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

Spectral fingerprinting for specific algal groups on sediments in situ: a new sensor

Currently it is still extremely difficult to adequately sample populations of microalgae on sediments for large-scale biomass determination. We have now devised a prototype of a new benthic sensor (BenthoFluor) for the quantitative and qualitative assessment of microphytobenthos populations in situ. This sensor enables a high spatial and temporal resolution and a rapid evaluation of the community structure and distribution. These determinations are based on the concept that five spectral excitation ranges can be used to differentiate groups of microalgae, in situ, within a few seconds. In addition, because sediments contain a lot of yellow substances, which can affect the fluorescence and optical differentiation of the algae, the device was equipped with a UV-LED for yellow substances correction. The device was calibrated against HPLC with cultures and tested in the field. Our real-time approach can be used to monitor algal assemblage composition on sediments and is an ideal tool for investigations on the large-scale spatial and temporal variation of algal populations in sediments. Apart from the differentiation of algal populations, the BenthoFluor allows instantaneous monitoring of the chlorophyll concentrations and determination of which algae are responsible for this on the uppermost surface of sediments in the field and in experimental set-ups

Effects of high CO2 and warming on a Baltic Sea microzooplankton community

Global warming and ocean acidification are among the most important stressors for aquatic ecosystems in the future. To investigate their direct and indirect effects on a near-natural plankton community, a multiple-stressor approach is needed. Hence, we set up mesocosms in a full-factorial design to study the effects of both warming and high CO2 on a Baltic Sea autumn plankton community, concentrating on the impacts on microzooplankton (MZP). MZP abundance, biomass, and species composition were analysed over the course of the experiment. We observed that warming led to a reduced time-lag between the phytoplankton bloom and an MZP biomass maximum. MZP showed a significantly higher growth rate and an earlier biomass peak in the warm treatments while the biomass maximum was not affected. Increased pCO2 did not result in any significant effects on MZP biomass, growth rate, or species composition irrespective of the temperature, nor did we observe any significant interactions between CO2 and temperature. We attribute this to the high tolerance of this estuarine plankton community to fluctuations in pCO2, often resulting in CO2 concentrations higher than the predicted end-of-century concentration for open oceans. In contrast, warming can be expected to directly affect MZP and strengthen its coupling with phytoplankton by enhancing its grazing pressure

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

Ocean acidification and global warming: Can we expect effects on microzooplankton communities?

The focus of this project was on the pivotal role of microzooplankton (MZP) as trophic intermediary between the microbial loop and higher trophic levels. At the base of the food web, MZP has a strong impact on phytoplankton standing stocks due to its high growth and grazing rates, leading to dietary competition with larger mesozooplankton. Simultaneously, higher trophic levels use MZP as food source and benefit from its ability to buffer nutritional imbalances especially at times when food quality of phytoplankton is low. Therefore, MZP abundance, biomass and taxonomic composition were investigated during three mesocosm experiments within the BIOACID II framework, using natural plankton communities. The KOSMOS 2013 Gullmar Fjord experiment in the North Sea was a long-term outdoor mesocosm study with an elevated CO2 level as single stressor. Contrastingly, the BIOACID Autumn 2012 and Summer 2013 Baltic Sea indoor mesocosm experiments investigated the combined effects of both high CO2 and warming. In conclusion, warming can be expected to directly affect MZP communities and enhance their growth and grazing pressure. Additionally, more complex responses of MZP to an increase in CO2 can be expected from the results. The present data points at predominately indirect effects on the MZP community via e.g. changes in phytoplankton community composition and/or standing stocks

Analyzing the Impacts of Elevated-CO2 Levels on the Development of a Subtropical Zooplankton Community During Oligotrophic Conditions and Simulated Upwelling

Ocean acidification (OA) is affecting marine ecosystems through changes in carbonate chemistry that may influence consumers of phytoplankton, often via trophic pathways. Using a mesocosm approach, we investigated OA effects on a subtropical zooplankton community during oligotrophic, bloom, and post-bloom phases under a range of different pCO2 levels (from ∼400 to ∼1480 μatm). Furthermore, we simulated an upwelling event by adding 650 m-depth nutrient-rich water to the mesocosms, which initiated a phytoplankton bloom. No effects of pCO2 on the zooplankton community were visible in the oligotrophic conditions before the bloom. The zooplankton community responded to phytoplankton bloom by increased abundances in all treatments, although the response was delayed under high-pCO2 conditions. Microzooplankton was dominated by small dinoflagellates and aloricate ciliates, which were more abundant under medium- to high-pCO2 conditions. The most abundant mesozooplankters were calanoid copepods, which did not respond to CO2 treatments during the oligotrophic phase of the experiment but were found in higher abundance under medium- and high-pCO2 conditions toward the end of the experiment, most likely as a response to increased phyto- and microzooplankton standing stocks. The second most abundant mesozooplankton taxon were appendicularians, which did not show a response to the different pCO2 treatments. Overall, CO2 effects on zooplankton seemed to be primarily transmitted through significant CO2 effects on phytoplankton and therefore indirect pathways. We conclude that elevated pCO2 can change trophic cascades with significant effects on zooplankton, what might ultimately affect higher trophic levels in the future

Grazer diversity effects in an eelgrass–epiphyte–microphytobenthos system

The dramatic loss of biodiversity and its consequences for ecosystem processes have been of considerable interest in recent ecological studies. However, the complex and interacting processes influencing diversity effects in multitrophic systems are still poorly understood. We used an experimental eelgrass system to study the effects of changing richness of three consumer species on the biomass, diversity and taxonomic composition of both epiphytic and benthic microalgal assemblages. After 1 week, consumer richness enhanced the grazing impact on epiphyte biomass relative to single consumer treatments and a positive effect of consumer richness on prey diversity was found. Moreover, strong effects of consumer species identity on taxonomic composition were found in both microalgal assemblages. However, the effects of consumer richness were not consistent over time. The consequences of high nutrient availability seemed to have masked consumer richness effects

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