Nutrient limitation of primary producers affects planktivorous fish condition

We investigated whether nutrient limitations of primary producers act upward through food webs only in terms of density effects or if there is a second pathway for nutrient limitation signals channelled upward to higher trophic levels. We used tritrophic food chains to assess the effects of nutrient-limited phytoplankters (the cryptophyte Rhodomonas salina) on herbivorous zooplankters (the calanoid copepod Acartia tonsa) and finally zooplanktivores (larval herring Clupea harengus) living on the herbivores. The primary producers� food quality had a significant effect on fish condition. Our experimental phosphorus-limited food chain resulted in larval fish with a significantly poorer condition than their counterparts reared under nitrogen-limited or nutrient-sufficient conditions. Our results show that mineral nutrient requirements of consumers have to be satisfied first before fatty acids can promote further growth. This challenges the match/mismatch hypothesis, which links larval fish survival probability solely to prey availability, and could imply that reduced nutrient releases into the environment may affect fish stocks even more severely than previously believed

Contrasting phytoplankton-zooplankton distributions observed through autonomous platforms, in-situ optical sensors and discrete sampling

Plankton distributions are remarkably ‘patchy’ in the ocean. In this study, we investigated the contrasting phytoplankton-zooplankton distributions in relation to wind mixing events in waters around a biodiversity-rich island (Runde) located off the western coast of Norway. We used adaptive sampling from AUV and shipboard profiles of in-situ phytoplankton photo-physiology and particle identification (copepods, fecal pellets and the dinoflagellate Tripos spp.) and quantification using optical and imaging sensors. Additionally, traditional seawater and net sampling were collected for nutrient and in-vitro chlorophyll a concentrations and phytoplankton and meso-zooplankton abundances. Persistent strong wind conditions (~5 days) disrupted the stratification in offshore regions, while stratification and a subsurface chlorophyll maximum (SCM) were observed above the base of the mixed layer depth (MLD ~30 m) in inshore waters. Contrasting phytoplankton and zooplankton abundances were observed between inshore (with the presence of a SCM) and offshore waters (without the presence of a SCM). At the SCM, phytoplankton abundances (Tripos spp., the diatom Proboscia alata and other flagellates) were half (average of 200 cell L-1) of those observed offshore. On the contrary, meso-zooplankton counts were ~6× higher (732 ind m-3 for Calanus spp.) inshore (where a SCM was observed) compared to offshore areas. In parallel, fecal pellets and ammonium concentrations were high (>1000 ind m-3 for the upper 20 m) at the SCM, suggesting that the shallow mixed layer might have increased encounter rates and promoted strong grazing pressure. Low nutrient concentrations (< 1μM for nitrate) were found below the MLD (60 m) in offshore waters, suggesting that mixing and nutrient availability likely boosted phytoplankton abundances. The size of the absorption cross-section (σPII’) and yield of photosystem II photochemistry under ambient light (φPII’) changed according to depth, while the depth-related electron flow (JPII) was similar between regions, suggesting a high degree of community plasticity to changes in the light regime. Our results emphasize the importance of using multiple instrumentation, in addition to traditional seawater and net sampling for a holistic understanding of plankton distributions.publishedVersio

Broad-scale distribution of the winter protozooplankton community in the North Sea.

Protozooplankton (PZP) (here size range: 12–200 μm) are rarely sampled over a broad scale, especially in ecosystem monitoring programs, despite their trophodynamic importance as grazers in the microbial loop and as prey for larger zooplankton and early life stages of fish. In this study we sampled PZP from Dutch, French,German and Norwegian research vessels taking part in the annual ICES coordinated International Bottom Trawl Survey (IBTS) which provides data on fish stock abundances and status for the entire North Sea. The abundance,biomass, composition and distribution of PZP were examined at 39 stations across the North Sea (from 3.2°W to 7.6°E and 50.5 to 59.8°N) in mid-winter (January–February 2014), a period of the year which is under-investigated so far. Twenty four taxa of dinoflagellates and ciliates were identified. Two groups comprised 89% of the total abundance of PZP: Gymnodinium spp. and other athecate dinoflagellates (68%) and Strombidium spp. and other naked ciliates (21%). The biomass of PZP at each station ranged between 0.08 and 2.4 μg C L−1, which is much lower than that reported for spring or summer (≥100 μg C L−1) in the North Sea. Relatively small-sized (< 40 μm) PZP contributed 46% of the total biomass. No significant spatial pattern in the composition of the PZP community was found, although the total abundance of tintinnids was highest in the southern North Sea, an important over-wintering area for marine fish larvae. Using this fish survey (IBTS) as a sampling platform allowed us to obtain a synoptic view of the PZP community over a large area. The present collaborative effort provides an example of how existing monitoring platforms can be augmented in the future to collect relevant data and potential ecological indicators needed to advance the ecosystem-based approach to managing marine systems.Broad-scale distribution of the winter protozooplankton community in the North Sea.publishedVersio

