Temperature-related timing of the spring bloom and match between phytoplankton and zooplankton

Global warming is causing changes in the food web structure and seasonal plankton dynamics. The Baltic Sea is one of the fastest-warming sea areas and warming consequently affects the timing and magnitude of phytoplankton blooms. Based on available Chlorophyll a data from nine years between 1979 and 2018, from the entrance to the Gulf of Finland, we studied the timing of the phytoplankton spring bloom in relation to spring seawater temperature. We found the peak of the bloom to occur earlier in years with higher spring seawater temperature. In warmer years, there was also a shorter time lag between phytoplankton and zooplankton biomass peaks. In addition, it seems as the spring bloom total biomass has decreased with time, with more extensive summer blooms during later years, in line with the general trend observed in the Baltic Sea. The spring bloom has traditionally been considered the most important part of the season, but we argue that the whole growth season should be investigated since summer blooms appear to increase with warming.Peer reviewe

Contrasting patterns of carbon cycling and dissolved organic matter processing in two phytoplankton-bacteria communities

Microbial consumption of phytoplankton-derived organic carbon in the pelagic food web is an important component of the global C cycle. We studied C cycling in two phytoplankton-bacteria systems (non-axenic cultures of a dinoflagellate Apocalathium malmogiense and a cryptophyte Rhodomonas marina) in two complementary experiments. In the first experiment we grew phytoplankton and bacteria in nutrient-replete conditions and followed C processing at early exponential growth phase and twice later when the community had grown denser. Cell-specific primary production and total community respiration were up to 4 and 7 times higher, respectively, in the A. malmogiense treatments. Based on the optical signals, accumulating dissolved organic C (DOC) was degraded more in the R. marina treatments, and the rate of bacterial production to primary production was higher. Thus, the flow of C from phytoplankton to bacteria was relatively higher in R. marina treatments than in A. malmogiense treatments, which was further supported by faster C-14 transfer from phytoplankton to bacterial biomass. In the second experiment we investigated consumption of the phytoplankton-derived DOC by bacteria. DOC consumption and transformation, bacterial production, and bacterial respiration were all higher in R. marina treatments. In both experiments A. malmogiense supported a bacterial community predominated by bacteria specialized in the utilization of less labile DOC (class Bacteroidia), whereas R. marina supported a community predominated by copiotrophic Alphaand Gammaproteobacteria. Our findings suggest that large dinoflagellates cycle relatively more C between phytoplankton biomass and the inorganic C pool, whereas small cryptophytes direct relatively more C to the microbial loop.Peer reviewe

Viability of pico- and nanophytoplankton in the Baltic Sea during spring

Phytoplankton cell death is an important process in marine food webs, but the viability of natural phytoplankton communities remains unexplored in many ecosystems. In this study, we measured the viability of natural pico- and nanophytoplankton communities in the central and southern parts of the Baltic Sea (55°21′ N, 17°06′ E–60°18′ N, 19°14′ E) during spring (4th–15th April 2016) to assess differences among phytoplankton groups and the potential relationship between cell death and temperature, and inorganic nutrient availability. Cell viability was determined by SYTOX Green cell staining and flow cytometry at a total of 27 stations representing differing hydrographic regimes. Three general groups of phytoplankton (picocyanobacteria, picoeukaryotes, and nanophytoplankton) were identified by cytometry using pigment fluorescence and light scatter characteristics. The picocyanobacteria and picoeukaryotes had significantly higher cell viability than the nanophytoplankton population at all depths throughout the study area. Viability correlated positively with the photosynthetic efficiency (Fv/Fm, maximum quantum yield of photosystem II) as measured on the total phytoplankton community. However, an anticipated correlation with dissolved organic carbon was not observed. We found that the abiotic factors suggested to affect phytoplankton viability in other marine ecosystems were not as important in the Baltic Sea, and other biotic processes, e.g. processes related to species succession could have a more pronounced role.peerReviewe

Seasonal Variability in Benthic-Pelagic Coupling : Quantifying Organic Matter Inputs to the Seafloor and Benthic Macrofauna Using a Multi-Marker Approach

