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The Elbe Estuary is strongly impacted by anthropogenic activities such as dredging and eutrophication. Together, these cause oxygen minimum zones (OMZ) regularly during summer in the Hamburg Port area, within the tidal freshwater region of the estuary. Over the last years, this OMZ has expanded spatially and temporally. We present an analysis of an extraordinary oxygen minimum event in June 2022, when an all-time lowest oxygen concentration was observed upstream of the Hamburg Port.We combine data from six transect cruises (early May -late June, 2022) and monitoring stations (2016-2022), to show the decrease of oxygen, and the increasing number of oxygen minimum events. In June 2022, the OMZ moved upstream due to the collapse of a phytoplankton bloom upstream of the tidal weir. This was accompanied by particularly warm temperatures and low river discharge, providing a glimpse into the potential future changes of central European estuaries under climate change.

In 2018, Europe experienced an unprecedented heatwave and drought, especially in central and northern Europe, which caused decreased terrestrial production and ecosystem health. The effects on the marine environment are yet unclear. Here we investigate the biogeochemical response to the event in the German Bight of the North Sea. Using time series data from FerryBoxes, research cruises, monitoring programs and remote sensing we compare conditions in 2018 to climatological values. We hypothesize that (1) the heatwave caused unusual warming of surface waters, (2) the drought reduced river discharge and nutrient loads to the coast, and (3) the combined effects altered coastal biogeochemistry and productivity. During 2018, both water discharge and nutrient loads from rivers discharging into the German Bight were extremely low from March onward. Throughout the study domain, water temperature was unusually low in March 2018, but extremely high in May 2018, representing the fastest spring warming on record. This extreme warming period saw concurrent unusually high peaks in chlorophyll a, dissolved oxygen and pH values, consistent with the development of a strong spring bloom. It appears that productivity was unusually strong in near-shore regions, while offshore it was unusually low in 2018. The drought related low discharge limited nutrient supply from the rivers, but likely enhanced near-shore water residence time, where a burst of effective nutrient utilization during the spring depleted nutrients available for transport off shore. There, the heatwave related rapid surface water warming resulted in the establishment of a stable thermal water column stratification, hindering vertical nutrient supply to the surface layer during the summer.

The partial pressure of carbon dioxide (pCO2) in surface waters is influenced by physical, chemical and biological variables. The interplay of these environmental variables is shown along a section in the central North Sea between Immingham (UK) and Halden (Norway) for the year 2013. Both, underway measurements by ships of opportunity and the results of a high resolution 3D – biogeochemical model show very high pCO2 values near the UK coast and reduced values near the Norwegian coast. This pattern is due to high UK river loads at the western part and the outflow of Baltic water along the Norwegian coast at the eastern part of the section. Seasonal variations within the central North Sea off the coastal waters exhibit both the influence of temperature and biological activities. During the phytoplankton growth seasons, the pCO2 is considerably lowered by the biological uptake of dissolved inorganic carbon and concurrently increased by higher sea surface temperatures.The model is able to reproduce observed winter pCO2 values using a variety of different bulk air-sea flux formulas. However, during spring and summer, the use of these formulas results in overestimated simulated pCO2 values. Only the application of a formula which takes the presence of surfactants into consideration improved the simulated pCO2 values. The latter formula exhibits lower piston velocities for the exchange of CO2 between atmosphere and ocean than the common formulas.

