Enter Food Webs Research Articles

Theoretical Review on Microplastic Pollution: A Multifaceted Threat to Marine Ecosystems, Human Health, and Environment

Microplastic pollution in oceans poses a complex global threat to marine ecosystems and human health. These ubiquitous plastic particles originate from diverse sources and become widely dispersed by ocean currents, contaminating remote regions worldwide. In the aquatic environment, microplastics cause physical harm to aquatic species through ingestion. They disrupt habitats and act as vectors for toxic chemicals through bioaccumulation in aquatic species, enter food webs, and impair marine ecosystem functions. For humans, microplastic exposure via seafood consumption, drinking water, and inhalation raises concerns about potential inflammatory, cytotoxic, and endocrine-disrupting diseases that are linked to cancer, reproductive problems, and metabolic disorders. Mitigating this pervasive pollution requires a multi-pronged approach, including policy interventions that minimize plastic use, improve waste management, encourage technological innovations for biodegradable alternatives, provide public awareness campaigns, and improve coordinated efforts across plastic stakeholders. Comprehensive strategies combining research, regulations, and public engagement are crucial to protect marine life and human health from the detrimental impacts of microplastic contamination. This paper provides an overview of the origins and pathways of microplastics and assesses the impacts of microplastic pollution on human health and the environment. This study also highlights the methodology, knowledge gaps, and future directions to facilitate research on microplastic removal and risk assessment in the oceans.

Dynamics of trace element enrichment in blue carbon ecosystems in relation to anthropogenic activities

Blue carbon ecosystems (BCEs), located at the land-sea interface, provide critical ecological services including the buffering of anthropogenic pollutants. Understanding the interactions between trace element (TE) loads in BCEs and socioeconomic development is imperative to informing management plans to address pollution issues. However, the identification of anthropogenic TE pollution in BCEs remains uncertain due to the complex geochemical and depositional processes and asynchronous socioeconomic development along continental coastlines. Here, priority-controlled TE (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) concentrations in the mangrove, saltmarsh and seagrass soils and plant tissues along the coastline of China were investigated while taking bare flat and upland soils as corresponding references. We demonstrate that blue carbon (BC) soils accumulated markedly higher concentrations of anthropogenic TEs than the reference soils, mainly due to the effective trapping of fine-grained particles and higher binding capacities. We identify the time course of TE changes over the last 100 years which shows increasing anthropogenic TE accumulation resulting from military activities (1930–1950) and the growth of industrial and agricultural activities (1950–1980), then reaching a maximum after national economic reform (1980–2000). Since the 2000s, decreases in TE discharges driven by socioeconomic reform and strengthened environmental regulations have led to a widespread reversal of anthropogenic TE concentrations in BC soils. Based on the current TE flux we estimate that BCEs can filter over 27.3–100 % of the TEs emitted in industrial wastewaters from Chinese coastal provinces annually. However, the uptake of these TEs by plants can be substantially reduced through various mechanisms offered by edaphic properties such as organic carbon, clay, and sulfur contents. Therefore, enhancing TE filtering while preventing TEs from entering food webs through the conservation and restoration of BCEs will greatly aid in achieving the sustainable development goal of the coastal zone under intensified anthropogenic activities.

Review of Marine Cyanobacteria and the Aspects Related to Their Roles: Chemical, Biological Properties, Nitrogen Fixation and Climate Change.

Marine cyanobacteria are an ancient group of photosynthetic microbes dating back to 3.5 million years ago. They are prolific producers of bioactive secondary metabolites. Over millions of years, natural selection has optimized their metabolites to possess activities impacting various biological targets. This paper discusses the historical and existential records of cyanobacteria, and their role in understanding the evolution of marine cyanobacteria through the ages. Recent advancements have focused on isolating and screening bioactive compounds and their respective medicinal properties, and we also discuss chemical property space and clinical trials, where compounds with potential pharmacological effects, such as cytotoxicity, anticancer, and antiparasitic properties, are highlighted. The data have shown that about 43% of the compounds investigated have cytotoxic effects, and around 8% have anti-trypanosome activity. We discussed the role of different marine cyanobacteria groups in fixing nitrogen percentages on Earth and their outcomes in fish productivity by entering food webs and enhancing productivity in different agricultural and ecological fields. The role of marine cyanobacteria in the carbon cycle and their outcomes in improving the efficiency of photosynthetic CO2 fixation in the chloroplasts of crop plants, thus enhancing the crop plant's yield, was highlighted. Ultimately, climate changes have a significant impact on marine cyanobacteria where the temperature rises, and CO2 improves the cyanobacterial nitrogen fixation.

