Feeding of Calanus finmarchicus and Oithona similis on the microplankton assemblage in the Irminger Sea, North Atlantic

The present investigation reviews published data on the feeding rates and prey selection of Oithona similis females, Calanus finmarchicus nauplii and females in the Irminger Sea in April/May and July/August 2002. Our aim was to examine how the feeding rates and prey selection of these three copepod stages respond to concomitant changes in microplankton community composition and prey abundance. Copepods typically ingested prey overall according to its ambient concentration although significant species and stage-specific differences in prey-type ingestion and selection were apparent. Despite being of comparable weight, the ingestion rates of C. finmarchicus nauplii were always higher than those of the O. similis females. Moreover, C. finmarchicus nauplii and O. similis females fed preferentially on diatoms and ciliates respectively, whereas adult female C. finmarchicus showed limited prey selectivity. Copepod grazing impact on total and on ciliates/dinoflagellates standing stock was <0.5 and <2%, respectively. We attribute this result to a combination of low grazing rates, low copepod abundance and low microplankton biomass, all of which are indicative of the non-bloom conditions under which these experiments were conducted. The differences in copepod feeding rates and prey selection we report reflect species and stage-specific eco-physiological adaptations, which may act as important driving forces for marine ecosystem structuring and functioning.

Feeding and overwintering of Antarctic krill across its major habitats: The role of sea ice cover, water depth, and phytoplankton abundance

Antarctic krill (Euphausia superba) were sampled in contrasting habitats: a seasonally ice-covered deep ocean (Lazarev Sea), ice-free shelves at their northern range (South Georgia) and the Antarctic Peninsula (Bransfield Strait), and shelf and oceanic sites in the Scotia Sea. Across 92 stations, representing a year-round average, the food volume in krill stomachs comprised 71 +/- 29% algae, 17 +/- 21% protozoans, and 12 +/- 25% metazoans. Fatty acid trophic markers showed that copepods were consistently part of krill diet, not a switch food. In open waters, both diatom and copepod consumption increased with phytoplankton abundance. Under sea ice, ingestion of diatoms became rare, whereas feeding on copepods remained constant. During winter, larvae contained high but variable proportions of diatom markers, whereas in postlarvae the role of copepods increased with krill body length. Overwintering differed according to habitat. Krill from South Georgia had lower lipid stores than those from the Bransfield Strait or Lazarev Sea. Feeding effort was much reduced in Lazarev Sea krill, whereas most individuals from the Bransfield Strait and South Georgia contained phytoplankton and seabed detritus in their stomachs. Their retention of essential body reserves indicates that krill experienced most winter hardship in the Lazarev Sea, followed by South Georgia and then Bransfield Strait. This was reflected in the delayed development from juveniles to adults in the Lazarev Sea. Circumpolar comparisons of length frequencies suggest that krill growth conditions are more favorable in the southwest Atlantic than in the Lazarev Sea or off East Antarctica because of longer phytoplankton bloom periods and rewarding access to benthic food.

Flexible C : N ratio enhances metabolism of large phytoplankton when resource supply is intermittent

Phytoplankton cell size influences particle sinking rate, food web interactions and biogeographical distributions. We present a model in which the uptake, storage and assimilation of nitrogen and carbon are explicitly resolved in different-sized phytoplankton cells. In the model, metabolism and cellular C :N ratio are influenced by the accumulation of carbon polymers such as carbohydrate and lipid, which is greatest when cells are nutrient starved, or exposed to high light. Allometric relations and empirical data sets are used to constrain the range of possible C : N, and indicate that larger cells can accumulate significantly more carbon storage compounds than smaller cells. When forced with extended periods of darkness combined with brief exposure to saturating irradiance, the model predicts organisms large enough to accumulate significant carbon reserves may on average synthesize protein and other functional apparatus up to five times faster than smaller organisms. The advantage of storage in terms of average daily protein synthesis rate is greatest when modeled organisms were previously nutrient starved, and carbon storage reservoirs saturated. Small organisms may therefore be at a disadvantage in terms of average daily growth rate in environments that involve prolonged periods of darkness and intermittent nutrient limitation. We suggest this mechanism is a significant constraint on phytoplankton C :N variability and cell size distribution in different oceanic regimes.

Feeding 9 billion by 2050 – Putting fish back on the menu

Fish provides more than 4.5 billion people with at least 15 % of their average per capita intake of animal protein. Fish's unique nutritional properties make it also essential to the health of billions of consumers in both developed and developing countries. Fish is one of the most efficient converters of feed into high quality food and its carbon footprint is lower compared to other animal production systems. Through fish-related activities (fisheries and aquaculture but also processing and trading), fish contribute substantially to the income and therefore to the indirect food security of more than 10 % of the world population, essentially in developing and emergent countries. Yet, limited attention has been given so far to fish as a key element in food security and nutrition strategies at national level and in wider development discussions and interventions. As a result, the tremendous potential for improving food security and nutrition embodied in the strengthening of the fishery and aquaculture sectors is missed. The purpose of this paper is to make a case for a closer integration of fish into the overall debate and future policy about food security and nutrition. For this, we review the evidence from the contemporary and emerging debates and controversies around fisheries and aquaculture and we discuss them in the light of the issues debated in the wider agriculture/farming literature. The overarching question that underlies this paper is: how and to what extent will fish be able to contribute to feeding 9 billion people in 2050 and beyond?