Estuaries are an important contributor to the global carbon budget, facilitating carbon removal, transfer and transformation between land and coastal ocean. Estuaries are also susceptible to global climate change and anthropogenic perturbations. We find that a long-term significant increase in dissolved inorganic carbon (DIC) of 6–21 µmol kg-1 yr-1 (1997–2020) in a temperate estuary in Germany (Elbe Estuary), was driven by an increase in upper estuary particulate organic carbon (POC) content of 8–14 µmol kg-1 yr-1. The temporal POC increase was due to an overall improvement in water quality observed in the form of high rates of primary production and a significant drop in biological oxygen demand. The magnitude of mid-estuary DIC gain was equivalent to the increased POC production in the upper estuary, suggesting that POC is efficiently remineralized and retained as DIC in the mid-estuary, with the estuary acting as an efficient natural filter for POC. In the context of the significant DIC increase, the impact of a recent extensive drought period (2014–2020) significantly lowered the annual mean river discharge (468 ± 234 m3 s-1) compared to the long-term mean (690 ± 441 m3 s-1, 1960–2020). During the drought period, the late spring internal DIC load in the estuary doubled. This suggests that the drought induced a longer dry season, starting in May (earlier than normal), increased the residence time in the estuary and allowed for a longer remineralization period for POC. Annually, 77–94 % of the total DIC export was laterally transported to coastal water, reaching 89 ± 4.8 Gmol C yr-1, and thus only a maximum of 23 %, at 10 Gmol C yr-1, was released via carbon dioxide (CO2) evasion, between 1997 and 2020. Export of DIC to coastal waters decreased significantly during the drought (2014–2020: 38 ± 5.4 Gmol C yr-1), on average by 24 % compared to the non-drought period. In addition, we have identified that seasonal changes in DIC processing in an estuary require consideration in order to understand both the long-term and future changes in air-water CO2 flux, DIC export to coastal waters, as well as the impacts of prolonged droughts on the land-ocean carbonate system.

Estuaries are nutrient filters and change riverine nutrient loads before they reach coastal oceans. They have been extensively changed by anthropogenic activities like draining, deepening, and dredging to meet economic and social demand, causing significant regime changes like tidal amplifications and in some cases to hyper-turbid conditions. Furthermore, increased nutrient loads, especially nitrogen, mainly by agriculture cause coastal eutrophication. Estuaries can either act as a sink or as a source of nitrate, depending on environmental and geomorphological conditions. These factors vary along an estuary, and change nitrogen turnover in the system. Here, we investigate the factors controlling nitrogen turnover in the hyper-turbid Ems estuary (Northern Germany) that has been strongly impacted by human activities. During two research cruises in August 2014 and June 2020, we measured water column properties, dissolved inorganic nitrogen, dual stable isotopes of nitrate and dissolved nitrous oxide concentration along the estuary. Overall, the Ems estuary acts as a nitrate sink in both years. However, three distinct biogeochemical zones exist along the estuary. A strong fractionation (~ 26 ‰) of nitrate stable isotopes points towards nitrate removal via water column denitrification in the hyper-turbid Tidal River, driven by anoxic conditions in deeper water layers. In the Middle Reaches of the estuary nitrification gains in importance turning this section into a net nitrate source. The Outer Reaches are dominated by mixing with nitrate uptake in 2020. We find that the overarching control on biogeochemical nitrogen cycling, zonation and nitrous oxide production in the Ems estuary is exerted by suspended particulate matter concentrations and the linked oxygen deficits.

Nitrous oxide (N2O) is a greenhouse gas, with a global warming potential 298 times that of carbon dioxide. Estuaries can be sources of N2O, but their emission estimates have significant uncertainties due to limited data availability and high spatiotemporal variability. We investigated the spatial and seasonal variability of dissolved N2O and N2O emissions along the Elbe estuary (Germany). During nine research cruises done between 2017 and 2022, we measured dissolved N2O concentrations, as well as dissolved nutrients and oxygen concentrations along the estuary and calculated N2O saturation, flux densities and emissions. We found intense N2O production along the Elbe estuary that compensated the effect of decreasing dissolved inorganic nitrogen (DIN) loads since the 1990s. Two hot-spots areas of N2O production have been identified in the estuary: the Port of Hamburg and the mesohaline estuary near the estuarine turbidity maximum (MTZ). N2O production was enhanced by warmer temperatures and fueled by riverine organic matter in the Hamburg Port or marine organic matter in the MTZ. Surprisingly, estuarine N2O emissions where equally high in winter and summer. In winter, high riverine N2O concentrations led to high N2O emissions from the estuary, whereas in summer, estuarine biological N2O production led to equally high N2O emissions. Overall, we find that the Elbe estuary is a year-round source of N2O with estimated annual emissions of 0.24 ± 0.06 Gg yr−1.