Food Quality Affects Secondary Consumers Even at Low Quantities: An Experimental Test with Larval European Lobster

The issues of food quality and food quantity are crucial for trophic interactions. Although most research has focussed on the primary producer – herbivore link, recent studies have shown that quality effects at the bottom of the food web propagate to higher trophic levels. Negative effects of poor food quality have almost exclusively been demonstrated at higher food quantities. Whether these negative effects have the same impact at low food availability in situations where the majority if not all of the resources are channelled into routine metabolism, is under debate. In this study a tri-trophic food chain was designed, consisting of the algae Rhodomonas salina, the copepod Acartia tonsa and freshly hatched larvae of the European lobster Homarus gammarus. The lobster larvae were presented with food of two different qualities (C∶P ratios) and four different quantities to investigate the combined effects of food quality and quantity. Our results show that the quality of food has an impact on the condition of lobster larvae even at very low food quantities. Food with a lower C∶P content resulted in higher condition of the lobster larvae regardless of the quantity of food. These interacting effects of food quality and food quantity can have far reaching consequences for ecosystem productivity

Climate change effects on phytoplankton depend on cell size and food web structure

We investigated the effects of warming on a natural phytoplankton community from the Baltic Sea, based on six mesocosm experiments conducted 2005–2009. We focused on differences in the dynamics of three phytoplankton size groups which are grazed to a variable extent by different zooplankton groups. While small-sized algae were mostly grazer-controlled, light and nutrient availability largely determined the growth of medium- and large-sized algae. Thus, the latter groups dominated at increased light levels. Warming increased mesozooplankton grazing on medium-sized algae, reducing their biomass. The biomass of small-sized algae was not affected by temperature, probably due to an interplay between indirect effects spreading through the food web. Thus, under the higher temperature and lower light levels anticipated for the next decades in the southern Baltic Sea, a higher share of smaller phytoplankton is expected. We conclude that considering the size structure of the phytoplankton community strongly improves the reliability of projections of climate change effects

Influence of Ocean Acidification on a Natural Winter-to-Summer Plankton Succession : First Insights from a Long-Term Mesocosm Study Draw Attention to Periods of Low Nutrient Concentrations

Every year, the oceans absorb about 30% of anthropogenic carbon dioxide (CO2) leading to a re-equilibration of the marine carbonate system and decreasing seawater pH. Today, there is increasing awareness that these changes-summarized by the term ocean acidification (OA)-could differentially affect the competitive ability of marine organisms, thereby provoking a restructuring of marine ecosystems and biogeochemical element cycles. In winter 2013, we deployed ten pelagic mesocosms in the Gullmar Fjord at the Swedish west coast in order to study the effect of OA on plankton ecology and biogeochemistry under close to natural conditions. Five of the ten mesocosms were left unperturbed and served as controls (similar to 380 mu atm pCO(2)), whereas the others were enriched with CO2-saturated water to simulate realistic end-of-the-century carbonate chemistry conditions (mu 760 mu atm pCO(2)). We ran the experiment for 113 days which allowed us to study the influence of high CO2 on an entire winter-to-summer plankton succession and to investigate the potential of some plankton organisms for evolutionary adaptation to OA in their natural environment. This paper is the first in a PLOS collection and provides a detailed overview on the experimental design, important events, and the key complexities of such a "long-term mesocosm" approach. Furthermore, we analyzed whether simulated end-of-the-century carbonate chemistry conditions could lead to a significant restructuring of the plankton community in the course of the succession. At the level of detail analyzed in this overview paper we found that CO2-induced differences in plankton community composition were non-detectable during most of the succession except for a period where a phytoplankton bloom was fueled by remineralized nutrients. These results indicate: (1) Long-term studies with pelagic ecosystems are necessary to uncover OA-sensitive stages of succession. (2) Plankton communities fueled by regenerated nutrients may be more responsive to changing carbonate chemistry than those having access to high inorganic nutrient concentrations and may deserve particular attention in future studies.Peer reviewe

