Copepods controlling bacterial communities on fecal pellets

The traditional view of the marine food web depicts bacteria and copepods (mainly planktonic species) as separate units, indirectly connected via nutrient cycling and trophic cascade processes. In contrast, several recent studies have demonstrated that zooplankton and bacteria directly interact, physically, e.g. bacteria attached to zooplankton bodies and biologically, e.g. zooplankton feeding supports bacterial growth through their excretions. Copepods produce large numbers of fecal pellets in the marine environment. Almost immediately after egestion, pellets host extensive bacterial communities. Low amounts of fecal material in sediment traps indicate most part of fecal pellet production is retained in the water column as a result of high microbial degradation rates and planktonic copepods reworking the fecal pellets. First observations on the re-use of feces by benthic copepods points out that these crustaceans profit in a yet unknown way from fecal pellet bacteria. Recently it was illustrated that the benthic species Paramphiascella fulvofasciata increases its fecal pellet production according to its food source. Presumably the bacteria associated with fecal pellets create a trophic upgrading of the fecal material. A detailed characterization of these bacteria is crucial to understand the trophic pathways in the lower marine food web. Culture-independent molecular techniques (e.g. DGGE) showed the specificity of these communities. Shifts in the bacterial communities are caused by age, original food source (e.g. diatoms) and producer of the fecal pellet. Moreover, an additional grazing experiment illustrated the importance of the freshness of the initial food source for grazing preferences but also for the bacterial communities on the fecal pellets. Food of low quality was compensated by more diverse bacterial communities that were available for additional grazing. These results illustrated the importance of fecal bacteria in the transformation of organic matter and energy transfer in marine sediments

A view from above : changing seas, seabirds and food sources

In this review we summarize what is known about mechanisms by which climate change may be affecting the populations of seabirds around the UK. Breeding success and adult survival are the key factors affecting changes in seabird populations, and food intake is implicated as a major determinant of both. The diet of most UK seabird species is almost exclusively sandeels, small clupeoid fish or zooplankton and it is clear that the marine pelagic food web is the key ecological system determining food supply. Hence, we develop the review by first considering how climate changes may affect primary production, and then examine how this propagates through the food web to zooplankton and fish culminating in fluctuations in seabird numbers. A trend of increasing numbers of many seabird species since 1970, particularly puffins, guillemots and razorbills, appears to have been reversed since 2000. The proximate cause of the recent declines seems to be a succession of 5 years of low breeding success for a range of species due to a shortage of food, especially sandeels. However, the connection with climate change remains uncertain, though there are indications that declines in the productivity of sandeel populations may be linked in some complex way to warming sea temperatures. The main conclusion is that no part of the marine food web, including fisheries, can be considered in isolation when trying to understand and predict the consequences of climate change for seabirds. Impacts can be expected in all parts of the system, and all parts of the system are interconnected

Marine plastic debris emits a keystone infochemical for olfactory foraging seabirds.

Plastic debris is ingested by hundreds of species of organisms, from zooplankton to baleen whales, but how such a diversity of consumers can mistake plastic for their natural prey is largely unknown. The sensory mechanisms underlying plastic detection and consumption have rarely been examined within the context of sensory signals driving marine food web dynamics. We demonstrate experimentally that marine-seasoned microplastics produce a dimethyl sulfide (DMS) signature that is also a keystone odorant for natural trophic interactions. We further demonstrate a positive relationship between DMS responsiveness and plastic ingestion frequency using procellariiform seabirds as a model taxonomic group. Together, these results suggest that plastic debris emits the scent of a marine infochemical, creating an olfactory trap for susceptible marine wildlife

Spatial analysis of demersal food webs through integration of eDNA metabarcoding with fishing activities