The exchange between the water column and the seafloor is a complex process, and is particularly intensive in the shallow waters of highly productive coastal areas, where the temporal variability in the inputs of pelagic organic matter will determine many aspects of the benthic community structure. However, few studies have focused on the seasonality of inputs of organic matter to the seafloor, and on the consequent dynamics and time scales of response of benthic consumers. We conducted a 1-year study where we repeatedly sampled multiple organic compounds traditionally used as markers to study the link between the pelagic organic matter inputs and the seafloor, and the potential response of benthic macrofauna to seasonal trends in phytoplankton biomass. We simultaneously quantified the particulate organic matter in the water column, the sinking material and different seafloor compartments, and analyzed it for pigments, organic carbon and nitrogen content, C/N ratio, and stable isotopes. Seafloor sediment was also analyzed for total lipids, and the dominant macrobenthic species for isotopic signatures. Results showed a major deposition of fresh organic matter during the spring bloom followed by more degraded organic matter inputs during the late summer bloom and even lower quality of the organic matter reaching the seafloor during winter. Strong positive relationships between water column and sedimentary pigments suggest that phytoplankton was the main source of carbon to the seafloor. The isotopic signatures of the dominant macrobenthic species suggest a fast response to the organic matter inputs from the water column. However, different species responded differently to the deposition of organic matter. Macoma balthica and Marenzelleria spp. fed on more reworked and degraded sedimentary material, while Monoporeia affinis showed a shift in the feeding habits according to its life stage, with adult individuals feeding on fresher material than juveniles did. Our study highlights the seasonal variability of the benthic-pelagic coupling and the utility of a multi-marker approach to follow the temporal inputs of organic matter from the water column to the seafloor and benthic organisms.Peer reviewe

Shifting diatom—dinoflagellate dominance during spring bloom in the Baltic Sea and its potential effects on biogeochemical cycling

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Community composition and extracellular enzyme activity of bacteria associated with suspended and sinking particles in contrasting arctic and sub-arctic marine environments

11 pages, 5 figures, 1 tableHeterotrophic bacteria contribute to controlling the oceans' biological carbon pump by solubilising organic particles and by using the dissolved organic matter for growth. To better understand how bacteria colonise and degrade vertically exported organic matter, spatial and temporal variations in the bacterial community composition (BCC) and cell-specific extracellular enzyme activity were investigated in water column samples and in sinking material collected by short-term deployed sediment traps in the Barents Sea (Arctic Ocean) and in a North-Norwegian fjord. Denaturing gradient gel electrophoresis showed that both the suspended and sinking BCC differed among locations (Arctic Water, Atlantic Water and fjord) and with time (summer vs. autumn in the fjord). Within stations, the BCC differed between shallow (20 to 50 m) and deep (120 or 200 m) suspended samples. The sediment traps revealed no depth-related difference but had high similarity to the shallow suspended samples, indicating that the exported community largely derived from suspended particles in the upper mixed layer and that little new colonisation or succession of the resident community took place during transfer to depth. Bacteria in the sediment traps tended to have higher enzyme activities compared to suspended bacteria, likely reflecting a response to aggregation and different composition (carbon, nitrogen and chlorophyll) of suspended and sinking organic matter. While the present study demonstrates that particle export from the surface layer contributes to microbial diversity in the deep sea, future studies using high taxonomical resolution are required to further evaluate the importance of this dispersal pathway. © Inter-Research 2013The present study was funded by Tromsø forskningsstiftelse through the project CONFLUX, by the Research Council of Norway through the project MERCLIM (Nr. 184860/S30), and by the European Commission’s 7th FP through the project BacPac (Marie Curie Actions, No. 297849)Peer Reviewe

Effects of Spring Bloom Phytoplankton Dynamics and Hydrography on the Composition of Settling Material in the Coastal Northern Baltic Sea

