Nutrient dynamics in coastal and shelf oceans - sediments as a regulator of eutrophication feedbacks

Coastal and shelf systems are under increasing pressure from human activities. Many coastal systems currently suffer from excessive algae growth following increased nutrient input from land, a process called eutrophication. The algae eventually sink to the seafloor, where they are degraded, which consumes oxygen (O2). In severe cases, O2 can become depleted entirely, resulting in mass mortality of animals and substantial changes in the system’s chemical conditions. When the external nutrient input decreases, O2 conditions should improve. However, feedback mechanisms that strengthen eutrophication and O2-depletion can lead to non-linear recovery. Processes that occur in the seafloor account for a considerable part of this non-linearity, as the shallow water depth in coastal systems leads to a strong coupling between processes in the water column and the sediments. Although we know much about the processes that cause eutrophication, we lack detailed and mechanistic knowledge about many of the feedback mechanisms that keep systems in a eutrophic state. Methods to mitigate coastal eutrophication are being developed, but due to the non-linearity response of many systems, the results of restoration efforts are difficult to predict. A marine system that exhibits strong, non-linear eutrophication feedbacks is the Baltic Sea. Efforts to reduce the nutrient load to the Baltic Sea over the last decades have been successful, yet the system remains eutrophic and suffers from severe O2 depletion. The main basins of the Baltic Sea differ substantially in environmental conditions as well as input rates and sources of organic matter, nitrogen and phosphorus. This spatial variability makes the Baltic Sea ideal for assessing factors that affect organic matter and nutrient cycling in sediment, which can determine the recovery trajectory for eutrophic systems. In this thesis, I use the Baltic Sea as a study site to investigate the role of sedimentary feedback mechanisms that regulate coastal eutrophication. I show that the shuttling of sediment particles can redistribute organic matter from shallow to deep parts of basins. The reactivity of organic matter depends on the nature of the compound and its environment. Therefore, particle shuttling substantially affects the pattern of O2 consumption as well as degradation and burial of organic matter. I further show that natural oxygenation events can be insufficient to break the feedback between eutrophication and O2 depletion in long-term eutrophic systems where the sedimentary capacity to remove or retain nitrogen and phosphorus is decreased. Finally, I show that when placed in a suitable environment, mussel farms used for eutrophication mitigation only have a minor impact on the underlying sediment. However, in terms of eutrophication mitigation, other techniques may be more efficient than mussel farming. In conclusion, the results presented in this thesis inform us about sediment mechanisms that regulate eutrophication and can be used to find locally adapted solutions for systems around the world

Ökad bentisk produktion av kvävedioxid och ammonium efter naturlig syresättning av långtidsanoxiska sediment - Processer och pooler i den bentiska kvävecykeln efter naturlig syresättning av långtidsanoxiskt sediment

In this study, we describe the long-term impact of a natural oxygenation event on the fate of fixed nitrogen in the sediment. We investigated whether the newly oxygenated sediments were sites of efficient removal of fixed nitrogen through denitrification and anammox, which environmental factors affected the division between nitrate reduction processes, and whether nitrate from the water column or sedimentary nitrification was fuelling nitrate reduction processes in the sediment. We conducted three yearly samplings in 2016, 2017, and 2018, where we performed in situ measurements of nitrate reduction process rates and sediment–water fluxes of nutrients, oxygen and dissolved inorganic carbon . We additionally collected sediment samples and measured sediment microprofiles of oxygen, nitrous oxide and hydrogen sulphide to gain further insights into the spatial distribution of the processes in surface sediments. The data set contains 5 files with the following data: - Station information; locations, sediment surface and bottom water conditions - Sediment properties; carbon and nitrogen in the sediment solid phase, porosity, pore water concentrations of NOx and NH4. - Microsensor measurements of O2, N2O and H2S - Sediment-water fluxes of oxygen, dissolved inorganic carbon, NH4 and NOx - Nitrate reduction rates in the sediment Full descriptions of the data can be found in the corresponding readme files accessible from the Data and documentation tab.Studiens mål var att utvärdera hur ett syresatt vatteninflöde till de djupa delarna av Östersjön påverkade kvävecykeln i tidigare långtidsanoxiska sediment. Se utförlig beskrivning i den engelska versionen av katalogposten. Datasetet innehåller 5 filer med följande data: - Stationsinformation; koordinater, kemisk information om sedimentytan och bottenvattnet - Sedimentegenskaper; kol och kväve i sedimentets fasta fas, porositet, koncentrationer av NOx och NH4 i porvattnet - Mikrosensormätningar av O2, N2O och H2S - Sediment-vattenflöden av syre, löst oorganiskt kol, NH4 och NOx - Nitratreduktionshastigheter i sedimentet Utförlig beskrivning av data finns i de tillhörande readme-filerna under fliken Data och dokumentation