In fall 2020 and 2021, two field surveys examined the water column dynamics and surface mixing in a shallow lagoon, Szczecin (Stettin) Lagoon, located at the border between Germany and Poland. This was part of a larger experiment, looking into water column and air-sea interactions, and momentum fluxes, but this study is focused on how the presence of proposed Langmuir circulation affects the carbon and oxygen dynamics, and primary production in this shallow lagoon.Measurements were collected from a station in Szczecin Lagoon, located near the Polish border, with water depth of about 4 meters. Measurements at and around the station were made using mobile FerryBox systems, or Pocket FerryBoxes, which measured almost continuously water temperature, salinity, dissolved oxygen, chlorophyll fluorescence, pH, turbidity, colored dissolved organic matter (CDOM) and in 2021 partial pressure of CO2 (pCO2). In addition, water column measurements of currents (ADCP) and water level were available, as well as surface drifters, and drone aerial measurements.We found that during low wind conditions, the water column was well-mixed to a depth controlled by expected Langmuir cells, and bottom waters below this depth were quite different in most of the biogeochemical parameters measured. Therefore Langmuir circulation most likely controlled water column structure in large regions of the Szczecin Lagoon, consequently influencing the community, carbon and dissolved gas distributions in this shallow lagoon, and most likely the air-sea gas exchange rate. Only during short storm events, these conditions changed, and the water column structure and concentrations of biogeochemical parameters were altered.

Coastal regions are highly variable ecosystems and play a crucial role in the global carbon cycle. In the North Sea, the CARBOSTORE project aims to investigate some of the benthic and pelagic reservoirs of carbon. This study focuses on the seasonal and inter-annual changes in the Southern North Sea and the North Frisian Islands in the Wadden Sea. Large regional and seasonal variability has been documented in previous studies of total alkalinity and carbon in these regions, but the driving factors are still being investigated.Since the start of the CARBOSTORE project in spring 2021, two seasonal cruises were completed in July and October 2021. The dissolved inorganic carbon (DIC) and total alkalinity (TA) in the southern North Sea and intertidal regions around the North Frisian Islands were measured, along with several other biogeochemical parameters measured using a Ferry Box. These surveys allow insights into the regional distribution and the seasonal cycle of TA and DIC and will help elucidate the potential sources of carbon. In addition, a close collaboration to colleagues investigating the benthic processes in this project will allow for coupling the benthic and pelagic dynamics.Preliminary results show a gradient in TA and DIC from land to sea, as well as regional variability. In the intertidal zones, TA and DIC values are higher overall than in the southern North Sea. Higher TA and DIC values were measured in July compared to October. What is more, the intertidal regions near the Ems River show some of the highest TA and DIC values, suggesting a potential influence from riverine inputs.

Marine uptake of carbon dioxide limits the atmospheric concentration growth. Continental shelf seas are important areas for this uptake, but are also highly variable environments, with indications that their sink capacity is weakening. A way to reduce uncertainty of budgeting is to increase our observational capacity, such as through FerryBox installations on Ships-of-Opportunity. Here, we compare FerryBox observations in the North Sea for periods of interest in the autumn seasons of 2019 and 2020. We show that short-lived and small to medium-scale events can be identified when the sampling resolution is adequately high, and that these events cause changes in some essential environmental variables on the same magnitude as seasonal cycles. In particular, these events rapidly lowered seawater pCO2 by 8-10%. In September 2019, an advectively-driven event caused a previously carbon source area (flux of 1.3 ± 0.6 mmol m-2 day-1) to be in relative balance with the atmosphere (new flux of −0.04 ± 0.34 mmol m-2 day-1). In November 2020, a late autumn bloom caused another previously carbon source area (flux of 2.7 ± 2.1 mmol m-2 day-1) to potentially become a carbon sink (flux of −0.6 ± 1.4 mmol m-2 day-1 during the bloom). We demonstrate the importance of including such events 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.

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.

