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

The effects of the 2004 Tsunami on mainland India and the Andaman and Nicobar Islands

Mortality from the tsunamis was high, with more than 7,000 deaths in the Nicobar group alone (the final number may never be known as many indigenous people on remote islands may have perished). On the mainland, there were a similar number of fatalities ; The greatest losses were in fishing communities although the waves destroyed roads, jetties, other basic infrastructure and entire villages; There was major damage to the coastal resources of southeast India, particularly to mangrove and coastal forests. On the Andaman and Nicobar Islands there was considerable damage to the coral reefs and beaches, as well as the forests; The earthquakes changed the bathymetry of the coral reefs and coasts of the Andaman and Nicobars: reefs in the South Andamans to the Nicobars subsided by 1 - 3 metres; many reefs in the northern Andamans were uplifted out of the water and died; and some beaches have almost disappeared, while new beaches have formed; There was major damage to large areas of coral reefs of the Andamans and Nicobars, particularly due to debris being washed off the land and smothering by sediments; Mainland coral reefs in the Gulf of Mannar and elsewhere suffered very minor, localised damage. Many mainland beaches were seriously eroded; and The affected reefs are expected to recover within 5-10 years, if there is effective resource management and enforcement of legislation controlling destructive fishing, coral mining, over-harvesting of reef resources, coastal development, sedimentation and pollution

Organic matter remineralization in marine sediments : A Pan-Arctic synthesis

Natural Environment Research Council (GrantNumber(s): NE/J023094/1; Grant recipient(s): Ursula Witte) ArcticNet (GrantNumber(s): Hotspot biodiversity project; Grant recipient(s): Philippe Archambault)Peer reviewedPublisher PD

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

Tracking seasonal changes in North Sea zooplankton trophic dynamics using stable isotopes

Trophodynamics of meso-zooplankton in the North Sea (NS) were assessed at a site in the southern NS, and at a shallow and a deep site in the central NS. Offshore and neritic species from different ecological niches, including Calanus spp., Temora spp. and Sagitta spp., were collected during seven cruises over 14 months from 2007 to 2008. Bulk stable isotope (SI) analysis, phospholipid-derived fatty acid (PLFA) compositions, and δ 13CPLFA data of meso-zooplankton and particulate organic matter (POM) were used to describe changes in zooplankton relative trophic positions (RTPs) and trophodynamics. The aim of the study was to test the hypothesis that the RTPs of zooplankton in the North Sea vary spatially and seasonally, in response to hydrographic variability, with the microbial food web playing an important role at times. Zooplankton RTPs tended to be higher during winter and lower during the phytoplankton bloom in spring. RTPs were highest for predators such as Sagitta sp. and Calanus helgolandicus and lowest for small copepods such as Pseudocalanus elongatus and zoea larvae (Brachyura). δ 15NPOM-based RTPs were only moderate surrogates for animals’ ecological niches, because of the plasticity in source materials from the herbivorous and the microbial loop food web. Common (16:0) and essential (eicosapentaenoic acid, EPA and docosahexaenoic acid, DHA) structural lipids showed relatively constant abundances. This could be explained by incorporation of PLFAs with δ 13C signatures which followed seasonal changes in bulk δ 13CPOM and PLFA δ 13CPOM signatures. This study highlighted the complementarity of three biogeochemical approaches for trophodynamic studies and substantiated conceptual views of size-based food web analysis, in which small individuals of large species may be functionally equivalent to large individuals of small species. Seasonal and spatial variability was also important in altering the relative importance of the herbivorous and microbial food webs

The importance of benthic-pelagic coupling for marine ecosystem functioning in a changing world

Benthic-pelagic coupling is manifested as the exchange of energy, mass, or nutrients between benthic and pelagic habitats. It plays a prominent role in aquatic ecosystems, and it is crucial to functions from nutrient cycling to energy transfer in food webs. Coastal and estuarine ecosystem structure and function are strongly affected by anthropogenic pressures; however, there are large gaps in our understanding of the responses of inorganic nutrient and organic matter fluxes between benthic habitats and the water column. We illustrate the varied nature of physical and biological benthic-pelagic coupling processes and their potential sensitivity to three anthropogenic pressures - climate change, nutrient loading, and fishing - using the Baltic Sea as a case study and summarize current knowledge on the exchange of inorganic nutrients and organic material between habitats. Traditionally measured benthic-pelagic coupling processes (e.g., nutrient exchange and sedimentation of organic material) are to some extent quantifiable, but the magnitude and variability of biological processes are rarely assessed, preventing quantitative comparisons. Changing oxygen conditions will continue to have widespread effects on the processes that govern inorganic and organic matter exchange among habitats while climate change and nutrient load reductions may have large effects on organic matter sedimentation. Many biological processes (predation, bioturbation) are expected to be sensitive to anthropogenic drivers, but the outcomes for ecosystem function are largely unknown. We emphasize how improved empirical and experimental understanding of benthic-pelagic coupling processes and their variability are necessary to inform models that can quantify the feedbacks among processes and ecosystem responses to a changing world.Peer reviewe

Net primary productivity estimates and environmental variables in the Arctic Ocean: An assessment of coupled physical-biogeochemical models

The relative skill of 21 regional and global biogeochemical models was assessed in terms of how well the models reproduced observed net primary productivity (NPP) and environmental variables such as nitrate concentration (NO3), mixed layer depth (MLD), euphotic layer depth (Zeu), and sea ice concentration, by comparing results against a newly updated, quality-controlled in situ NPP database for the Arctic Ocean (1959-2011). The models broadly captured the spatial features of integrated NPP (iNPP) on a pan-Arctic scale. Most models underestimated iNPP by varying degrees in spite of overestimating surface NO3, MLD, and Zeu throughout the regions. Among the models, iNPP exhibited little difference over sea ice condition (ice-free vs. ice-influenced) and bottom depth (shelf vs. deep ocean). The models performed relatively well for the most recent decade and towards the end of Arctic summer. In the Barents and Greenland Seas, regional model skill of surface NO3 was best associated with how well MLD was reproduced. . Regionally, iNPP was relatively well simulated in the Beaufort Sea and the central Arctic Basin, where in situ NPP is low and nutrients are mostly depleted. Models performed less well at simulating iNPP in the Greenland and Chukchi Seas, despite the higher model skill in MLD and sea ice concentration, respectively. iNPP model skill was constrained by different factors in different Arctic Ocean regions. Our study suggests that better parameterization of biological and ecological microbial rates (phytoplankton growth and zooplankton grazing) are needed for improved Arctic Ocean biogeochemical modeling