Sedimentära biogeokemiska data från en musselodling i As Vig, Danmark

The goal of the study was to evaluate how a newly re-established mussel farm in As Vig, Denmark, affected the sediment-water nutrient exchange. A mussel farm and a reference site were sampled on four occasions in two years, spanning the entire first year of production. Nitrate reduction rates and sediment-water fluxes of nutrients, methane, dissolved inorganic carbon and oxygen were measured in situ with benthic chamber landers. Sediment cores were collected for measurements of carbon, nitrogen and pigments and sedimentation rates were measured with sediment traps. Currents, illuminance at the seafloor and water column profiles of salinity, temperature and chlorophyll a were measured to describe the environmental characteristics of the stations. The dataset contains 8 files with the following data: - CTD profiles and chlorophyll a concentrations in the water column - Current speed and current directions - Illuminance at the sediment surface - Sedimentation rates (total, organic matter, inorganic matter) - Sediment-water fluxes of oxygen, nutrients, methane and dissolved inorganic carbon - Nitrate reduction rates in the sediment - Sedimentary pigment content (chlorophyll a, fucoxanthin, pheopigments) - Carbon and nitrogen in the sediment solid phase, porosity Full descriptions of the data can be found in the corresponding readme files accessible from the Description tab.Studiens mål var att utvärdera hur en nyligen återetablerad musselodling i As Vig, Danmark, påverkade utbytet av närsalter mellan sediment och vatten. Se utförlig beskrivning på engelska. Datasetet innehåller 8 filer med följande data: - CTD-profiler och klorofyll a-koncentrationer i vattenpelaren - Strömhastigheter och strömriktningar - Ljusmätningar vid havsbotten - Sedimentationshastigheter (total, organiskt material, oorganiskt material) - Sediment-vattenflöden av syre, närsalter, metan, och löst oorganiskt kol - Nitratreduktionshastigheter i sedimentet - Pigmentinnehåll i sedimentet (klorofyll a, fukoxantin, pheopigment) - Kol och kväve i sedimentets fasta fas, porositet Utförlig beskrivning av data finns i de tillhörande readme-filerna under fliken Beskrivning

Elevated sedimentary removal of Fe, Mn, and trace elements following a transient oxygenation event in the Eastern Gotland Basin, central Baltic Sea

info:eu-repo/semantics/publishe

The fate of fixed nitrogen in marine sediments with low organic loading : an in situ study

Over the last decades, the impact of human activities on the global nitrogen (N) cycle has drastically increased. Consequently, benthic N cycling has mainly been studied in anthropogenically impacted estuaries and coasts, while in oligotrophic systems its understanding is still scarce. Here we report on benthic solute fluxes and on rates of denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) studied by in situ incubations with benthic chamber landers during two cruises to the Gulf of Bothnia (GOB), a cold, oligotrophic basin located in the northern part of the Baltic Sea. Rates of N burial were also inferred to investigate the fate of fixed N in these sediments. Most of the total dissolved fixed nitrogen (TDN) diffusing to the water column was composed of organic N. Average rates of dinitrogen (N-2) production by denitrification and anammox (range: 53-360 mu mol Nm(-2) day(-1)) were comparable to those from Arctic and subarctic sediments worldwide (range: 34-344 mu mol Nm(-2) day(-1)). Anammox accounted for 18-26% of the total N2 production. Absence of free hydrogen sulfide and low concentrations of dissolved iron in sediment pore water suggested that denitrification and DNRA were driven by organic matter oxidation rather than chemolithotrophy. DNRA was as important as denitrification at a shallow, coastal station situated in the northern Bothnian Bay. At this pristine and fully oxygenated site, ammonium regeneration through DNRA contributed more than one-third to the TDN efflux and accounted, on average, for 45% of total nitrate reduction. At the offshore stations, the proportion of DNRA in relation to denitrification was lower (0-16% of total nitrate reduction). Median value and range of benthic DNRA rates from the GOB were comparable to those from the southern and central eutrophic Baltic Sea and other temperate estuaries and coasts in Europe. Therefore, our results contrast with the view that DNRA is negligible in cold and well-oxygenated sediments with low organic carbon loading. However, the mechanisms behind the variability in DNRA rates between our sites were not resolved. The GOB sediments were a major source (237 kt yr(-1), which corresponds to 184% of the external N load) of fixed N to the water column through recycling mechanisms. To our knowledge, our study is the first to document the simultaneous contribution of denitrification, DNRA, anammox, and TDN recycling combined with in situ measurements

