The hydrography of the Arctic Seas is being altered by ongoing climate change, with knock‐on effects to nutrient dynamics and primary production. As the major pathway of exchange between the Arctic ...and the Atlantic, the Fram Strait hosts two distinct water masses in the upper water column, northward flowing warm and saline Atlantic Waters in the east, and southward flowing cold and fresh Polar Surface Water in the west. Here, we assess how physical processes control nutrient dynamics in the Fram Strait using nitrogen isotope data collected during 2016 and 2018. In Atlantic Waters, a weakly stratified water column and a shallow nitracline reduce nitrogen limitation. To the west, in Polar Surface Water, nitrogen limitation is greater because stronger stratification inhibits nutrient resupply from deeper water and lateral nitrate supply from central Arctic waters is low. A historical hindcast simulation of ocean biogeochemistry from 1970 to 2019 corroborates these findings and highlights a strong link between nitrate supply to Atlantic Waters and the depth of winter mixing, which shoaled during the simulation in response to a local reduction in sea‐ice formation. Overall, we find that while the eastern Fram Strait currently experiences seasonal nutrient replenishment and high primary production, the loss of winter sea ice and continued atmospheric warming has the potential to inhibit deep winter mixing and limit primary production in the future.
Plain Language Summary
The Fram Strait is the main gateway of the Arctic Ocean. In the east, warm, salty waters from the Atlantic flow north into the Arctic basin, and in the west, cold, fresh waters flow south from the central Arctic into the North Atlantic. We examined how changes to the availability of nutrients (which are essential for algae to grow) may limit algae growth in the Fram Strait, both as a result of changes to their source and also how easily the upper ocean mixes nutrients from depth. In the eastern Fram Strait, there is a high availability of nitrate, one of the main nutrients to support algae growth, and winter mixing sustains nutrient supply and biological production in recent decades. However, in the western Fram Strait, the outflowing surface waters do not easily mix with deeper waters and are depleted in nitrate, and nutrient supply from the central Arctic has been declining in recent decades. Our work suggests that although the eastern Fram Strait is sustaining higher levels of algae growth, which supports fisheries and higher trophic levels, warming over the coming decades could shoal winter mixed layers enough to decrease summertime nutrients and limit biological production.
Key Points
Nitrate isotope signatures in the western Arctic outflow of Polar Surface Water and eastern inflow of Atlantic Water are characterized
Western Fram Strait is strongly stratified and nitrate deplete compared to the east where winter mixing sustains nutrient supply
Future warming may shoal the winter mixed layer in the east, decreasing nitrate supply, and reducing primary production below current rates
Marine dissolved organic matter (DOM) was separated by reversed-phase (RP) liquid chromatography method and analyzed with fluorescence/absorption detection and Fourier transform ion cyclotron mass ...spectrometry (FT-ICR-MS). The three key characteristics of the RP method are: (a) The C18-RP column chosen provides enhanced separation when the aqueous phase is 100% buffer-free water, and it does not degrade over time; (b) the water eluent adjusted to pH 7 significantly improves the resolution of water soluble compounds; (c) the initial flow maintained at low levels improves the separation of polar compounds. In samples, containing “fresh” DOM, specific peaks were detected, which were absent in “old” DOM samples. The combination with size exclusion chromatography (SEC) also demonstrated the relation between polarity and molecular size of DOM. FT-ICR-MS was applied to evaluate the quality of separation on a molecular scale demonstrating that physico-chemical characteristics of DOM can be related to molecular formulas. Sample extracts were separated into 4 preparative fractions, and a large suite of the identified molecular formulas only occurred in specific fractions. This is an important basis for the application of further analytical techniques in order to perform a more target-oriented analysis aiming at the determination of source and process biomarkers for DOM.
Total suspended matter was collected along the Yangtze River (Changjiang) and in the East China Sea in April to May and in September 2003, respectively, to study origin and fate of particulate ...organic nitrogen. Concentrations of particulate organic carbon (POC), nitrogen (PN) and hydrolyzable particulate amino acids (PAA;
d- and
l-enantiomers) were higher in the Yangtze Estuary than in the river and decreased offshore towards the shelf edge. In the coastal area, higher values of PAA were observed in the surface layer than in the bottom water. Stable carbon isotope ratios (
δ
13C) of POC increased from −
24.4‰ in the river to values around −
21‰ on the East China Sea Shelf. Dominant amino acids were aspartic acid
+
aspartine (Asx), glutamic acid
+
glutamine (Glx), glycine, alanine and serine. The proportions of Asx, Glx and isoleucine were higher in the marine than in the riverine samples contrary to the distributions of glycine, alanine, threonine and arginine. The proportions of
d-amino acids were highest in the riverine suspended organic matter (6% of PAA) decreasing towards the shelf edge (1.5% of PAA).
d-arginine, not reported in natural aquatic samples so far, was the most abundant
d-amino acid in the river. The amino acid composition of the particulate organic matter (POM) in the Yangtze River indicates an advanced stage of degradation of POM. Highly degraded organic matter from soils is probably a main source of POM in the Yangtze River, but the relatively high
δ
13C values and low C/N ratios (7.7
±
1.6) also indicate contribution from anthropogenic sources. The degraded riverine material was a dominant organic matter source in the estuary, where aquatic primary production had only a small overall contribution. In the East China Sea, gradual settling of riverine organic matter and the addition of fresher phytoplankton impacted the amino acid composition and
δ
13C values, and on the outer shelf relatively fresh phytoplankton-derived organic matter dominated.