Estuaries are crucial in transforming matter fluxes from land to sea. To better understand and quantify these processes and respective fluxes, it is important to determine the input into an estuary accurately. To allow for such studies in the Elbe estuary in Germany, a state-of-the-art research platform is currently being set-up just upstream of the weir in Geesthacht at the entrance of the estuary. Here, we report on small-scale spatial dynamics of organic matter and associated processes from several cross and longitudinal profiles around the planned location and the implications for the set-up of the aforementioned research platform.Based on preliminary data obtained in August 2020 during a period of relatively low discharge, we present the following results: (1) In three cross profiles along a 10 km transect of the Elbe upstream of the weir, we observed considerable small-scale gradients regarding currents and various biogeochemical parameters. In comparison to the fairway, water from the riverbanks was depleted in suspended particulate matter, chlorophyll a, dissolved oxygen, and nitrate, and enhanced in ammonium, phosphate and silicate, as well as total alkalinity and dissolved inorganic carbon paralleled by decreasing pH. This suggests that in the summer, organic matter is deposited and remineralised at the riverbanks, resulting in the release of ammonium, phosphate and silicate, and in the removal of nitrate, presumably by denitrification. (2) Along the 10 km transect towards the weir, we observed that concentrations of suspended particulate matter, chlorophyll a, dissolved oxygen, nitrate and pH were decreasing. In contrast, we found that ammonium, phosphate and silicate, total alkalinity and dissolved inorganic carbon increased towards the weir. This suggests an increased sedimentation and subsequent remineralisation due to the reduced flow velocities in front of the weir. (3) An analysis of a 10-year time series from the weir supports this by showing higher ammonium concentrations when discharges were relatively low. The implications of these findings for the set-up of the research platform in this area, as well as for optimising estimates of budgets are discussed. The research platform will contribute to understand further such variations in biogeochemical parameters at the entrance of the Elbe estuary over time.The research platform is set-up in cooperation with the Helmholtz initiative MOSES (“Modular Observation Solutions for Earth Systems“) and will be incorporated in the Elbe-North Sea Supersite of DANUBIUS-RI (“International Centre for Advanced Studies on River-Sea Systems“). Funding is provided by European Regional Development Funds, the federal state of Schleswig-Holstein, the Helmholtz Association and the Helmholtz-Zentrum Geesthacht. The research platform, planned to be operational in autumn 2021, will also be open for users e.g. to develop and test new methods and technologies. Data will be made available through the “Helmholtz Coastal Data Centre” (HCDC).

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.

The partial pressure of carbon dioxide (pCO2) in surface seawater is an important biogeochemical variable because, together with the pCO2 in the atmosphere, it determines the direction of air–sea carbon dioxide exchange. Large-scale observations of pCO2 are facilitated by Ships-of-Opportunity (SOOP-CO2) equipped with underway measuring instruments. The need for expanding the observation capacity and the challenges involving the sustainability and maintenance of traditional equilibrator systems led the community toward developing simpler and more autonomous systems. Here we performed a comparison between a membrane-based sensor and a showerhead equilibration sensor installed on two SOOP-CO2 between 2013 and 2018. We identified time- and space-adequate crossovers in the Skagerrak Strait, where the two ship routes often crossed. We found a mean total difference of 1.5 ± 10.6 μatm and a root mean square error of 11 μatm. The pCO2 values recorded by the two instruments showed a strong linear correlation with a coefficient of 0.91 and a slope of 1.07 (± 0.14), despite the dynamic nature of the environment and the difficulty of comparing measurements from two different vessels. The membrane-based sensor was integrated with a FerryBox system on a ship with a high sampling frequency in the study area. We showed the strength of having a sensor-based network with a high spatial coverage that can be validated against conventional SOOP-CO2 methods. Proving the validity of membrane-based sensors in coastal and continental shelf seas and using the higher frequency measurements they provide can enable a thorough characterization of pCO2 variability in these dynamic environments. publishedVersion