Microplastic ingestion affects hydrogen production and microbiomes in the gut of the terrestrial isopod Porcellio scaber.

Microplastic (MP) is an environmental burden and enters food webs via ingestion by macrofauna, including isopods (Porcellio scaber) in terrestrial ecosystems. Isopods represent ubiquitously abundant, ecologically important detritivores. However, MP-polymer specific effects on the host and its gut microbiota are unknown. We tested the hypothesis that biodegradable (polylactic acid [PLA]) and non-biodegradable (polyethylene terephthalate [PET]; polystyrene [PS]) MPs have contrasting effects on P. scaber mediated by changes of the gut microbiota. The isopod fitness after an 8-week MP-exposure was generally unaffected, although the isopods showed avoidance behaviour to PS-food. MP-polymer specific effects on gut microbes were detected, including a stimulation of microbial activity by PLA compared with MP-free controls. PLA stimulated hydrogen emission from isopod guts, while PET and PS were inhibitory. We roughly estimated 107 kg year-1 hydrogen emitted from the isopods globally and identified their guts as anoxic, significant mobile sources of reductant for soil microbes despite the absence of classical obligate anaerobes, likely due to Enterobacteriaceae-related fermentation activities that were stimulated by lactate generated during PLA-degradation. The findings suggest negative effects of PET and PS on gut fermentation, modulation of important isopod hydrogen emissions by MP pollution and the potential of MP to affect terrestrial food webs.

PFASs in Soil: How They Threaten Human Health through Multiple Pathways and Whether They Are Receiving Adequate Concern.

Per- and polyfluoroalkyl substances (PFASs) have been mass-produced and widely applied in consumer and industrial products, resulting in their widespread presence in the environment. Features such as environmental persistence, bioaccumulation, and high toxicity even at low doses have made PFASs an increasing concern. This brief review focuses on soil PFASs, especially the effect of soil PFASs on other environmental media and their potential threats to human health through daily diet. Specifically, soil PFASs contamination caused by different pathways was first investigated. Soil pollution from application of aqueous film-forming foams (AFFFs) is generally more severe than that from fluorochemical manufacturing plants, followed by biosolid land use, landfill, and irrigation. Factors, such as carbon chain length of PFASs, wastewater treatment technology, geographical conditions, and regional development level, are related to soil PFASs' pollution. Then, the migration, bioaccumulation, and toxicity characteristics of soil PFASs were analyzed. Short-chain PFASs have higher solubility, mobility, and bioavailability, while long-chain PFASs have higher bioaccumulation potential and are more toxic to organisms. Factors such as soil texture, solution chemistry conditions, enzymes, and fertilization conditions also influence the environmental behavior of PFASs. The risk of human exposure to PFASs through agricultural and animal products is difficult to control and varies depending on living region, age, eating habits, lifestyle, ethnicity, etc. Soil PFASs threaten drinking water safety, affect soil function, and enter food webs, threatening human health. Knowledge gaps and perspectives in these research fields are also included in current work to assist future research to effectively investigate and understand the environmental risks of soil PFASs, thereby reducing human exposure.

Effects on Growth of Juvenile Abalones Haliotis tuberculata Under Chronic Exposition to Metals Released from the Dissolution of an Aluminium-Based Galvanic Anode