The evaluation of the status of marine communities, and especially the monitoring of those heavily exploited by fisheries, is a key, challenging task in marine sciences. Fishing activities are a major source of disruption to marine food webs, both directly, by selectively removing components at specific trophic levels (TL), and indirectly, by altering habitats and production cycles. Food web analysis can be very useful in the context of an Ecosystem Approach to Fisheries, but food web reconstructions demand large and expensive data sets, which are typically available only for a small fraction of marine ecosystems. Recently, new technologies have been developed to easily, quickly and cost-effectively collect environmental DNA (eDNA) during fishing activities. By generating large, multi-marker metabarcoding data from eDNA samples obtained from commercial trawlers, it is possible to produce exhaustive taxonomic inventories for the exploited ecosystems, which are suitable for food-web reconstructions. Here, we integrate and re-analyse the data of a recent study in which the α diversity was investigated using the eDNA opportunistically collected during fishing operations. Indeed, we collect highly resolved information on species feeding relationships to reconstruct the food webs at different sites in the Strait of Sicily (Mediterranean Sea) from eDNA and catch data. After observing that the trophic networks obtained from eDNA metabarcoding data are more consistent with the available knowledge, a set of food web indicators (species richness, number of links, direct connectance and generality) is computed and analysed to unravel differences in food webs structure through different areas (spatial variations). Species richness, number of links and generality (positively) and direct connectance (negatively) are correlated with increasing distance from the coast and fishing effort intensity. The combined effects of environmental gradients and fishing effort on food web structure at different study sites are then examined and modelled. Taken together, these findings indicate the suitability of eDNA metabarcoding to assist and food web analysis, obtain several food web-related ecological indicators, and tease out the effect of fishing intensity from the environmental gradients of marine ecosystems

Global change in the trophic functioning of marine food webs

The development of fisheries in the oceans, and other human drivers such as climate warming, have led to changes in species abundance, assemblages, trophic interactions, and ultimately in the functioning of marine food webs. Here, using a trophodynamic approach and global databases of catches and life history traits of marine species, we tested the hypothesis that anthropogenic ecological impacts may have led to changes in the global parameters defining the transfers of biomass within the food web. First, we developed two indicators to assess such changes: the Time Cumulated Indicator (TCI) measuring the residence time of biomass within the food web, and the Efficiency Cumulated Indicator (ECI) quantifying the fraction of secondary production reaching the top of the trophic chain. Then, we assessed, at the large marine ecosystem scale, the worldwide change of these two indicators over the 1950-2010 time-periods. Global trends were identified and cluster analyses were used to characterize the variability of trends between ecosystems. Results showed that the most common pattern over the study period is a global decrease in TCI, while the ECI indicator tends to increase. Thus, changes in species assemblages would induce faster and apparently more efficient biomass transfers in marine food webs. Results also suggested that the main driver of change over that period had been the large increase in fishing pressure. The largest changes occurred in ecosystems where 'fishing down the marine food web' are most intensive

New insights into the Weddell Sea ecosystem applying a quantitative network approach

Network approaches can shed light on the structure and stability of complex marine communities. In recent years, such approaches have been successfully applied to study polar ecosystems, improving our knowledge on how they might respond to ongoing environmental changes. The Weddell Sea is one of the most studied marine ecosystems outside the Antarctic Peninsula in the Southern Ocean. Yet, few studies consider the known complexity of the Weddell Sea food web, which in its current form comprises 490 species and 16041 predator-prey interactions. Here we analysed the Weddell Sea food web, focusing on the species and trophic interactions that underpin ecosystem structure and stability. We estimated the strength for each interaction in the food web, characterised species position in the food web using unweighted and weighted food web properties, and analysed species&rsquo; roles with respect to the stability of the food web. We found that the distribution of the interaction strength (IS) at the food web level is asymmetric, with many weak interactions and few strong ones. We detected a positive relationship between species mean IS and two unweighted properties (i.e., trophic level and the total number of interactions). We also found that only a few species possess key positions in terms of food web stability. These species are characterised by high mean IS, mid to high trophic level, relatively high number of interactions, and mid to low trophic similarity. In this study, we integrated unweighted and weighted food web information, enabling a more complete assessment of the ecosystem structure and function of the Weddell Sea food web. Our results provide new insights, which are important for the development of effective policies and management strategies, particularly given the ongoing initiative to implement a Marine Protected Area (MPA) in the Weddell Sea.</p