Feeding rates and prey selectivity of planktonic decapod larvae in the Western English Channel

Meroplankton are seasonally important contributors to the zooplankton, particularly at inshore sites, yet their feeding ecology is poorly known relative to holoplankton. While several studies have measured feeding in decapod larvae, few studies have examined the feeding rates of decapod larvae on natural prey assemblages throughout the reproductive season. We conducted 8 feeding experiments with Necora puber, Liocarcinus spp. and Upogebia spp. zoea larvae collected from the L4 monitoring site off Plymouth (50°15.00′N, 4°13.02′W) during spring–summer 2009 and 2010. This period spanned moderate-to-high food availability (0.5–1.6 µg chl-a L−1), but a great range in food composition with small cells <20 µm dominating in 2010. Daily rations averaged 17, 60 and 22 % of body C for the 3 respective decapod species. Clearance rates differed according to prey type, and all 3 decapod genera showed evidence of selection of dinoflagellates. Importantly, small cells including nano- and pico-plankton were ingested, this being demonstrated independently by flow cytometric analysis of the feeding experiments and molecular analysis. PCR-based analysis of the haptophyte portion of the diet revealed ingestion of Isochrysis galbana by decapod larvae in the bottle incubations and Isochrysis galbana and Phaeocystis globosa by decapod larvae collected directly from the field. This study has shown that pico- and nano-sized plankton form an important supplement to the diverse and variable diet of decapod larvae.

Feeding selectivity of bivalve larvae on natural plankton assemblages in the Western English Channel

Meroplankton, including bivalve larvae, are an important and yet understudied component of coastal marine food webs. Understanding the baseline of meroplankton ecology is imperative to establish and predict their sensitivity to local and global marine stressors. Over an annual cycle (October 2009–September 2010), bivalve larvae were collected from the Western Channel Observatory time series station L4 (50°15.00′N, 4°13.02′W). The morphologically similar larvae were identified by analysis of the 18S nuclear small subunit ribosomal RNA gene, and a series of incubation experiments were conducted to determine larval ingestion rates on natural plankton assemblages. Complementary gut content analysis was performed using a PCR-based method for detecting prey DNA both from field-collected larvae and those from the feeding experiments. Molecular identification of bivalve larvae showed the community composition to change over the course of the sampling period with domination by Phaxas in winter and higher diversity in autumn. The larvae selected for nanoeukaryotes (2–20 µm) including coccolithophores (<20 µm) which together comprised >75 % of the bivalve larvae diet. Additionally, a small percentage of carbon ingested originated from heterotrophic ciliates (<30 µm). The molecular analysis of bivalve larvae gut content provided increased resolution of identification of prey consumed and demonstrated that the composition of prey consumed established through bottle incubations conferred with that established from in situ larvae. Despite changes in bivalve larvae community structure, clearance rates of each prey type did not change significantly over the course of the experiment, suggesting different bivalve larvae species may consume similar prey.

The Impact of Polystyrene Microplastics on Feeding, Function and Fecundity in the Marine CopepodCalanus helgolandicus

Microscopic plastic debris, termed “microplastics”, are of increasing environmental concern. Recent studies have demonstrated that a range of zooplankton, including copepods, can ingest microplastics. Copepods are a globally abundant class of zooplankton that form a key trophic link between primary producers and higher trophic marine organisms. Here we demonstrate that ingestion of microplastics can significantly alter the feeding capacity of the pelagic copepod Calanus helgolandicus. Exposed to 20 μm polystyrene beads (75 microplastics mL–1) and cultured algae ([250 μg C L–1) for 24 h, C. helgolandicus ingested 11% fewer algal cells (P = 0.33) and 40% less carbon biomass (P < 0.01). There was a net downward shift in the mean size of algal prey consumed (P < 0.001), with a 3.6 fold increase in ingestion rate for the smallest size class of algal prey (11.6–12.6 μm), suggestive of postcapture or postingestion rejection. Prolonged exposure to polystyrene microplastics significantly decreased reproductive output, but there were no significant differences in egg production rates, respiration or survival. We constructed a conceptual energetic (carbon) budget showing that microplastic-exposed copepods suffer energetic depletion over time. We conclude that microplastics impede feeding in copepods, which over time could lead to sustained reductions in ingested carbon biomass.