Reevaluating the Role of Organic Matter Sources for Coastal Eutrophication, Oligotrophication, and Ecosystem Health

Organic matter (OM) in aquatic systems is either produced internally (autochthonous OM) or delivered from the terrestrial environment (ter-OM). For eutrophication (or the reverse – oligotrophication), the amount of autochthonous OM plays a key role for coastal ecosystem health. However, the influence of ter-OM on eutrophication or oligotrophication processes of coastal ecosystems is largely unclear. Therefore, ter-OM, or ter-OM proxies are currently not included in most policies or monitoring programs on eutrophication. Nevertheless, ter-OM is increasingly recognized as a strong driver of aquatic productivity: By influencing underwater light conditions and nutrient- and carbon availability, increased ter-OM input may shift systems from autotrophic toward heterotrophic production, but also alter the interactions between benthic, and pelagic habitats. Thus, by changing baseline conditions in coastal zones, ongoing, and predicted changes in inputs of ter-OM due to climate change (e.g., in precipitation) and anthropogenic activities (e.g., reduced sulfate deposition, damming, and coastal erosion) may strongly modify eutrophication symptoms within affected ecosystems, but also hinder recovery from eutrophication following a reduction in nutrient loadings (i.e., oligotrophication). In this review, we aim to shed light upon the role of ter-OM for coastal eutrophication and oligotrophication processes and ecosystem health. Specifically, we (1) discuss the theoretical interactions between ter-OM and eutrophication and oligotrophication processes in coastal waters, (2) present global case studies where altered ter-OM supply to coastal ecosystems has shifted baseline conditions, with implications for eutrophication and oligotrophication processes, and (3) provide an outlook and recommendations for the future management of coastal zones given changes in ter-OM input. We conclude that it is essential to include and target all OM sources (i.e., also ter-OM) in monitoring programs to better understand the consequences of both eutrophication and oligotrophication processes on coastal ecosystems. Our review strongly urges to include ter-OM, or ter-OM proxies in eutrophication monitoring, and policies to safeguard coastal ecosystem health also under changing climatic conditions and globally increasing anthropogenic perturbations of coastal ecosystems

Seasonal variation in marine C:N:P stoichiometry: can the composition of seston explain stable Redfield ratios?

Seston is suspended particulate organic matter, comprising a mixture of autotrophic, heterotrophic and detrital material. Despite variable proportions of these components, marine seston often exhibits relatively small deviations from the Redfield ratio (C:N:P = 106:16:1). Two time-series from the Norwegian shelf in Skagerrak are used to identify drivers of the seasonal variation in seston elemental ratios. An ordination identified water mass characteristics and bloom dynamics as the most important drivers for determining C:N, while changes in nutrient concentrations and biomass were most important for the C:P and N:P relationships. There is no standardized method for determining the functional composition of seston and the fractions of POC, PON and PP associated with phytoplankton, therefore any such information has to be obtained by indirect means. In this study, a generalized linear model was used to differentiate between the live autotrophic and non-autotrophic sestonic fractions, and for both stations the non-autotrophic fractions dominated with respective annual means of 76 and 55%. This regression model approach builds on assumptions (e.g. constant POC:Chl-a ratio) and the robustness of the estimates were explored with a bootstrap analysis. In addition the autotrophic percentage calculated from the statistical model was compared with estimated phytoplankton carbon, and the two independent estimates of autotrophic percentage were comparable with similar seasonal cycles. The estimated C:nutrient ratios of live autotrophs were, in general, lower than Redfield, while the non-autotrophic C:nutrient ratios were higher than the live autotrophic ratios and above, or close to, the Redfield ratio. This is due to preferential remineralization of nutrients, and the P content mainly governed the difference between the sestonic fractions. Despite the seasonal variability in seston composition and the generally low contribution of autotrophic biomass, the variation observed in the total seston ratios was low compared to the variation found in dissolved and particulate pools. Sestonic C:N:P ratios close to the Redfield ratios should not be used as an indicator of phytoplankton physiological state, but could instead reflect varying contributions of sestonic fractions that sum up to an elemental ratio close to Redfield

Organic nitrogen steadily increasing in Norwegian rivers draining to the Skagerrak coast

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Variation in the seston C:N ratio of the Arctic Ocean and pan-Arctic shelves