Ocean alkalinity enhancement through enhanced silicate weathering in coastal areas: a long-term mesocosm study

Enhanced silicate weathering (ESW) in coastal environments is a promising method for ocean alkalinity enhancement. The idea behind ESW is to generate alkalinity by application of silicate minerals in coastal areas, where waves, currents and bioturbation can speed up the weathering rate. Due to its potentially large CO2 sequestration capacity and relatively high technological readiness, allowing rapid upscaling, coastal ESW currently receives substantial interest from researchers and policymakers. However, the vast majority of studies on ESW have been conducted in idealised laboratory conditions, while research on the method in natural environments is lacking. As a result, the CO2 sequestration efficiency and environmental risks when applying ESW in the field remain largely unknown. Here we present results from the first and longest-running mesocosm experiment investigating ESW and associated CO2 uptake in coastal marine sediments. Using tanks containing one square meter of natural seafloor each, we have studied biogeochemical cycling in sediment treated with the fast-weathering silicate mineral olivine. Lugworms (Arenicola marina) were added to some tanks to investigate the effect of bioturbation on the olivine dissolution rate, as well as the impact of olivine addition on biota. In the mesocosms, we quantified the sedimentary release of alkalinity and other weathering end-products (trace metals and dissolved silicate). Five years into the experiment, olivine dissolution is obvious from an elevated sedimentary alkalinity release and decreased average olivine grain size. The elevated alkalinity release has further led to higher CO2 sequestrations in tanks with olivine. Based on the results from this unique mesocosm setup, we will discuss the large-scale effect of ESW on biogeochemical cycling in coastal ecosystems

Elevated sedimentary removal of Fe, Mn, and trace elements following a transient oxygenation event in the Eastern Gotland Basin, central Baltic Sea

Iron, manganese, and trace elements play an important role in the marine carbon cycle as they are limiting nutrients for marine primary productivity. Water column concentrations of these bio-essential elements are controlled by the balance between input and removal, with burial in marine sediments being the main sink. The efficiency of this burial sink is dependent on the redox state of the water column, with sediments underlying a sulphidic (euxinic) water column being the most efficient sinks for Fe, but also Mn and trace elements (Co, Cd, Ni, Mo, As, W, V, and U). Transient changes in ocean redox state can hence affect trace element burial, and correspondingly, the ocean's trace element inventory, but the impact of transient oxygenation events on trace element cycling is currently not well understood. Here, we investigate the impact of a natural oxygenation event on trace element release and burial in sediments of the Eastern Gotland Basin (EGB), a sub-basin of the Baltic Sea. After being anoxic (<0.5 µM O2) for ∼10 years, the deep waters of the EGB experienced a natural oxygenation event (Major Baltic Inflow, MBI) in 2015. Following this oxygenation event, we deployed benthic chamber landers along a depth transect in the EGB in April 2016, 2017 and 2018. We complemented these in situ flux measurements with analyses of water column, solid phase and pore water chemistry. Overall, the event increased the benthic effluxes of dissolved trace elements, though particular responses were element-specific and were caused by different mechanisms. Enhanced fluxes of Cd and U were caused by oxidative remobilisation, while Ni showed little response to the inflow of oxygen. In contrast, enhanced release of Co, Mo, As, W, and V was caused by the enhanced transient input of Mn oxides into the sediment, whereas Fe oxides were of minor importance. Following the dissolution of the oxides in the sediment, Mn and W were nearly completely recycled back to the water column, while fractions of Fe, Co, Mo, As, and V were retained in the sediment. Our results suggest that transient oxygenation events in euxinic basins may decrease the water column inventory of certain trace elements (Fe, Co, Mo, As, and V), thus potentially affecting global marine primary productivity on longer timescales.Accepted Author ManuscriptBT/Environmental Biotechnolog