In the marine environment, the galvanic anode cathodic protection system (GACP) undergoes oxidation and releases metals in the forms of ions or oxy-hydroxides into the environment. The objective of the present study was to investigate the toxicity of a cocktail of metals released from the dissolution of an aluminium-based galvanic anode (~ 95% Al, ~ 5% Zn) on the abalone Haliotis tuberculata. Juveniles were exposed for 16weeks (i.e. 12weeks of exposure and 4weeks of decontamination phase) and their growth, intake rate, conversion rate and metallic concentrations were monitored. A total of 6 conditions were tested: a control, 4 concentrations based on Al and a trophic control. Results showed that the mortality reached 57% for individuals exposed to 1125µg L-1 of Al, and the abalone growth significantly decreased for an Al concentration greater than 495µg L-1. At the highest exposure concentration, intake rate measurements revealed that the appetite of abalones was affected, supported by the large increase in the conversion rate which was indicative of a poor feed efficiency. The monitoring of metallic concentrations showed that H. tuberculata strongly bioconcentrated Al relative to zinc. The diet did not appear to be the primary pathway for metal entry. Concentrations that significantly impacted abalone growth and survival during the experiment were higher than those found in natural environment, but the bioconcentration of Al into the tissues of a primary consumer such as abalone may be a potential pathway for Al to enter food webs.

First detection of microplastics in Xyrichtys novacula (Linnaeus 1758) digestive tract from Eivissa Island (Western Mediterranean)

Plastic waste and its ubiquity in the oceans represent a growing problem for marine life worldwide. Microplastics (MPs) are ubiquitous in the sea and easily enter food webs. Xyrichtys novacula L. is one of the main target species of recreational fishing in the Balearic Islands, Spain. In the present study, the quantity of MPs in gastrointestinal tracts of X. novacula from two different areas (a marine protected area (MPA) and a non-protected area) of Eivissa Island (in the Balearic archipelago) has been assessed, as well as MPs evaluation within the sediment of both areas. The results showed that over 80% of sampled individuals had MPs in their gut with an average of 3.9 ± 4.3 plastic items/individual. Eighty percent of these plastics were fibres, while the rest were fragments. Although the sediment of the non-protected area had a significant higher presence of MPs, no significant differences in the number of MPs were observed in X. novacula from both areas. The µ-FT-IR analysis showed that the main polymers in the sediments were polycarbonate (PC) and polypropylene (PP), whereas in the digestive tract of fish PC, PP, polyethylene, polystyrene and polyester. In conclusion, practically all X. novacula specimens presented MPs in their digestive tract regardless if the capture zone was in a MPAs or not. These results highlight the ubiquity of MPs in coastal marine areas, and further studies might be necessary to evaluate further implications of MP presence in this species.

Impact of Plastic Debris on the Gut Microbiota of Caretta caretta From Northwestern Adriatic Sea

Plastic pollution is nowadays a relevant threat for the ecological balance in marine ecosystems. Small plastic debris (PD) can enter food webs through various marine organisms, with possible consequences on their physiology and health. The loggerhead sea turtle (Caretta caretta), widespread across the whole Mediterranean Sea, is a “flagship species,” useful as indicator of the general pollution level of marine ecosystems. Ingested PD accumulate in the final section of turtles’ digestive tract before excretion. During their transit and accumulation, PD also interact with the residing microbial community, with possible feedback consequences on the host’s health. To explore the possible relationship between fecal microbial composition and PD ingestion, we collected fecal samples from 45 turtles rescued between 2017 and 2019 in the Northwestern Adriatic Sea (Italy), assessing occurrence and content of PD in the samples and in parallel the microbiome structure by 16S rRNA gene sequencing. According to our findings, almost all samples contained PD, mirroring the high level of plastic pollution in the area. We identified phylotypes associated to a high amount of PD, namelyCetobacterium someraeand other taxa, possibly responding to contamination by plastic-associated chemicals. Furthermore, putative marine pathogens were found associated to higher plastic contamination, supporting the hypothesis that PD can act as a carrier for environmental pathogenic bacteria into marine organisms. Besides confirming the role of the sea turtle as relevant flagship species for plastic pollution of the marine environment, our study paves the way to the exploration of the impact that PD ingestion can have on the microbial counterpart of large marine organisms, with potential feedback consequences on the animal and ecosystem health.