Studying more than 3600 observations of particulate organic carbon (POC) and particulate organic nitrogen (PON), we evaluate the applicability of the classic Redfield C:N ratio (6.6) and the recently proposed Sterner ratio (8.3) for the Arctic Ocean and pan-Arctic shelves. The confidence intervals for C:N ranged from 6.43 to 8.82, while the average C:N ratio for all observations was 7.4. In general, neither the Redfield or Sterner ratios were applicable, with the Redfield ratio being too low and the Sterner ratio too high. On a regional basis, all northern high latitude regions had a C:N ratio significantly higher than the Redfield ratio, except the Arctic Ocean (6.6), Chukchi (6.4) and East Siberian (6.5) Seas. The latter two regions were influenced by nutrient-rich Pacific waters, and had a high fraction of autotrophic (i.e. algal-derived) material. The C:N ratios of the Laptev (7.9) and Kara (7.5) Seas were high, and had larger contributions of terrigenous material. The highest C:N ratios were in the North Water (8.7) and Northeast Water (8.0) polynyas, and these regions were more similar to the Sterner ratio. The C:N ratio varied between regions, and was significantly different between the Atlantic (6.7) and Arctic (7.9) influenced regions of the Barents Sea, while the Atlantic dominated regions (Norwegian, Greenland and Atlantic Barents Seas) were similar (6.7–7). All observations combined, and most individual regions, showed a pattern of decreasing C:N ratios with increasing seston concentrations. This meta-analysis has important implications for ecosystem modelling, as it demonstrated the striking temporal and spatial variability in C:N ratios and challenges the common assumption of a constant C:N ratio. The non-constant stoichiometry was believed to be caused by variable contributions of autotrophs, heterotrophs and detritus to seston, and a significant decrease in C:N ratios with increasing Chlorophyll a concentrations supports this view. This study adds support to the use of a power function model, where the exponent is system-specific, but we suggest a general Arctic relationship, where POC = 7.4 PON0.89

Fluxes of carbon and nutrients to the Iceland Sea surface layer and inferred primary productivity and stoichiometry

This study evaluates long-term mean fluxes of carbon and nutrients to the upper 100 m of the Iceland Sea. The study utilises hydro-chemical data from the Iceland Sea time series station (68.00° N, 12.67° W), for the years between 1993 and 2006. By comparing data of dissolved inorganic carbon (DIC) and nutrients in the surface layer (upper 100 m), and a sub-surface layer (100–200 m), we calculate monthly deficits in the surface, and use these to deduce the long-term mean surface layer fluxes that affect the deficits: vertical mixing, horizontal advection, air–sea exchange, and biological activity. The deficits show a clear seasonality with a minimum in winter, when the mixed layer is at the deepest, and a maximum in early autumn, when biological uptake has removed much of the nutrients. The annual vertical fluxes of DIC and nitrate amounts to 2.9 ± 0.5 and 0.45 ± 0.09 mol m−2 yr−1, respectively, and the annual air–sea uptake of atmospheric CO2 is 4.4 ± 1.1 mol C m−2 yr−1. The biologically driven changes in DIC during the year relates to net community production (NCP), and the net annual NCP corresponds to export production, and is here calculated as 7.3 ± 1.0 mol C m−2 yr−1. The typical, median C : N ratio during the period of net community uptake is 9.0, and clearly higher than the Redfield ratio, but is varying during the season.publishedVersio

Three decades of change in the Skagerrak coastal ecosystem, shaped by eutrophication and coastal darkening

Global coastal ecosystems are under accelerating pressure from human activities and climate change. In this study we explore a long-term time series (mostly 1990–2016) from major Norwegian rivers, together with coastal time series from the Norwegian Skagerrak coast. The aims are to: 1) analyse long-term trends in riverine loadings to Skagerrak, changes in coastal water quality and pelagic and benthic species composition, and 2) to describe the relationships between human drivers (eutrophication and coastal darkening) and community structure of benthic communities. Riverine discharge and transport increased by 23–85% over the time period, corresponding to a 40–78% increase in concentrations of suspended particulate material in coastal waters and reduced surface salinity, connected to the reported coastal darkening of coastal waters. There was a worsening in ecological status for hard-bottom benthic communities (0–30 m) and a reduction in the lower growth depth limit of several macroalgae species. A structural shift in the community composition from macroalgae towards increased abundance of animals was found to be related to coastal darkening and reduced eutrophication. The concentration of coastal inorganic nutrients (DIN, PO4) declined by 27–36%, in response to management efforts to reduce eutrophication in European regional seas. Results indicate that reduced eutrophication was related to increased ecological status of the deep soft-bottom community (350 m), with a reduction in opportunistic polychaetes and an increase in filter feeding bivalves. This work highlights how climate change and other human-induced changes in a boreal ecosystem intensifies land-ocean interactions, and calls for more adaptive monitoring, where traditional water quality programs and policies need to evolve iteratively as new information emerges and the major drivers of the systems change.publishedVersio

