ALBERT

Description: Diazotrophic dinitrogen (N2) fixation contributes ~76% to "new" nitrogen inputs to the sunlit open ocean, but environmental factors determining N2 fixation rates are not well constrained. Excess phosphate (phosphate-nitrate/16 〉 0) and iron availability control N2 fixation rates in the eastern tropical North Atlantic (ETNA), but it remains an open question how excess phosphate is generated within or supplied to the phosphate-depleted sunlit layer. Our observations in the ETNA region (8°N-15°N, 19°W-23°W) suggest that Prochlorococcus and Synechococcus, the two ubiquitous non-diazotrophic cyanobacteria with cellular N:P ratios higher than the Redfield ratio, create an environment of excess phosphate, which cannot be explained by diapycnal mixing, atmospheric, and riverine inputs. Thus, our results unveil a new biogeochemical niche construction mechanism by non-diazotrophic cyanobacteria for their diazotrophic phylum group members (N2 fixers). Our observations may help to understand the prevalence of diazotrophy in low-phosphate, oligotrophic regions.

Description: Fecundity of marine fish species is highly variable, but trade-offs between fecundity and egg quality have rarely been observed at the individual level. We investigated spatial differences in reproductive investment of individual European sprat Sprattus sprattus (Linnaeus 1758) females by determining batch fecundity, condition indices (somatic condition index and gonadosomatic index) as well as oocyte dry weight, protein content, lipid content, spawning batch energy content, and fatty acid composition. Sampling was conducted in five different spawning areas within the Baltic Sea between March and May 2012. Spawning sprat from the Kiel Bight were in a better nutritional condition compared to sprat from the Arkona Basin, Bornholm Basin, and the Gdansk Deep. These females were also producing up to twice as many oocytes, and invested significantly more energy in reproduction, than their counterparts sampled in the eastern part of the Baltic Sea. Still, oocytes produced by Kiel Bight sprat contained significantly lower fractions of the essential fatty acids 20:4 (n-6) and 22:6 (n-3). A seasonal trade-off between oocyte weight/lipid content and fecundity was found for Baltic sprat, albeit the gross energy invested into spawning remained constant. Observed spatial and seasonal differences in sprat reproductive investment may be linked to hydrographic conditions and food availability and will impact the survival probability of yolk-sac and first feeding larvae. These findings indicate that Baltic sprat is able to adapt its reproductive tactics to the highly variable pelagic habitat of the Baltic Sea.

Description: Inorganic dissolved macronutrient (nitrogen, N, and phosphorus, P) supply to surface waters in the eastern tropical South Pacific is influenced by expanding oxygen minimum zones, since N loss occurs due to microbial processes under anoxic conditions while P is increasingly released from the shelf sediments. To investigate the impact of decreasing N:P supply ratios in the Peruvian Upwelling, we conducted nutrient manipulation experiments using a shipboard mesocosm setup with a natural phytoplankton community. In a first experiment, either N or P or no nutrients were added with mesozooplankton present or absent. In a second experiment, initial nutrient concentrations were adjusted to four N:P ratios ranging from 2.5 to 16 using two "high N" and two "high P" levels in combination (i.e., +N, +P, +N and P, no addition). Over six and seven days, respectively, microalgal biomass development as well as nutrient uptake was monitored. Phytoplankton biomass strongly responded to N addition, in both mesozooplankton-grazed and not grazed treatments. The developing diatom bloom in the "high N" exceeded that in the "low N" treatments by a factor of two. No modulation of the total biomass by P-addition was observed, however, species-specific responses were more variable. Notably, some organisms were able to benefit from low N:P fertilization ratios, especially Heterosigma sp. and Phaeocystis globosa which are notorious for forming blooms that are toxic or inadequate for mesozooplankton nutrition. After the decline of the diatom bloom, the relative contribution of unsaturated fatty acid to the lipid content of seston was positively correlated to diatom biomass in the peak bloom, indicating that positive effects of diatom blooms on food quality of the protist community to higher trophic levels remain even after the phytoplankton biomass was incorporated by grazers. Our results indicate an overall N-limitation of the system, especially in the case of dominating diatoms, which were able to immediately utilize the available nitrate (within two days) and develop a biomass maximum within three days of incubation. After the decline of diatom biomass, detection of the cyanobacterial marker pigment aphanizophyll indicated the occurrence of diazotrophs, especially in those enclosures initially provided with high N supply. This was surprising, as diazotrophs are thought to play a role in compensating to some extent the N deficit above OMZs in the succession of phytoplankton after an upwelling event

