 vs. 1,687 h^-1, 1,018 vs. 214 h^-1, 551 vs. 165 h^-1 and 74 vs. 9 individuals h^-1, respectively). In contrast, crab megalopae were more abundant in catches by small traps than by large traps (3,134 vs. 1,687 individuals h^-1, respectively). The catch rate of fishes in moored traps was higher than in drifting traps (105 vs. 20 fishes h^-1) and was dominated by baitfishes (86% of total catch). Reef fishes were also captured in greater numbers by small traps than by the large design (10.17 vs. 4.4 individuals h^-1) in this mode of deployment. Despite these differences in catch rates, multivariate analysis showed that cross-shelf patterns in catches of fishes and invertebrates were mapped equally well by both trap designs. Variation in the efficiency of trap designs thus appears to be small when compared to changes in the composition and abundance of zooplankton assemblages that occur at scales of tens of kilometers.     

Effects of the 1996 and 1998 positive sea-surface temperature anomalies on corals,coral diseases and fish in the Arabian Gulf (Dubai,UAE)

Two positive sea-surface temperature anomalies occurred in the Arabian Gulf in short sequence. Between May and August 1996 and 1998, sea-surface temperatures in the southern Arabian Gulf were elevated by 2°C above average. The consequences for coral fauna, coral diseases and coral regeneration were studied in Dubai (United Arab Emirates) between Jebel Ali and Ras Hasyan. In 1996, coral death was widespread, affecting primarily the genus Acropora. In Acropora-dominated areas, live coral cover was reduced from 90% to about 26% in 1996, while in 1998 only a reduction from 26% to 22% of the remaining coral cover occurred. In the study area, all six Acropora species suffered total mortality in 1996, thus the coral fauna was reduced from 34 species to 27. The nearest areas with surviving Acropora were 30 km to the east (Deira) and 20 km to the west (Al Jazira). Massive coral species suffered negligible mortality, and slowly increased in space cover. The Acropora overkill turned 7.9 km² (19.7% of total coral-covered area) of previously lush coral gardens into a dead framework that was increasingly bioeroded. Acropora recruitment only started in 1998, average recruit size in 1999 was 7Dž cm, and recruits were rare. Prior to the mass mortality event, coral diseases were common and seasonal (14LJ% of corals, mainly Acropora, affected in summer, in winter 7Lj%, mainly massives), after the mortality event seasonality was lost and infection remained below winter levels (6LJ%, only massives infected). In fish, overall species richness decreased from 95 to 64 species in point counts, but frequency only decreased in one species (Pseudochromis persicus). Guild structure changed inasmuch as herbivores and planktivores increased, and invertivores decreased, although differences were not statistically significant. The most abundant family, both prior to and after the coral mass mortality, was Lutjanidae. It appears that even though much of the coral was dead, the maintenance of structural complexity allowed the fish assemblage to avoid a similar catastrophic change to that experienced by the coral assemblage.