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Marine uptake of carbon dioxide reduces the accumulation of carbon dioxide in the atmosphere. Continental shelf seas are essential for carbon uptake from the atmosphere, but are also highly variable environments, for which uncertainties of carbon budget estimates are large. Recent studies indicate that their carbon sink capacity is weakening. A way to reduce the uncertainty of carbon budgets is to increase our observational capacity, for example through FerryBox installations on Ships-of-Opportunity. Here, we compare FerryBox observations in the North Sea for the fall seasons of 2019 and 2020. We show that short-lived mesoscale events can be characterized when the sampling resolution is adequately high, and that these events cause changes in essential environmental variables on the same magnitude as seasonal cycles. Whether advective or biological in origin, these events rapidly lowered seawater pCO2 by 8–10% and influenced the carbon uptake capacity. We demonstrate the importance of resolving and integrating the variability of these smaller features in regional carbon budget assessments and advocate for the tuning of models in order to capture this small-scale variability.

The ability to make measurements of phosphate (PO43–) concentrations at temporal and spatial scales beyond those offered by shipboard observations offers new opportunities for investigations of the marine phosphorus cycle. We here report the first in situ PO43– dataset from an underwater glider (Kongsberg Seaglider) equipped with a PO43– Lab-on-Chip (LoC) analyser. Over 44 days, a 120 km transect was conducted in the northern North Sea during late summer (August and September). Surface depletion of PO43– (43– concentrations being associated with elevated salinities in northernmost regions, consistent with nutrient rich North Atlantic water intruding onto the shelf. Our study represents a significant step forward in autonomous underwater vehicle sensor capabilities and presents new capability to extend research into the marine phosphorous cycle and, when combined with other recent LoC developments, nutrient stoichiometry.

One key challenge of marine monitoring programs is to reasonably combine information from different in situ observations spread in space and time. In that context, we suggest the use of Lagrangian transport simulations extending both forward and backward in time to identify the movements of water bodies from the time they were observed to the time of their synopsis. We present examples of how synoptic maps of salinity generated by this method support the identification and tracing of river plumes in coastal regions. We also demonstrate how we can use synoptic maps to delineate different water masses in coastal margins. These examples involve quasi-continuous observations of salinity taken along ferry routes. A third application is the synchronization of measurements between fixed stations and nearby moving platforms. Both observational platforms often see the same water body, but at different times. We demonstrate how the measurements from a fixed platform can be synchronized to measurements from a moving platform by taking into account simulation-based time shifts.

Surface seawater carbon dioxide partial pressure (pCO2) in the North Sea, a large temperate shelf sea, was measured between 2014 and 2018 using FerryBox-integrated membrane sensors on ships of opportunity. The use of commercial vessels ensured a high spatio-temporal resolution, with data available year-round in areas belonging to all the stratification regime types found in the North Sea. Average annual cycles revealed a dominant biological control on pCO2 variability, with thermal effects modulating its amplitude. In the regions of freshwater influence, the biogeochemical characteristics of the riverine end-member also influenced the pCO2 measured near shore. Deseasonalized winter trends of seawater pCO2 were positive (ranging from 4.4 ± 2.0 µatm yr-1 to 8.4 ± 2.9 µatm yr-1 depending on the region), while the trends calculated including all deseasonalized monthly averages were even higher (ranging from 9.7 ± 2.8 µatm yr-1 to 12.2 ± 1.4 µatm yr-1). All these trends were stronger than the atmospheric pCO2 trend. Consequently, during our study period, the southern North Sea became a stronger source and the northern North Sea became a weaker sink for atmospheric carbon with implications for the Northwestern European Shelf carbon uptake capacity.

Surface seawater carbon dioxide partial pressure (pCO2) in the south-central North Sea was measured between 2014 and 2018 using FerryBox-integrated membrane sensors on ships-of-opportunity. Average annual pCO2 variability was biologically controlled, with thermal effects modulating its amplitude. Deseasonalized winter trends of seawater pCO2 were positive (4.4 ± 2.0–8.4 ± 2.9 µatm yr−1), biogeochemically driven, stronger than the atmospheric pCO2 trend, and more pronounced than previous analyses. The trends calculated including all deseasonalized monthly averages were even higher (9.7 ± 2.8–12.2 ± 1.4 µatm yr−1). During our investigation, the southern study area became a stronger source and the northern part became a weaker sink for atmospheric carbon. Overall, average sea-air CO2 flux in our study area, from the Skagerrak to the Southern Bight (53°N), changed from −0.75 ± 0.61 mmol m−2 day−1 in 2014 to +0.20 ± 0.96 mmol m−2 day−1 in 2018.