Description: The Eastern Tropical North Atlantic (ETNA) is characterised by a strong east to west gradient in the vertical upward flux of dissolved inorganic nitrogen to the photic zone. We measured the stable nitrogen isotope (δ15N) signatures of various zooplankton taxa covering twelve stations in the ETNA (04°–14°N, 016–030°W) in fall 2009, and observed significant differences in δ15N values among stations. These spatial differences in δ15N within zooplankton taxa exceeded those between trophic levels and revealed an increasing atmospheric input of nitrogen by N2 fixation and Aeolian dust in the open ocean as opposed to remineralised NO3− close to the NW African upwelling. In order to investigate the spatial distribution of upwelling-fuelled versus atmospheric-derived nitrogen more closely, we examined the δ15N signatures in size-fractionated zooplankton as well as in three widely distributed epipelagic copepod species on a second cruise in fall 2010 in the ETNA (02-17°35′N, 015–028°W). Copepods were sampled for δ15N and RNA/DNA as a proxy for nutritional condition on 25 stations. At the same stations, vertical profiles of chlorophyll-a and dissolved nutrients were obtained. High standing stocks of chl-a were associated with shallow mixed layer depth and thickening of the nutricline. As the nitracline was generally deeper and less thick than the phosphacline, it appears that non-diazotroph primary production was limited by N rather than P throughout the study area, which is in line with enrichment experiments during these cruises. Estimated by the δ15N in zooplankton, atmospheric sources of new N contributed less than 20% close to the African coast and in the Guinea Dome area and up to 60% at the offshore stations, depending on the depth of the nitracline. δ15N of the three different copepod species investigated strongly correlated with each other, in spite of their distinct feeding ecology, which resulted in different spatial patterns of nutritional condition as indicated by RNA/DNA. Highlights: ► We studied δ15N and RNA/DNA of eastern tropical Atlantic zooplankton along with nutrients and Chl-α. ► Zooplankton −δ15N was decreasing from east (West African Shelf) to west (oligotrophic open ocean). ► Total integrated Chl-a depended mainly on nutricline depth and was N-limited throughout the area. ► Zooplankton δ15N and nutricline depth were used to estimate atmospheric N sources to the food web. ► Estimated atmospheric nitrogen sources were less than 20% at the shelf slope and up to 60% offshore.

Description: Gelatinous zooplankton hold key functions in the ocean and have been shown to significantly influence the transport of organic carbon to the deep sea. We discovered a gelatinous, flux‐feeding polychaete of the genus Poeobius in very high abundances in a mesoscale eddy in the tropical Atlantic Ocean, where it co‐occurred with extremely low particle concentrations. Subsequent analysis of an extensive in situ imaging dataset revealed that Poeobius sp. occurred sporadically between 5°S–20°N and 16°W–46°W in the upper 1000 m. Abundances were significantly elevated and the depth distribution compressed in anticyclonic modewater eddies (ACMEs). In two ACMEs, high Poeobius sp. abundances were associated with strongly reduced particle concentrations and fluxes in the layers directly below the polychaete. We discuss possible reasons for the elevated abundances of Poeobius sp. in ACMEs and provide estimations showing that a single zooplankton species can completely intercept the downward particle flux by feeding with their mucous nets, thereby substantially altering the biogeochemical setting within the eddy.

Description: Baltic sprat (Sprattus sprattus balticus, Schneider 1908) is a key species in the Baltic Sea ecosystem, where it is the most abundant planktivorous fish. In the present study, we applied the daily egg production method (DEPM) for the years 1999–2008 to estimate the size of the stock component reproducing in the Bornholm Basin, a major spawning ground for sprat and cod. This is the first study assessing this stock with a fishery independent egg production method for a consecutive time series of ten years. DEPM stock size estimates were compared with those obtained by a multi species virtual population analysis for the same stock component and results from an acoustic survey. In general, the results obtained by the DEPM were in the same order of magnitude compared to the other methods and most similar to the acoustic estimate. However, in some years differences between methods were substantial. With respect to previous egg production methods to assess Baltic sprat stock components our approach takes several aspects into account which were ignored before, e.g. effect of ambient temperature range on sprat egg stage duration and mortality and interannual variability of adult stock parameters. Since the accurate determination of the daily spawning fraction bears major uncertainties, different scenarios were tested for this parameter. Least deviation compared to the other assessment methods was obtained when using a daily female spawning fraction of 24%, which corresponds well to values described in literature. The applicability of the DEPM to Baltic sprat was clearly demonstrated. Thus, it can serve as valuable tool for the estimation of Baltic sprat stock sizes independent of data obtained from commercial fisheries.

Description: 1. In a warming world, both the metabolic rates of ectotherm predators and the phenology of their prey organisms is subject to change. Knowledge on how intrinsic and extrinsic factors govern predator-prey interactions is essential in order to understand how the environment regulates the vital rates of consumers. Controlled experiments, however, simultaneously testing behavioural and growth responses of the larvae of fish and other ectotherm organisms in different feeding regimes are scarce. 2. Prey size (PS) selection was determined for young Atlantic herring (Clupea harengus L.) larvae offered 100- to 850-μm copepods (Acartia tonsa) at five different concentrations. In separate, 4- (13°C) or 7-day (7°C) trials, the effect of prey size on larval foraging behaviour, specific growth rate (SGR) and biochemical condition (RNA:DNA, RD, a proxy for individual instantaneous growth) was tested. 3. Preferred (selected) PS was similar at all prey concentrations but increased from 3 to 5% predator length with increasing larval size. At various temperatures, dome-shaped relationships existed between PS and larval RD (and accordingly SGR). Compensatory changes in foraging behaviour (pause and feeding strike frequencies) existed but were not adequate to maintain positive SGR when available prey were substantially smaller than those preferred by larvae. 5. A physiology-based model predicted that larvae depended more heavily on optimal prey sizes at the colder versus warmer temperature to grow well and that the profitable prey niche breadth (the range in prey sizes in which growth was positive) increased at warmer temperatures. 6. Seemingly subtle match-mismatch dynamics between ectotherm predators and their preferred prey size can have large, temperature-dependent consequences for rates of growth and likely survival of the predator. To the best knowledge, this was the first study to directly quantify the “costs and trade-offs” of optimal foraging in marine fish larvae.