Assessing size-based exposure to microplastic particles and ingestion pathways in zooplankton and herring in a coastal pelagic ecosystem of British Columbia, Canada

Microplastic particles (hereafter 'microplastics') are a widespread class of pollutants in marine environments that can become embedded in food webs. Due to their diverse composition and size, microplastics can enter food webs both directly through consumption and indirectly via trophic transfer. In this study, we investigated potential ingestion pathways of microplastics in an important pelagic food web in coastal British Columbia, Canada. Between March and September 2019, we completed repeat surveys of water, zooplankton, and larval Pacific herring Clupea pallasii at 11 locations in Baynes Sound, Strait of Georgia. Five zooplankton taxa were isolated from each zooplankton sample for specific analysis. Juvenile herring were sampled once in September. Samples were cold-digested with KOH or H2O2 and suspected microplastics isolated. Suspected microplastics were confirmed using μ-Raman spectroscopy and were subsequently identified from the collected samples. The average microplastic concentration in surface waters was 0.59 microplastic particles l-1, and no clear spatial pattern was evident. Average microplastic particle loads were 0.0007 ind.-1 in zooplankton, 0.0017 ind.-1 in larval herring, and 0.089 ind.-1 in juvenile herring. There was a clear difference in the biological:microplastic particle ratio across size fractions (125-250, 250-500, 500-1000, 1000-2000, 2000-4000 µm) in the water column. In size classes <1000 µm, biological particles outnumbered microplastic particles by up to 4 orders of magnitude, whereas for size classes >1000 µm, the ratio decreased to nearly 1. Zooplanktivorous consumers like juvenile herring are more likely to consume microplastics than prey since the ratio of microplastic particles >1000 µm to potential food, and therefore encounter rate, is higher.

Arabidopsis halleri shows hyperbioindicator behaviour for Pb and leaf Pb accumulation spatially separated from Zn.

Lead (Pb) ranks among the most problematic environmental pollutants. Background contamination of soils is nearly ubiquitous, yet plant Pb accumulation is barely understood. In a survey covering 165 European populations of the metallophyte Arabidopsis halleri, several field samples had indicated Pb hyperaccumulation, offering a chance to dissect plant Pb accumulation. Accumulation of Pb was analysed in A.halleri individuals from contrasting habitats under controlled conditions to rule out aerial deposition as a source of apparent Pb accumulation. Several elemental imaging techniques were employed to study the spatial distribution and ligand environment of Pb. Regardless of genetic background, A.halleri individuals showed higher shoot Pb accumulation than A.thaliana. However, dose-response curves revealed indicator rather than hyperaccumulator behaviour. Xylem sap data and elemental imaging unequivocally demonstrated the in planta mobility of Pb. Highest Pb concentrations were found in epidermal and vascular tissues. Distribution of Pb was distinct from that of the hyperaccumulated metal zinc. Most Pb was bound by oxygen ligands in bidentate coordination. A.halleri accumulates Pb whenever soil conditions render Pb phytoavailable. Considerable Pb accumulation under such circumstances, even in leaves of A.thaliana, strongly suggests that Pb can enter food webs and may pose a food safety risk.

Isotope fractionation approach to characterize the reactive transport processes governing the fate of hexachlorocyclohexanes at a contaminated site in India

The transformation processes of hexachlorocyclohexane isomers (HCHs) from production sites of Lindane across the landscape and along the food web were studied as an example to understand the fate of POPs in the environment. Therefore, we studied the concentration and isotope composition of HCHs in different matrices in the vicinity of a dumpsite and a chemical plant producing HCHs in India. Carbon isotope compositions (δ13C) of HCHs and the enantiomer fraction (EF) of α-HCH were used as indicators to characterize in situ degradation in soil, groundwater, and sediment as well as along the food web. The HCHs were detected in plants growing on contaminated soil. Elevated concentrations of HCHs were found in a number of crops, which indicates an important transfer pathway of HCHs entering food webs. The EF value of α-HCH and the δ13C signature of HCHs indicated that degradation processes occurred in the rhizosphere or within the plants potentially attenuating the contamination of HCHs. The isotope enrichment of HCHs in dung and milk samples showed that degradation of HCHs may take place in the digestive track of cow and buffalo as well as during their metabolism. The δ13C of HCHs was used to analyze the potential dispersion routes on the landscape scale in order to understand the reactive transport pathways starting at the source of HCHs. In this study, the potential of carbon isotope fractionation and EF for characterizing uptake of HCHs into plants and accumulation in the food web were examined. To the best of our knowledge, this is the first study using the combination of stable isotope fractionation and EF to track the reactive transport processes in a complex environment including the food web.