Empty Oceans

How does the human population affect the population of marine species? What can citizens do to sustain seafood populations? In this lesson, students will learn how pieces of the ocean food web, fish, are being removed faster than they can be replenished. Students will also learn how they can become informed consumers to promote sustainable seafood. Educational levels: Middle school, High school

Empty Oceans

How does the human population affect the population of marine species? What can citizens do to sustain seafood populations? In this lesson, students will learn how pieces of the ocean food web, fish, are being removed faster than they can be replenished. Students will also learn how they can become informed consumers to promote sustainable seafood. Educational levels: Middle school, High school

[Corrigendum to] Effects of small-scale turbulence on lower trophic levels under different nutrient conditions [vol 32, pg 197, 2010]

Small-scale turbulence affects the pelagic food web and energy flow in marine systems and the impact is related to nutrient conditions and the assemblage of organisms present. We generated five levels of turbulence (2*10 29 to 1*10 24 W kg 21 ) in land-based mesocosms (volume 2.6 m 3 ) with and without additional nutrients (31:16:1 Si:N:P m M) to asses the effect of small-scale turbulence on the lower part of the pelagic food web under different nutrient conditions. The ecological influence of nutrients and small-scale turbulence on lower trophic levels was quantified using multivariate statistics (RDA), where nutrients accounted for 31.8% of the observed biological variation, while 7.2% of the variation was explained by small-scale turbulence and its interaction with nutrients. Chlorophyll a, primary production rates, bacterial production rates and diatom and dinoflagellate abundance were positively correlated to turbulence, regardless of nutrient conditions. Abundance of autotrophic flagellates, total phytoplankton and bacteria were positively correlated to turbulence only when nutrients were added. Impact of small-scale turbulence was related to nutrient con- ditions, with implications for oligotrophic and eutrophic situations. The effect on community level was also different compared to single species level. Microbial processes drive biogeochemical cycles, and nutrient-controlled effects of small-scale turbulence on such processes are relevant to foresee altered carbon flow in marine systems

Food web-mediated interaction between marine mammals and fisheries in the Norwegian and Barents seas.

Ecological systems, such as marine ecosystems, are complex adaptive systems in which large scale system properties (e.g., trophic structure, energy flux patterns, etc.) emerge from interactions between ecosystem components or species. This makes them difficult to understand, predict, and model. The Norwegian and Barents Seas support multiple commercial fisheries, including those for herring, cod, and mackerel. Fisheries extract around 2.61 million tonnes of fish annually and marine mammals consume 11.7 million tonnes of fish and zooplankton annually in the region. The gradual change from conventional fisheries management towards ecosystem-based fisheries management (EBFM) requires that interactions and trade-offs between exploitation and conservation of fish and marine mammals be considered. Such trade-offs occurs when there is direct or food-web mediated competition for resources between fisheries and marine mammals. Recent diet studies suggest that there is limited direct competition between marine mammals and fisheries. Food-web mediated interactions may occur when mammals consume the prey of commercial fish species, or when fisheries target the prey of marine mammals, but evidence for such interactions is still lacking. Using Chance and Necessity modelling (CaN) we reconstruct possible dynamics of 12 trophospecies in the Norwegian and Barents Sea ecosystems during the period 1988-2021. The reconstructed dynamics are consistent with multiple observations of biomass, diet, consumption, and life-history characteristics of the species groups. We use these reconstructions to establish the level of empirical support for food-web mediated interactions between marine mammals and fisheries. The results of the model analysis indicate that there is limited evidence to support direct competitive interactions between marine mammals and fisheries in the Norwegian Sea, and mixed evidence for such interactions in the Barents Sea. The results showed that most direct interactions between the two groups were bottom-up driven, and that only demersal fish, aside from cod, demonstrated a direct competitive interaction. As for food-web mediated interactions, the model provided evidence in support of a competitive interaction between marine mammals and capelin, between marine mammals and Barents Sea fish, and between marine mammals and all fish included in the model domain. However, the analysis also revealed the presence of a bottom-up trophic control in the food-web mediated interactions, particularly involving capelin and juvenile herring as prey. Thus, our model results show the presence of both opportunistic feeding and food-web mediated competitive interactions in the Norwegian and Barents Sea ecosystem