The effects of the juvenile hormone analog fenoxycarb (CAS No. 72490-01-8) were investigated in artificial indoor streams. The results from aufwuchs and two mayfly species (
Rhithrogena semicolorata ...and
Ephemerella ignita) are presented. Four concentrations ranging from 0.05 to 50
μg/L (with a spacing factor of 10) were tested. Fenoxycarb disappeared rapidly from the water phase (DT
50≈5 days in the highest concentration, less in the other concentrations). Physico-chemical parameters and aufwuchs were not affected by fenoxycarb. The mayfly
R. semicolorata, introduced at the start of the experiment, was affected by treatments of 5 and 50
μg/L. For the larvae in the streams a LC
50 of 3.3
μg/L and for the larvae in the enclosures a LC
50 of 2.5
μg/L were calculated. The second species (
E. ignita) was introduced 72 days after the application, at which time no fenoxycarb was detectable in the water of the streams (limit of detection of 0.5
ng/L). The emergence of
E. ignita was affected in the highest treatment (50
μg/L). Ninety percent of the emerged imagoes showed morphological abnormalities at the abdomen.
The effects of the world wide-distributed chemical bisphenol A (BPA) on the endocrine system of vertebrates have been demonstrated in several studies. Here, we report on the impact of BPA (0, 5, 50 ...and 500 μg/l nominally, deduced effective concentrations 0, 0.24, 2.4, and 24.1 μg/l, respectively, all at 15 °C) on the 70 kD stress protein family (hsp70), the 90 kD stress protein family (hsp90), and gonad histology of the crustacean Gammarus fossarum exposed in artificial indoor streams. The animals were exposed for a maximum of 103 days and samples were taken at the beginning and at days 34, 69 and 103 of the experiment. Exposure to BPA resulted in accelerated maturation of oocytes in females and in a decline in the number and size of early vitellogenic oocytes. The level of hsp90, which plays a pivotal role in vertebrate sex steroid signal transduction, was significantly reduced by BPA. In all five streams, measured parameters did not indicate any captivity stress for a period of 69 days. Beyond this time, the mortality rate and proteotoxic effects, the latter measured by hsp70 expression, were found to be elevated.
The disappearance (DT50) from water-phase and effects on aufwuchs of three nominal concentrations (5, 50 and 500 microg/l) of bisphenol-A (BPA) were investigated in artificial indoor streams over a ...period of 103 days. HPLC was used for analyses of BPA in water. Because of the disappearance of BPA from the water-phase, a pulse dosed exposure in weekly intervals was established. At the beginning of the exposure, a lag-phase of approximately 3-8 days was noted. Afterwards DT50 values (time, when 50% of initial BPA disappeared) were about 1 day with no clear trend to lower values during the experiment. The dynamics of aufwuchs was investigated on artificial substrates (unglazed ceramic tiles) in 14-day intervals and quantified by ash free dry weight (AFDW). As an ecotoxicological endpoint the area under the biomass/time curve (AUC) was calculated for: (a) the absolute AFDW-values and (b) values as percentage of start biomass (value of day-2 set as 100%). The reduction of AUC by 10% (EC10) and 50% (EC50) for absolute values was 11 microg/l (nominal: 38) and 46 microg/l (450), respectively; for values as percentage of start biomass: EC10 20 microg/l (239) and EC50 73 microg/l (806). These values are 20-70-fold lower compared to results from standard algae tests (EbC, 96 h).
Toxicity of Microcystis blooms to warm‐blooded animals generated by microcystins has bew reported world wide. The ecological relevance of microcystin production for cyanobacteria remains unknown. The ...microcystin concentration in Microcystis blooms occurring in the Bautzen reservoir Was investigated. The microcystin content of samples were determined by HPLC and ranged from undetectabel to 14.7 μg mg−1. Various chemical and physical parameters were monitored at the same time as Microcystis sampling, however, there was no correlation between these parameters and microcystin dynamics. The spatial distribution of microcystin in the Microcystis population was investigated once and showed no difference between samples taken at five stations. The microcystin concentration in ihc cell free water from the reservoir was below the detection threshold (< 1 μg L−1). Size dependent Fractions of the Microcystis population analyzed for microcystin concentration correlated with colony sim. In the small fraction (>30 <66 μm) microcystin was not detected. In the medium fraction (> 6 h < 100μm) lower microcystin concentrations were detected than in large fraction (>100μm) in which the highest microcystin concentrations were found.
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