Isotope and fatty acid trends along continental shelf depth gradients: Inshore versus offshore hydrological influences on benthic trophic functioning

Anthropogenic activities and land-based inputs into the sea may influence the trophic structure and functioning of coastal and continental shelf ecosystems, despite the numerous opportunities and services the latter offer to humans and wildlife. In addition, hydrological structures and physical dynamics potentially influence the sources of organic matter (e.g., terrestrial versus marine, or fresh material versus detrital material) entering marine food webs. Understanding the significance of the processes that influence marine food webs and ecosystems (e.g., terrestrial inputs, physical dynamics) is crucially important because trophic dynamics are a vital part of ecosystem integrity. This can be achieved by identifying organic matter sources that enter food webs along inshore–offshore transects. We hypothesised that regional hydrological structures over wide continental shelves directly control the benthic trophic functioning across the shelf. We investigated this issue along two transects in the northern ecosystem of the Bay of Biscay (north-eastern Atlantic). Carbon and nitrogen stable isotope analysis (SIA) and fatty acid analysis (FAA) were conducted on different complementary ecosystem compartments that include suspended particulate organic matter (POM), sedimentary organic matter (SOM), and benthic consumers such as bivalves, large crustaceans and demersal fish. Samples were collected from inshore shallow waters (at ∼1m in depth) to more than 200m in depth on the offshore shelf break. Results indicated strong discrepancies in stable isotope (SI) and fatty acid (FA) compositions in the sampled compartments between inshore and offshore areas, although nitrogen SI (δ15N) and FA trends were similar along both transects. Offshore the influence of a permanently stratified area (described previously as a “cold pool”) was evident in both transects. The influence of this hydrological structure on benthic trophic functioning (i.e., on the food sources available for consumers) was especially apparent across the northern transect, due to unusual carbon isotope compositions (δ13C) in the compartments. At stations under the cold pool, SI and FA organism compositions indicated benthic trophic functioning based on a microbial food web, including a significant contribution of heterotrophic planktonic organisms and/or of SOM, notably in stations under the cold pool. On the contrary, inshore and shelf break areas were characterised by a microalgae-based food web (at least in part for the shelf break area, due to slope current and upwelling that can favour fresh primary production sinking on site). SIA and FAA were relevant and complementary tools, and consumers better medium- to long-term system integrators than POM samples, for depicting the trophic functioning and dynamics along inshore–offshore transects over continental shelves.

Quantifying Salmon‐Derived Nutrient Loads from the Mortality of Hatchery‐Origin Juvenile Chinook Salmon in the Snake River Basin

AbstractHatchery supplementation of anadromous salmon is extensive across the Pacific Northwest region with millions of juvenile salmon stocked annually. The influence of hatchery‐origin fish as prey items in recipient ecosystems has been explored, but influences of these fish on broader stream nutrient dynamics has not been well‐studied. Salmon‐derived nutrients (SDN) associated with the mortality of adult anadromous salmon provide key subsidies to freshwater habitats. While a number of studies have estimated current and historic SDN loading from returning wild salmon, SDN contributions from the mortality of hatchery‐origin juveniles (many of which die in the stream prior to emigration) remains largely unknown. We conducted a mass balance analysis of SDN input and export via hatchery activities (stocking and broodstock collection) in the Snake River watershed. Using Chinook salmon Oncorhynchus tshawytscha as a model species, we accounted for yearly SDN input (via hatchery‐origin juvenile fish mortality) and export (via broodstock collections and presmolt growth) over 6 years (2002–2007) in the portion of the Snake River upstream from Lower Granite Dam accessible to anadromous fish. In the year with highest smolt mortality (2003), hatchery‐origin smolt mortality provided a net input of SDN equivalent to approximately 8,100 returning adults. In the year with lowest smolt mortality (2004), hatchery activities collectively yielded a net loss of nutrients. Although the mass of SDN from hatchery‐origin smolts may be presented in adult equivalencies, functional influences of SDN from hatchery smolt mortality are likely to differ. Salmon‐derived nutrients from hatcheries enter food webs through largely piscivorous pathways whereas SDN from adult carcasses enter food webs through multiple pathways at multiple trophic levels. The SDN from hatchery‐origin smolts probably influence different components of the food web more than do adult carcasses and have the potential to more directly affect predator populations.