Inter- and intraspecific variations in stable isotope fractionation experimental studies simulating pelagic multi-trophic systems

Stable isotope signatures of primary producers are known to show high inter-specific variations which are assigned to species-specific differences in isotope fractionation. Furthermore, fractionation processes can be altered by abiotic conditions e.g. pH, light, and temperature, thus increasing intra-specific variability. Since consumers reflect the isotopic signature of their food source, such variations have direct impacts on the interpretation of stable isotope data in aquatic ecology. We used a three-trophic level system in order to elucidate the variability of isotope fractionation at the primary producer level and the transfer of the signal through food webs. Strong inter-specific variations and a direct change of stable isotope enrichment in response to nutrient limitation for the different algal species were observed. The &#948;13C-signatures of primary and secondary consumers (copepods and larval fish) reflected only to moderate degrees the isotopic composition of their diets. The theoretical degree of enrichment of 1 &#948;13C and 3.5 &#948;15N per trophic level could only be confirmed in certain cases. The majority of isotope enrichments in the three-trophic food chain showed rather large discrepancies to the theoretical framework

Trophic variations in littoral fish species along latitudinal gradients

Differences in the trophic ecology of several littoral fish species along thermal gradients at the Western European shoreline were investigated as part of the EU/BMBF funded ERANET project MarinERA Marine phylogeographic structuring during climate change: the signature of leading and rear edge of range shifting populations. The aim was to assess the impact of climate change on the adaptability and changes in the feeding behaviour of specific littoral fish populations at their northern and southern distribution limits (cold- and warm-adapted species). During a first sampling campaign in 2009 the target fish species Pomatoschistus minutus, Ctenolabrus rupestris and Symphodus melops as well as herbivores (Littorina littorea and Mytilus edulis) from different populations were sampled along a gradient from 36°N to 58°N thus experiencing different thermal conditions. Fatty acid profiles and stable isotope signatures (&#948;13C, &#948;15N) as indicators for the trophic ecology of target fish species and herbivores were analysed and variations along latitudinal gradients assessed. Furthermore, issues such as the degree of trophic overlap between target fish species, their niche widths and their adaptive potential will be addressed in the light of ongoing climate change

Seasonal dynamics of microzooplankton communities in the Sea of Oman (Arabian Sea)

Seasonal dynamics of microzooplankton and changes in environmental condition were analysed during a one-year field sampling campaign in the Sea of Oman at two different stations. Monsoon winds in this region cause distinct seasonality patterns with high primary productivity during the south-west monsoon in summer (June to October) and north-east monsoon periods in winter (November to March). Microzooplankton in the Sea of Oman showed several biomass peaks throughout the year. In general, higher biomass occurred during the south-west monsoon when compared to the north-east monsoon period with maxima of 190 µg C l−1at the inshore station Bandar Al-Khyran at 1m and 308 µg C l−1 at 10m water depth. At the offshore-station, peaks of 372 µg C l−1 (1m) and 256 µg C l−1 (20m) occurred during the south-west monsoon. A strong coupling between phytoplankton and microzooplankton was observed during monsoon periods but some microzooplankton peaked during inter-monsoon periods when chlorophyll concentration was low (Bandar Al-Khyran: 372 µg C l−1 at 1m and 196 µg C l−1, 10m; Offshore-station: 419 µg C l−1, 20 m). The initiation of phytoplankton blooms in the Sea of Oman was bottom-up controlled due to strong seasonal nutrient influx during south-west and north-east monsoon periods. Highest microzooplankton biomass occurred during monsoon periods with a dominance of Noctiluciphyceae and peaks of 7596 µg C l−1 at Bandar Al-Khyran (1m) and 5942 µg C l−1 (10m). Copepod nauplii, Amoebozoa and Larvacea contributed substantially to microzooplankton biomass throughout the year. Ciliophora contributed low proportion to the total microzooplankton biomass peaking both during monsoon and inter-monsoon periods. During the spring inter-monsoon, choreotrich ciliates (tintinnids) showed distinct peaks of 15.9 µg C l−1 at Bandar Al-Khyran (1m) and 17.7 µg C l−1 (10m) as well as 18.2 µg C l−1 at Offshore-station (20m). The interplay between bottom-up controlled primary production and top-down control mechanisms regulates the phenology patterns of specific microzooplankton groups in the Sea of Oman thus pointing at complex trophodynamic interactions at the lowermost foodweb level in this low-latitude ecosystem