The phytoplankton species succession and sedimentation characteristics were studied on a sheltered and an open coastal station during a spring bloom in the northern Baltic Sea. Biomass (phytoplankton carbon and chlorophyll a), inorganic nutrients and particulate organic carbon and nitrogen (POC and PON) were determined in the suspended material. Sediment traps moored at different depths were used to determine the vertical flux of total particulate material (TPM), POC and PON, chlorophyll a, phaeopigments and phytoplankton carbon. The spring phytoplankton biomass was dominated by dinoflagellates that formed a dense bloom of short duration, whereas diatoms were present in the water column throughout the spring period in more moderate biomasses. The vertical flux of phytoplankton carbon was however dominated by diatoms at all times. The formation of resting stages increased the sedimentation of dinoflagellates, but compared to the suspended biomass of vegetative cells, their sinking rates were low. The ambient silicate concentration did probably not limit diatom growth; these were rather removed from the surface layer through sinking, a situation beneficial for dinoflagellates capable to exploit deep nutrient reserves through vertical migration. Due to rapid sinking soon after bloom formation and high specific loss rates, diatoms can be considered important contributors to the vertical flux of autochtonous material. Dinoflagellates mostly disintegrate in the water column and may settle as phytodetritus, except for the fraction of the populations that form rapidly sinking cysts. In addition to vertical export, advection of water from the stations seems to have been an important loss factor in the phytoplankton community. The two stations differed in that resuspension and input from littoral sources to the vertical flux were more important in the inner and shallower archipelago zone. This was also reflected in the C:N ratio of the settling material and in the bottom surface layer. In our study area, both the hydrographical regime and the species composition of the phytoplankton community were found to affect sedimentation characteristics and the composition of the settling material during the spring period.JRC.H.5-Rural, water and ecosystem resource

Spring-to-summer changes and regional variability of benthic processes in the western Canadian Arctic

Seasonal dynamics in the activity of Arctic shelf benthos have been the subject of few local studies, and the pronounced among-site variability characterizing their results makes it difficult to upscale and generalize their conclusions. In a regional study encompassing five sites at 100–595 m water depth in the southeastern Beau- fort Sea, we found that total pigment concentrations in surficial sediments, used as proxies of general food supply to the benthos, rose significantly after the transition from ice-covered conditions in spring (March–June 2008) to open-water conditions in summer (June–August 2008), whereas sediment Chl a concentrations, typical markers of fresh food input, did not. Macrobenthic biomass (including agglutinated foraminifera [500 lm) varied significantly among sites (1.2–6.4 g C m-2 in spring, 1.1–12.6 g C m-2 in summer), whereas a general spring-to-summer increase was not detected. Benthic carbon remineralisation also ranged significantly among sites (11.9–33.2 mg C m-2 day-1 in spring, 11.6–44.4 mg C m-2 day-1 in summer) and did in addition exhibit a general significant increase from spring-to-summer. Multiple regression analysis suggests that in both spring and summer, sediment Chl a concentration is the prime determinant of benthic carbon remineralisation, but other factors have a significant secondary influence, such as foraminiferan biomass (negative in both seasons), water depth (in spring) and infaunal biomass (in summer). Our findings indicate the importance of the combined and dynamic effects of food supply and benthic community patterns on the carbon remineralisation of the polar shelf benthos in seasonally ice-covered seas

New Production Regulates Export Stoichiometry in the Ocean

8 pages, 4 figures, 4 tablesThe proportion in which carbon and growth-limiting nutrients are exported from the oceans' productive surface layer to the deep sea is a crucial parameter in models of the biological carbon pump. Based on >400 vertical flux observations of particulate organic carbon (POC) and nitrogen (PON) from the European Arctic Ocean we show the common assumption of constant C:N stoichiometry not to be met. Exported POC:PON ratios exceeded the classical Redfield atomic ratio of 6.625 in the entire region, with the largest deviation in the deep Central Arctic Ocean. In this part the mean exported POC:PON ratio of 9.7 (a:a) implies c. 40% higher carbon export compared to Redfield-based estimates. When spatially integrated, the potential POC export in the European Arctic was 10-30% higher than suggested by calculations based on constant POC:PON ratios. We further demonstrate that the exported POC:PON ratio varies regionally in relation to nitrate-based new production over geographical scales that range from the Arctic to the subtropics, being highest in the least productive oligotrophic Central Arctic Ocean and subtropical gyres. Accounting for variations in export stoichiometry among systems of different productivity will improve the ability of models to resolve regional patterns in carbon export and, hence, the oceans' contribution to the global carbon cycle will be predicted more accurately. © 2013 Tamelander et al.This study was financed by the Research Council of Norway (http://www.forskningsradet.no) through the project 184860/S30 MERCLIM (T.T., D.S., P.W.) and by Tromsø forskningsstiftelse through the project CONFLUX (M.R., D.S.)Peer Reviewe