Utilization of invasive tamarisk by salt marsh consumers

Plant invasions of coastal wetlands are rapidly changing the structure and function of these systems globally. Alteration of litter dynamics represents one of the fundamental impacts of an invasive plant on salt marsh ecosystems. Tamarisk species (Tamarix spp.), which extensively invade terrestrial and riparian habitats, have been demonstrated to enter food webs in these ecosystems. However, the trophic impacts of the relatively new invasion of tamarisk into marine ecosystem have not been assessed. We evaluated the trophic consequences of invasion by tamarisk for detrital food chains in the Tijuana River National Estuarine Research Reserve salt marsh using litter dynamics techniques and stable isotope enrichment experiments. The observations of a short residence time for tamarisk combined with relatively low C:N values indicate that tamarisk is a relatively available and labile food source. With an isotopic (15N) enrichment of tamarisk, we demonstrated that numerous macroinvertebrate taxonomic and trophic groups, both within and on the sediment, utilized 15N derived from labeled tamarisk detritus. Infaunal invertebrate species that took up no or limited 15N from labeled tamarisk (A. californica, enchytraeid oligochaetes, coleoptera larvae) occurred in lower abundance in the tamarisk-invaded environment. In contrast, species that utilized significant 15N from the labeled tamarisk, such as psychodid insects, an exotic amphipod, and an oniscid isopod, either did not change or occurred in higher abundance. Our research supports the hypothesis that invasive species can alter the trophic structure of an environment through addition of detritus and can also potentially impact higher trophic levels by shifting dominance within the invertebrate community to species not widely consumed.

Macroalgal palatability and the flux of ciguatera toxins through marine food webs

The benthic dinoflagellate Gambierdiscus toxicus produces polyether toxins that cause ciguatera fish poisoning in humans. The toxins initially enter food webs when fish forage on macroalgae, or other substrates, hosting this epiphytic dinoflagellate. Population studies of G. toxicus and risk assessments in ciguatera-prone regions often rely on quantifying dinoflagellates on macroalgae. Underlying these studies is the assumption that the algae sampled represent a readily consumable resource equally available for benthic grazers. However, many algal hosts of G. toxicus possess a variety of defenses against grazing, and host–dinoflagellate associations may act as toxin sources or sinks depending on their palatability. Marine macroalgae may tolerate or avoid herbivory by exhibiting fast growth, by having poor nutritional quality, by utilizing spatial or temporal escapes or by using chemical or structural defenses. Thus, rapidly consumed algae that cope with herbivores by growing fast, such as many filamentous turfs, could be responsible for a high toxin flux even at low dinoflagellate densities. In contrast, ubiquitous unpalatable algae with much higher dinoflagellate densities might contribute little to toxin flux, and effectively act as refuges for G. toxicus. To date, G. toxicus has been reported from 56 algal genera, two cyanobacteria, one diatom, and one seagrass; 63% of these contain species that are defended from fish grazing and other grazers via chemical, morphological or structural defenses, by low nutritional quality, or by a combination of defensive strategies. High dinoflagellate densities on unpalatable macroalgae could indicate passive accumulation of cells on undisturbed hosts, rather than population explosions or active toxin sources for food webs. Understanding the flow of ciguatoxins in nature requires consideration of the ecology of both G. toxicus and its algal hosts. The complexity of marine algal–herbivore interactions also has consequences for other benthic dinoflagellates that produce toxins, which accumulate in consumers.

Dominance of autochthonous autotrophic carbon in food webs of heterotrophic rivers

This paper addresses the river heterotrophy paradox, “How can animal biomass within riverine food webs be fueled primarily by autochthonous autotrophic production if the ecosystem as a whole is heterotrophic?”. Reviewed, stable isotope data from tropical, temperate, and arctic rivers provide evidence consistent with the revised riverine productivity model (RPM): “The primary, annual energy source supporting overall metazoan production and species diversity in mid‐ to higher‐trophic levels of most rivers (≥4th order) is autochthonous primary production entering food webs via algal‐grazer and decomposer pathways”. The revised RPM does not conflict with the heterotrophy paradox because: (a) the decomposer (microbial loop) food pathway processes most of the transported, allochthonous and autochthonous carbon and, with algal respiration in some cases, is primarily responsible for a river's heterotrophic state (P/R<1); but (b) biomass production of mid‐ to higher‐trophic levels is principally supported by an algal‐grazer (phytoplankton and benthic microalgae) pathway that is only weakly linked to the decomposer pathway. The reason the algal‐grazer pathway supports the majority of metazoan biomass is that allochthonous carbon is mostly recalcitrant, whereas carbon from autochthonous primary production, though much less plentiful, is commonly more labile (easier to assimilate), contains more energy per unit mass, and is typically preferred by metazoa.

