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Sinking particles were studied by analyzing samples collected in a sediment trap at 180 m depth in the Gotland Basin, eastern Baltic Sea between 1999 and 2020. The aim of this study was to determine the temporal variability of the particle flux and its components and how their changes are linked to phytoplankton blooms. The variables studied included total particle flux, particulate organic carbon and nitrogen, biogenic silica, C:N ratio and the isotopic composition of organic carbon and nitrogen. The total particle flux and its components reached maximum values in 2003, 2012 and 2015. Long-term means over the 22-year period of the total particle flux and its components particulate organic carbon and nitrogen, biogenic silica were estimated at around 152, 22, 3 and 8 mg m−2 d−1, respectively. The C:N ratio and the isotopic composition of organic carbon and nitrogen showed high variability around their long-term means of 9, -25‰ and 4‰, respectively. The annual variability of the components of the flux particulate organic carbon (3–65 mg m−2 d−1), particulate organic nitrogen (0.4–9 mg m−2 d−1) and biogenic silica (1–24 mg m−2 d−1) exhibited the same general pattern as the total particle flux (11–450 mg m−2 d−1) over the study period. On the seasonal scale, sinking material in summer contributed roughly one-third (31%) to the total particle flux, followed by winter (27%), spring (24%) and autumn (19%). The highest particle flux occurred mostly in April, July and November, during and after the appearance of phytoplankton blooms in the Gotland Basin. The phytoplankton community changed from silicon-rich species to nitrogen-fixing cyanobacteria, indicating a shift in nitrogen sources from nitrate-based to N2-based over the year. The spring bloom, dominated by diatoms, was characterized by a lighter carbon and heavier nitrogen isotopic composition, while the summer bloom, mainly of diazotrophic cyanobacteria, was characterized in contrary by heavier carbon and lighter nitrogen isotopes. Although no trend was found in the data, the variability observed in the sinking material was related to the changes over time in the phytoplankton community in the Gotland Basin. The findings of this study provide new and valuable information for our understanding of the temporal variability of sinking material linked to the development of phytoplankton blooms and nutrient sources in the Gotland Basin, and underscore the importance of continued monitoring to understand the potential impacts of environmental changes on this fragile ecosystem.

The Amazon River plume (ARP) has been shown to support high rates of nitrogen fixation and primary production. However, nitrogen fixation alone cannot account for total primary production determined in the region, hinting that other nitrogen uptake processes might play a role. For the first time, we measured nitrate uptake rates in the ARP during three cruises in May 2018, June 2019 and April/May 2021, along with primary production rates and an analysis of phytoplankton community composition via high performance liquid chromatography. Based on a classification according to the salt content the region was divided into estuarine (ES), mesohaline (MH) and oceanic (OC) stations. Primary production was light limited near the river mouth at ES stations and was maximal off the coasts of French Guiana and Suriname, where also nitrate uptake was highest with rates of 11.4 mmol m−2 d−1. The role of eddies pinching off a deflecting plume are discussed as possible reason for higher nutrient concentrations at the MH stations. Surprisingly, at most MH stations north of 5°N, nitrate uptake rates were low despite the presence of sufficient substrate concentration (up to 1.44 μM nitrate). Diatoms, dinoflagellates or Synechococcus sp. dominated phytoplankton communities. OC stations showed lowest productivity rates in accordance with oligotrophic conditions. However, rates seem to be sufficient to completely deplete the remaining riverine nitrate, preventing its export to the open ocean. Plain Language Summary: The Amazon River is thought to deliver important nutrients to the Western Tropical North Atlantic, which enable high phytoplankton growth and especially favor nitrogen fixers. Previous studies have focused on nitrogen fixation, but we propose that other nitrogen sources could also be important in the river plume to support the high phytoplankton growth there. In this study, we measured the uptake of nitrate by phytoplankton in the plume for the first time. We found that nitrate uptake is of variable importance for phytoplankton growth in the Amazon River plume; it seems to be more