Within the context of predicted and observed increase in droughts and floods with climate change, large summer floods are likely to become more frequent. These extreme events can alter typical biogeochemical patterns in coastal systems. The extreme Elbe River flood in June, 2013 not only caused major damages in several European countries, but also generated large scale biogeochemical changes in the Elbe Estuary and the adjacent German Bight. Due to a number of well documented and unusual atmospheric conditions, the early summer of 2013 in Central and Eastern Europe was colder and wetter than usual, with saturated soils, and higher than average cumulative precipitation. Additional precipitation at the end of May, and beginning of June, 2013, caused widespread floods within the Danube and Elbe Rivers, as well as billions of euros in damages. The floods generated the largest summer discharge on record within the last 140 years. The high-frequency monitoring network in the German Bight available within the Coastal Observing System for Northern and Arctic Seas (COSYNA) captured the flood influence on the German Bight. Monitoring data from a FerryBox station in the Elbe Estuary (Cuxhaven) and from a FerryBox platform aboard the M/V Funny Girl Ferry (traveling between Büsum and Helgoland) documented the salinity changes on the German Bight, which persisted for about 2 months after the peak discharge. The flood generated a large influx of nutrients, dissolved and particulate organic carbon on the coast. These conditions subsequently led to the onset of a chlorophyll bloom within the German Bight, observed by dissolved oxygen supersaturation, and higher than usual pH in surface coastal waters. The prolonged stratification also led to widespread bottom water dissolved oxygen depletion, unusual for the south eastern German Bight in the summer.

The Murderkill Estuary (Delaware, USA) receives NO3− principally from its upland watershed and from a wastewater treatment facility. Due to disparate NO3− sources, one-dimensional salinity-based mixing models were inadequate for describing distributions of NO3−, δ15 NNO3−, and δ18 ONO3−. Distance-based mixing models with multiple, spatially-specified inputs were, therefore, applied to describe conservative mixing of these constituents and determine the extent to which biogeochemical reactions lead to non-conservative behavior of NO3−. These models closely matched Si observations in both winter and summer, consistent with high wastewater silicate loads and light limitation, and serve to validate modeling parameters for both seasons. A close fit of distance-based models to estuarine NO3− observations suggested a lack of uptake and fractionation in early winter. In the summer, modeled predictions of NO3−, δ15 NNO3−, and δ18 ONO3− diverged from estuarine observations, particularly in the oligohaline and polyhaline regions, consistent with in situ nitrogen cycling or additional sources and sinks. Effluent from an adjacent marsh in the lower estuary contained NO3− with low δ15 NNO3− and δ18ONO3, low DO and high NH4+ concentrations in late summer. This data and previous studies of adjacent Delaware Bay suggest that reactions in marshes and Bay waters likely drove the non-conservative behavior of NO3− and its stable isotopes. Potential uncertainty in watershed discharge, however, limited explicit quantitation of NO3− loss in the estuary. Nonetheless, distance-based models are useful tools for the study of NO3−, δ15 NNO3− and δ18 ONO3− distributions and cycling patterns in complex marsh-lined estuaries with multiple NO3− inputs.

A small, turbid and nutrient-rich tributary, the Murderkill Estuary, and a large estuarine ecosystem, the Delaware Bay, are tightly linked and form an efficient, tidally-coupled biogeochemical reactor during the summer. Nitrate loading from the Murderkill Estuary generates an instantaneous increase in biological oxygen production in the adjacent Delaware Bay. We are able to capture this primary production response with continuous hourly measurements of dissolved oxygen, chlorophyll, and nitrate. The nitrate influxes from the Murderkill support primary production rates in the Delaware Bay margins that are twice as high as the average production rates measured in the central Bay regions. This elevates chlorophyll in the Bay margins in the summer and fuels metabolism. Tidal transport of the newly produced autochthonous chlorophyll particles from the Bay into the Estuary could also provide a source of labile material to the marshes surrounding the Murderkill, thus perhaps fueling marsh respiration. As a consequence of the tidal coupling between Delaware Bay and the Murderkill Estuary, ecosystem productivity and metabolism in the Bay and Estuary are linked, generating an ecosystem feedback mechanism. Storms modulate this tidally-coupled biogeochemical reactor, by generating significant nitrate and salinity changes. Depending on their magnitude and duration, storms induce large phytoplankton blooms in the Delaware Bay. Such large phytoplankton blooms may occur more often with climate change, since century-long discharge records document an increase in storm frequency.

[1] We synthesized historical records of wind, temperature, nutrients, phytoplankton, and zooplankton to produce a climatological view of the biological response to upwelling in an estuarine/coastal system, the Delaware Bay, and surrounding coastal ocean. We find a persistent, rather than episodic, impact of upwelling near the mouth of the bay from May to September. During the upwelling season, the average sea surface temperature for the upwelling region is 2–3°C colder than the adjacent Delaware Bay and coastal ocean waters. This temperature difference is well correlated with the alongshore wind stress in the summer, making temperature a suitable upwelling index for the region. The upwelling apparently delivers subsurface nutrients to the lower bay and surrounding coastal ocean that help support phytoplankton primary production. Despite high primary production, the phytoplankton biomass (chlorophyll) does not increase accordingly. The lack of biomass increase is well correlated to seasonal increases in zooplankton biovolume. This result suggests that persistent upwelling near the Delaware Bay is characterized by an efficient transfer of carbon to primary consumers.