Population dynamics of a dense assemblage of marine detritivores

Accumulations of macrophyte detritus provide food and refuge for a dense (up to 3.5·10 6 ind. m −2) assemblage of amphipod and leptostracan crustaceans in a Southern California submarine canyon. The objective of this work was to determine how the physical environment and biological interactions limit these populations, that rarely, if ever, appear to be food limited. Preliminary observations suggested that the detritus-crustaceans would incur significant losses during the Winter, resulting from wave disturbance. Predation by dense schools of fishes associated with this habitat also appeared to be important. Laboratory and field experiments demonstrated that the detritus provided an effective refuge for the crustaceans. Predation rate of fishes upon detritus-associated crustaceans increased as the density of prey populations increased and/or as the thickness of the detritus mat was reduced (increasing number per m 3 but not per m 2). Density (m −2) of the mat crustaceans fluctuated seasonally, being greatest in the Winter and Spring following storms that eliminated large portions of the habitat. Few animals appeared to be directly eliminated by the storm disturbance. The important effect of storms was to reduce the quantity and quality of the detritus refuge by concentrating the crustaceans into smaller patches with less detrital cover. During the calm summer months the bacterium Beggiatoa sp. spread out over large portions of the detritus mat. Beneath the bacteria oxygen concentration was reduced, and infaunal density was two orders of magnitude lower than in unaffected portions of the mat. The summer increase in bacterial cover constituted a biological disturbance that functionally reduced the habitat area available to the mat fauna and left them more vulnerable to predation. Secondary production in the detrital mats is among the highest reported from natural environments. This is possible because the mat crustaceans are unable to graze down their food supply (detritus and associated microbes). The refuge provided by the detritus for large populations of invertebrates also is crucial; larger refugia translated into increased carrying capacity which allowed both greater population sizes and production, much of which became available to fishes. Most marine ecosystems are supported by allochthonous material that enters food webs via detritivores. Seafloor features that collect detritus constitute an important source of patchiness over a wide range of scales because detrital delivery is governed by physical processes. The La Jolla/Scripps Canyon system, by accumulating organic debris, provides a large food source from shallow to continental slope depths. This resource supports large numbers of fishes and presumably increases local production in higher trophic levels.

Changes in food web structure under the influence of increased anthropogenic nitrogen inputs to estuaries

Anthropogenic nitrogen loads to shallow coastal waters have been linked to shifts from seagrass- to algae-dominated communities in many regions of the world, yet the influence that these shifts have on the structure of nearshore food webs remains unclear. We used stable C and N isotope ratios to assess the relative importance of phytoplankton, macroalgae, and eelgrass Zostera marina as food sources to consumers in estuaries of Waquoit Bay, Massachusetts, USA, that receive low versus high nitrogen loads from their surrounding watersheds. We found that, in general, the diets of herbi- vores, suspension feeders, and detritlvores (collectively referred to as primary consumers) in the Waquoit Bay estuaries are influenced by the dominant forms of production that they are exposed to. Phytoplankton and macroalgae are the major food sources of primary consumers under both low and high N loading conditions, but eelgrass is also an important food so'urce where N loading is low Most of the primary consumers at the estuary with low N loading have between 10 and 16 % eelgrass in their diets. Some species, however, appear to have no eelgrass in their diets, while others have diets consist- ing of as much as 31 % eelgrass. Eelgrass C and N are passed on to benthic as well as pelagic secondary consumers. With losses of eelgrass as a consequence of nitrogen loading, an important pathway through which land-derived nitrogen enters food webs in the Waquoit Bay estuaries is eliminated. This fundamental change probably affects the rate at which land-derived nitrogen is cycled within estuar- ies, as well as its ultimate fate.

SESSION III. Summary and Overview: Long-Term Effects of Oil Spills on Marine Intertidal Communities

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