important close to the river mouth, where diatoms and dinoflagellates dominate, compared to the northern plume, where nitrogen fixers are dominant. Investigations like this will help us to better understand, which nitrogen sources are important for the growth of different phytoplankton species in the plume. With this information, we can improve predictions of competition of phytoplankton species for nitrogen and their general growth in the future. Some phytoplankton groups have a higher potential to contribute to oceanic CO2 storage than others, so understanding the future of these phytoplankton groups is critical to any assessment of the amount of CO2 that can be stored by the ocean. Key Points: Nitrate uptake rates were measured in the Amazon River plume for the first time to resolve its role for overall productivity in the plumeNitrate uptake displayed a pronounced gradient and was not only regulated by concentrationsNitrate from the Amazon River is completely consumed on the shelf and not exported to the open ocean [ABSTRACT FROM AUTHOR]

Coastal lagoons are important sites for nitrogen (N) removal via sediment burial and denitrification. Blooms of heterocystous cyanobacteria may diminish N retention as dinitrogen (N2) fixation offsets atmospheric losses via denitrification. We measured N2 fixation in the Curonian Lagoon, Europe's largest coastal lagoon, to better understand the factors controlling N2 fixation in the context of seasonal changes in phytoplankton community composition and external N inputs. Temporal patterns in N2 fixation were primarily determined by the abundance of heterocystous cyanobacteria, mainly Aphanizomenon flos-aquae, which became abundant after the decline in riverine nitrate inputs associated with snowmelt. Heterocystous cyanobacteria dominated the summer phytoplankton community resulting in strong correlations between chlorophyll a (Chl a) and N2 fixation. We used regression models relating N2 fixation to Chl a, along with remote-sensing-based estimates of Chl a to derive lagoon-scale estimates of N2 fixation. N2 fixation by pelagic cyanobacteria was found to be a significant component of the lagoon's N budget based on comparisons to previously derived fluxes associated with riverine inputs, sediment–water exchange, and losses via denitrification. To our knowledge, this is the first study to derive ecosystem-scale estimates of N2 fixation by combining remote sensing of Chl a with empirical models relating N2 fixation rates to Chl a.

Under increasing anthropogenic impacts, the hydrology, weathering, turbidity, and biogeochemistry of the Amazon and Pará river catchments are changing, potentially affecting the concentration and speciation of nitrogen (N) entering the estuary, as well as the subsequent primary productivity. Primary production and several N‐cycling rates have been studied in the northern river plume, and new studies focusing on specific processes in the river mouth emerged. Yet, a general overview is still lacking. For the first time, we provide an overview of primary production along with nitrification, ammonium (NH4+), nitrate (NO3−), and amino acid uptake rates in the Amazon and Pará river mouths which exhibit different degrees of catchment land use. Our results indicate that the Amazon supplies tenfold more NH4+ and NO3−, and its nitrification rates are four times higher than in the Pará River. The stable isotope composition of ambient water and NO3− confirmed that nitrification is a dominant pathway, with fluxes reaching 8.5 × 108 mol N d−1 in the Amazon. Nutrients are transported to regions with reduced turbidity and little nitrification, facilitating NH4+ (0.2–2.4 × 108 mol N d−1) and carbon uptake (4.6 and 14.0 × 108 mol C d−1 in the Pará and the Amazon, respectively) by phytoplankton. The lower N‐cycling rates measured in the Pará relative to the Amazon River mouth likely originate from differences in river discharge and watershed characteristics. Plain Language Summary: Riverine nitrogen can support intense production at the base of the food web in coastal oceans. Such productive ecosystems often show flourishing life and act as a strong atmospheric carbon sink. Since the extent of this nitrogen source is greatly influenced by the status of the river catchment area, it is crucial to have a good understanding of the amount and reactivity of riverine nitrogen at a given study site. Here, we used nitrogen isotope tracers to measure nitrogen assimilation and nitrification rates in the Amazon and Pará river mouths. We found that nitrification was happening at high rates in the Amazon River Mouth. In both estuaries, nitrate production is greater than consumption, allowing nitrate export beyond the turbid waters of the river mouths. Because the Amazon and Pará drainage basins are facing drastic changes, this nitrogen budget gives a baseline for nitrogen cycle pathways and export that will help to monitor the effect of these changes on estuarine nitrogen processes. Key Points: Nitrogen uptake and nitrification measurements reveal intense nitrification of autochthonous ammonium in the Amazon and Pará estuariesIn the turbid parts, nitrate produced via nitrification is not entirely consumed by primary production, leading to its exportRates of nitrogen cycle processes are 10 times higher in the Amazon River mouth, likely due to higher riverine nitrogen discharged [ABSTRACT FROM AUTHOR]