Utredning av behovet for å redusere tilførslene av nitrogen til Ytre Oslofjord

I store deler av Oslofjorden er det høye konsentrasjoner av nitrogen i vannmassene. I Frierfjorden, Tønsberg, Drammensfjorden, Mossesundet, Hvaler og Iddefjorden klassifiseres tilstanden til «moderat» eller dårlig for nitratnivået på sommeren, og det er ingen positiv trend i utviklingen. Det er en tydelig horisontal trend fra kildeområdet og ut mot nordlige deler av Skagerrak. I sentrale deler av fjorden, som i Breiangen som ser ut til å være på grensen mellom moderat og god økologisk tilstand, bør konsentrasjonen av nitrogen i vannmassene reduseres med minst 10 %, for å unngå at tilstanden havner permanent i klassen «moderat» eller dårligere.publishedVersio

Økokyst - DP Nordsjøen, Årsrapport 2022

Prosjektleder: Hilde Cecilie TrannumOvervåkingsprogrammet "Økosystemovervåking i Kystvann – Økokyst" har til hensikt å overvåke miljøtilstanden langs norskekysten i henhold til vannforskriften. Økokyst delprogram Nordsjøen dekker kyststrekningen fra Lista til Sognefjorden. 21 vannforekomster inngikk. Av disse fikk fem vannforekomster «moderat» tilstand, to «svært god» tilstand og de resterende fikk «god» tilstand. Det var hovedsakelig oksygen som trakk ned tilstanden, og i Jøsenfjorden også bløtbunnsfauna.MiljødirektoratetpublishedVersio

ØKOKYST – DP Norskehavet Sør, Årsrapport 2023

Prosjektleder Elianne EggeOvervåkingsprogrammet "Økosystemovervåking i Kystvann – Økokyst" har til hensikt å overvåke miljøtilstanden langs norskekysten i henhold til vannforskriften. Økokyst delprogram Norskehavet Sør dekker kyststrekningen fra Ulsteinvik i sør til Helgeland i nord. I 2023 ble det gjort undersøkelser av de biologiske kvalitetselementene hardbunn, ålegress, bløtbunn og planteplankton. Av de 16 vannforekomstene som kunne klassifiseres fikk fem “svært god” og 11 “god” samlet tilstand. De biologiske kvalitetselementene hadde alle “god” eller “svært god” tilstand. Fysisk-kjemiske parameter hadde også “god” eller “svært god” tilstand, med unntak av oksygen i bunnvannet ved Skinnbrokleia utenfor Ulsteinvik. The monitoring program "Ecosystem Monitoring in Coastal Water - Økokyst" monitors the environmental status along the Norwegian coast according to the Water Framework Directive. The subprogram Norskehavet Sør covers the coastal area along the Norwegian Sea, from Ulsteinvik in the South to Helgeland in the North. In 2023, the biological quality elements macroalgae, eel grass, soft bottom and phytoplankton were examined. Out of the 16 water bodies which could be classified, five obtained “very good” and 11 “good” status. The biological quality elements all had “good” or “very good” status. The physico-chemical parameters also classified as “good” or “very good”, except for the oxygen conditions in the deep water at Skinnbrokleia outside Ulsteinvik.MiljødirektoratetpublishedVersio

ØKOKYST – DP Norskehavet Sør, Årsrapport 2022

Prosjektleder: Elianne EggeOvervåkingsprogrammet "Økosystemovervåking i Kystvann – Økokyst" har til hensikt å overvåke miljøtilstanden langs norskekysten i henhold til vannforskriften. Økokyst delprogram Norskehavet Sør dekker kyststrekningen fra Ulsteinvik i sør til Helgeland i nord. Av de ni vannforekomstene som kunne klassifiseres i 2022 fikk én samlet «svært god» tilstand, sju «god» tilstand, mens én fikk «moderat» tilstand, der oksygenverdier i bunnvann trakk ned. De to ålegress-stasjonene var lokalisert i samme vannforekomst og fikk begge “god” tilstand. I tillegg til parameterne brukt til klassifisering ble klimaparameterne dyreplankton, lys og organisk materiale overvåket ved én stasjon i vannforekomst Steinsfjorden nær Ulsteinvik.MiljødirektoratetpublishedVersio