Seasonal and annual nitrate and phosphate loads were determined from FerryBox measurements to investigate the high seasonal and inter-annual variability of carbon and nutrient exchange between the Elbe estuary and North Sea. At the inner continental shelf, high biological activity is driven by riverine nutrient inputs, which can contribute to the net carbon dioxide (CO2) uptake. It is possible that in tidal systems this newly formed phytoplankton is transported back into the estuary over the flood tide, and this organic matter can be remineralized in the intertidal region. At present, the influence of this tidally driven mechanism on the nutrient exports and primary production in the coastal zone is not fully characterized, hence carbon sources and sinks at the estuary-coastal boundary may not be well accounted for.The coupling between nutrient inputs from the Elbe estuary to adjacent coastal waters and the subsequent biological activity are now being investigated with a high-frequency dataset provided by a FerryBox situated at the mouth of the estuary. The FerryBox continuously measures physical and biogeochemical parameters every 10 to 60 minutes. Prelimary seasonal and nutrient (nitrate and phosphate) loads from the Elbe estuary to the coastal waters were calculated with FerryBox data between 2014 and 2017. The nutrient loads exhibited high seasonal and inter-annual variability. For example, in summer 2014 nitrate loads reached 100 x 107 mol yr-1 whereas, in summer 2017 nitrate loads were 50 x 107 mol yr-1, which cannot be explained by river discharge alone. Such changes in nutrient loads are likely to influence primary production rates in the adjacent coastal waters and impact CO2 uptake and therefore carbon cycling.Time-series analysis is employed to determine patterns in oxygen changes in relation to photosynthesis and respiration, along with nutrient fluctuations, between 2014 and 2017. Salinity is used to differentiate between the coastal and estuarine end members, with low and high salinity representing flood tide (estuarine waters) and ebb tide (coastal waters), respectively. Changes in dissolved oxygen concentrations are used to estimate primary production (P) and community respiration (R) rates in the water column. The P/R ratio provides the ability to classify the community into autotrophic and heterotrophic systems. Results of this analysis will show the role of varying nutrient loads in supporting primary production in the coastal waters, along with estimating net ecosystem metabolism, and therefore give us a better understanding of nutrient and carbon cycling.

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.

Shelf seas play an important role in the global carbon cycle, absorbing atmospheric carbon dioxide (CO2) and exporting carbon (C) to the open ocean and sediments. The magnitude of these processes is poorly constrained, because observations are typically interpolated over multiple years. Here, we used 298500 observations of CO2 fugacity (fCO2) from a single year (2015), to estimate the net influx of atmospheric CO2 as 26.2 ± 4.7 Tg C yr−1 over the open NW European shelf. CO2 influx from the atmosphere was dominated by influx during winter as a consequence of high winds, despite a smaller, thermally-driven, air-sea fCO2 gradient compared to the larger, biologically-driven summer gradient. In order to understand this climate regulation service, we constructed a carbon-budget supplemented by data from the literature, where the NW European shelf is treated as a box with carbon entering and leaving the box. This budget showed that net C-burial was a small sink of 1.3 ± 3.1 Tg C yr−1, while CO2 efflux from estuaries to the atmosphere, removed the majority of river C-inputs. In contrast, the input from the Baltic Sea likely contributes to net export via the continental shelf pump and advection (34.4 ± 6.0 Tg C yr−1).

Shelf seas play an important role in the global carbon cycle, absorbing atmospheric carbon dioxide (CO2) and exporting carbon (C) to the open ocean and sediments. The magnitude of these processes is poorly constrained, because observations are typically interpolated over multiple years. Here, we used 298500 observations of CO2 fugacity (fCO2) from a single year (2015), to estimate the net influx of atmospheric CO2 as 26.2 ± 4.7 Tg C yr−1 over the open NW European shelf. CO2 influx from the atmosphere was dominated by influx during winter as a consequence of high winds, despite a smaller, thermally-driven, air-sea fCO2 gradient compared to the larger, biologically-driven summer gradient. In order to understand this climate regulation service, we constructed a carbon-budget supplemented by data from the literature, where the NW European shelf is treated as a box with carbon entering and leaving the box. This budget showed that net C-burial was a small sink of 1.3 ± 3.1 Tg C yr−1, while CO2 efflux from estuaries to the atmosphere, removed the majority of river C-inputs. In contrast, the input from the Baltic Sea likely contributes to net export via the continental shelf pump and advection (34.4 ± 6.0 Tg C yr−1).

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.