Abstract Increasing sea surface temperatures (SST) and blooms of lipid‐poor, filamentous cyanobacteria can change mesozooplankton metabolism and foraging strategies in marine systems. Lipid shortage and imbalanced diet may challenge the build‐up of energy pools of lipids and proteins, and access to essential fatty acids (FAs) and amino acids (AAs) by copepods. The impact of cyanobacterial blooms on individual energy pools was assessed for key species temperate Temora longicornis and boreal Pseudo‐/Paracalanus spp. that dominated field mesozooplankton communities isolated by seasonal stratification in the central Baltic Sea during the hot and the cold summer. We looked at (a) total lipid and protein levels, (b) FA trophic markers and AA composition, and (c) compound‐specific stable carbon isotopes (δ13C) in bulk mesozooplankton and in a subset of parameters in particulate organic matter. Despite lipid‐poor cyanobacterial blooms, the key species were largely able to cover both energy pools, yet a tendency of lipid reduction was observed in surface animals. Omni‐ and carnivory feeding modes, FA trophic makers, and δ13C patterns in essential compounds emphasized that cyanobacterial FAs and AAs have been incorporated into mesozooplankton mainly via feeding on mixo‐ and heterotrophic (dino‐) flagellates and detrital complexes during summer. Foraging for essential highly unsaturated FAs from (dino‐) flagellates may have caused night migration of Pseudo‐/Paracalanus spp. from the deep subhalocline waters into the upper waters. Only in the hot summer (SST>19.0°C) was T. longicornis submerged in the colder subthermocline water (~4°C). Thus, the continuous warming trend and simultaneous feeding can eventually lead to competition on the preferred diet by key copepod species below the thermocline in stratified systems. A comparison of δ13C patterns of essential AAs in surface mesozooplankton across sub‐basins of low and high cyanobacterial biomasses revealed the potential of δ13C‐AA isoscapes for studies of commercial fish feeding trails across the Baltic Sea food webs.

During three cruises to the Bay of Gdansk, Baltic Sea, the fauna, porewater and bottom water were sampled at stations parallel to the shore and along a transect offshore. Diffusive porewater fluxes were calculated and related to the total net fluxes (TNF) of nutrients. The TNF comprise all nutrients that reach the bottom water from the sediment including diffusive nutrient efflux, discharge from macrozoobenthos and microbial activity. They were determined during in situ incubations using a benthic chamber lander, which is rarely done in coastal research. The lander restricts the physical influence of currents and waves on the sediments and only allows nutrient fluxes due to bioturbation by natural communities. Strong benthic-pelagic coupling in the shallow coastal zone suggested a crucial filter function for the bioturbated coastal sediments, which are separated from muddy deep sediments with little or no fauna at a depth of 50 m; in between is a small intermediate zone. While diffusive fluxes were highest at intermediate and offshore stations, TNF were highest at sandy coastal stations, where reservoirs of dissolved nutrients were small and sediments almost devoid of organic material. The greatest impact of macrofauna on sedimentary fluxes was found at stations whose communities were dominated by deep-burrowing polychaetes. The largest TNF were measured directly at the mouth of the Vistula River, where riverine food and nutrients supplies were highest. Macrofauna communities and sediment variables can thus serve as descriptive indicator to estimate the extent of the coastal filter. Finally, based on the total areal size of the different sediment types, annual efflux for the complete coastal zone of the Gdansk Bay was estimated to be 6.9 kt N, 19 kt Si, and 0.9 kt P. Compared to the muddy offshore area, which is twice as large, these amounts were similar for P and threefold higher for N and Si.

Dinoflagellates readily use diverse inorganic and organic compounds as nitrogen sources, which is advantageous in eutrophied coastal areas exposed to high loads of anthropogenic nutrients, e.g. urea, one of the most abundant organic nitrogen substrates in seawater. Cell-to-cell variability in nutritional physiology can further enhance the diversity of metabolic strategies among dinoflagellates of the same species, but it has not been studied in free-living microalgae. We applied stable isotope tracers, isotope ratio mass spectrometry and nanoscale secondary ion mass spectrometry (NanoSIMS) to investigate the response of cultured nitrate-acclimated dinoflagellates Prorocentrum minimum to a sudden input of urea and the effect of urea on the concurrent nitrate uptake at the population and single-cell levels. We demonstrate that inputs of urea lead to suppression of nitrate uptake by P. minimum, and urea uptake exceeds the concurrent uptake of nitrate. Individual dinoflagellate cells within a population display significant heterogeneity in the rates of nutrient uptake and extent of the urea-mediated inhibition of the nitrate uptake, thus forming several groups characterized by different modes of nutrition. We conclude that urea originating from sporadic sources is rapidly utilized by dinoflagellates and can be used in biosynthesis or stored intracellularly depending on the nutrient status; therefore, sudden urea inputs can represent one of the factors triggering or supporting harmful algal blooms. Significant physiological heterogeneity revealed at the single-cell level is likely to play a role in alleviation of intra-population competition for resources and can affect the dynamics of phytoplankton populations and their maintenance in natural environments.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2006.12.009 Byline: Natalie Loick (a), Joachim Dippner (a), Hai Nhu Doan (b), Iris Liskow (a), Maren Voss (a) Keywords: Upwelling; Nitrogen isotopes; Carbon isotopes; Trophic relationships; Zooplankton; Nitrogen fixation; Vietnam; South China Sea Abstract: Upwelling and nitrogen (N) fixation provide new N for primary production off southern central Vietnam. Here we evaluate the roles of both N sources for zooplankton nutrition by comparing [delta].sup.15N and [delta].sup.13C values in nitrate, particulate organic matter (POM), and six net-plankton size fractions from monsoon and intermonsoon seasons. The [delta].sup.13C values in POM and the net-plankton size fractions differed by 2-4a[degrees] at any time. We assume that plankton from the POM filters was dominated by nano-and picoplankton as opposed to micro- and mesoplankton in the net-samples. The implications of this are discussed in terms of size differential pathways of C and N in the planktonic food web. We used [delta].sup.15N to estimate the differences in N nutrition between the actual upwelling region and the oligotrophic area further offshore. The [delta].sup.15N values of the net-plankton size fractions were depleted in [delta].sup.15N by ca. 2a[degrees] outside compared to inside the upwelling area during the monsoon season. We attribute these patterns to the additional utilization of N derived from N fixation. The concomitant findings of high N fixation rates reported earlier and low subthermocline nitrate (nitrate.sub.sub) values of 2.9-3.6a[degrees] support this conclusion. Net-plankton [delta].sup.15N values increased with size, pointing to the dominance of higher trophic levels in the larger size fractions. According to a two source mixing model N fixation may have provided up to 13% of the N demand in higher trophic levels. Author Affiliation: (a) Department of Marine Biology, Baltic Sea Research Institute, FR Germany, Seestr. 15, 18119 Rostock, FR Germany (b) Institute of Oceanography Nha Trang, Cau Da 01, Nha Trang, Khan Hoa, SR Vietnam Article History: Received 12 September 2005; Revised 13 December 2006; Accepted 16 December 2006