Modeling tropical Fresh Submarine Groundwater Discharge (FSGD) and its associated nitrogen fluxes at regional scale

Fresh submarine groundwater discharge (FSGD) is a poorly studied flux and water pathway that can discharge a fraction of terrestrial nitrogen (N) surplus into the ocean. It is assumed that FSGD plays a major role in driving water quality in the coastal ocean and marginal seas, especially in the tropics. Rapidly changing land use in subtropical and tropical catchments to farmland results with an increased fertilizer application in more N losses to aquatic ecosystems. Relatively high tropical monsoon rain amounts suggest a high potential for FSGD draining groundwater to the ocean. Furthermore, coastal urbanization is found to function as an additional source of coastal N especially in the tropics, were most coastal mega cities are found. While oceanic N inputs through rivers are well studied, because the integrated lumped catchment N loss is drained through one easily accessible point at the surface, diffusive groundwater discharge occurring at the interface between ocean and aquifer at long coastal stretches are essentially invisible and submarine, which makes them difficult to quantify. As a result, directly measuring FSGD requires great effort and is currently only feasible at local scale, which motivated me to develop the regional transient lumped water balance model CoCa-RFSGD to quantify daily FSGD at long stretches of coastlines at the whole island of Java, Indonesia. The model calculates water balances for and fluxes between top soil, sub soil and aquifer at areas, which are not part of a larger river catchment, discharging groundwater largely to the ocean. If top soil is saturated surface runoff occurs forming seasonal rivers, which drain additional precipitation into the ocean. To estimate N recharge into the aquifer the soil modules were extended by a mass balance model including N transformations hydrolysis, volatilization, nitrification, denitrification and sorption processes. This model was specifically adapted each for two research questions: (1) Is catchment N surplus derived from creek water quality measurement related to land use types in sub-tropical basins in NSW, Australia and (2) does fast-changing coastal land use patterns in tropical Kerala, India, from largely traditional home gardens to urbanization explain strongly raised nitrate and ammonium concentrations in coastal springs? Java generates a total FSGD of annualy 15.27 km3. Especially areas with high top soil waterholding capacity and precipitation surplus generate large FSGD locating Indonesia, Vietnam, Nigeria, Cameroon, Equatorial Guinea and Columbia as potential FSGD hotspots. The model can be applied anywhere in the world to locate regional FSGD hotspots without obtaining in situ data, because it can run solely with global data sets, which makes it especially feasible for the locations mentioned. Furthermore, CoCa-RFSGD extended by N mass balances revealed for sub-tropical basins in Australia that a horticulture proportion of 3% drove a 3.5-fold increase of N losses to water ways and a 6.7-fold increase of N losses to atmosphere compared to catchments without horticulture. The blueberry horticultural land use lost 92 kg-N/ha of which 85% evaded to the atmosphere and 15% discharged through surface waters. In Kerala, India, paddy fields showed the highest contribution of N to groundwater with annually 56 kg-N/ha (standard deviation (SD)=13 kg-N/ha), rubber plantations and home gardens contributing 7 kgN/ha (SD=2 kg-N/ha) and 14 kg-N/ha (SD=4 kg-N/ha), respectively, while urbanization resulted in an increased amount of N stored in pit latrines, depending on pH within the pit latrine. Analyzing N groundwater recharge and historical land use changes from 2002 to 2015 of that region suggests that while fertilizer application rates doubled, tourism increased 13-fold, coastal spring N discharge increased 10-fold. Large N inputs to groundwater must originate from pit latrines, explaining high ammonium concentrations just after the rainy seasons, which could not be reproduced modeling fertilizer N recharge. Overall, my lumped model provides a simple but effective tool to upscale FSGD point measurements to a full year and to upscale local aquatic water quality measurements to catchment scale. This allows me to assess the importance of a changing land use on aquatic nitrogen loads. Tropical catchments face large groundwater recharge, hence large FSGD, and at the same time, due to saturated soil and high soil temperatures, high atmospheric losses of N

Capacity for Cellular Osmoregulation Defines Critical Salinity of Marine Invertebrates at Low Salinity

Low-salinity stress can severely affect the fitness of marine organisms. As desalination has been predicted for many coastal areas with ongoing climate change, it is crucial to gain more insight in mechanisms that constrain salinity acclimation ability. Low-salinity induced depletion of the organic osmolyte pool has been suggested to set a critical boundary in osmoconforming marine invertebrates. Whether inorganic ions also play a persistent role during low-salinity acclimation processes is currently inconclusive. We investigated the salinity tolerance of six marine invertebrate species following a four-week acclimation period around their low-salinity tolerance threshold. To obtain complete osmolyte budgets, we quantified organic and inorganic osmolytes and determined fitness proxies. Our experiments corroborated the importance of the organic osmolyte pool during low-salinity acclimation. Methylamines constituted a large portion of the organic osmolyte pool in molluscs, whereas echinoderms exclusively utilized free amino acids. Inorganic osmolytes were involved in long-term cellular osmoregulation in most species, thus are not just modulated with acute salinity stress. The organic osmolyte pool was not depleted at low salinities, whilst fitness was severely impacted. Instead, organic and inorganic osmolytes often stabilized at low-salinity. These findings suggest that low-salinity acclimation capacity cannot be simply predicted from organic osmolyte pool size. Rather, multiple parameters (i.e. osmolyte pools, net growth, water content and survival) are necessary to establish critical salinity ranges. However, a quantitative knowledge of cellular osmolyte systems is key to understand the evolution of euryhalinity and to characterize targets of selection during rapid adaptation to ongoing desalination

Inorganic ion and metabolite concentrations in tissue of marine invertebrates exposed to low salinity in the laboratory

Laboratory experiments were conducted in the climate chambers at GEOMAR Helmholtz Centre for Ocean Research Kiel in the time between March and November 2018. Experiments were designed to study the effect of long-term (1 month) exposure to low salinity in osmoconforming invertebrates. The study organisms (Asterias rubens, Mytilus edulis, Littorina littorea, Diadumene lineata, Strongylocentrotus droebachiensis and Psammechinus milliaris) were collected in Kiel Fjord, Eckernförder Bight or the Kattegat from spring to autumn 2018. Organisms were acclimated to climate chamber conditions for 1 week (under habitat salinity, 14˚C, constant aeration) and then subjected to salinity acclimation for 1-2 weeks until the final salinity treatment level was reached. Then different salinity treatments were maintained for 4 weeks. Water physiochemistry (temperature, salinity, pH, nitrite, nitrate, phosphate) was recorded frequently. After the experiment, samples were taken from tissues to measure total osmolality (mosmol/kg) with an osmomat, and inorganic ions (mmol/kg or µmol/g wet mass). Anions were measured with a novel protocol via ion chromatography, cations were measured via flame photometry. Organic osmolytes were measured via 1H-NMR

Osmolyte and fitness parameters of marine invertebrates exposed to low salinity in the laboratory

Low-salinity stress can severely affect the fitness of marine organisms. As desalination has been predicted for many coastal areas with ongoing climate change, it is crucial to gain more insight in mechanisms that constrain salinity acclimation ability. Low-salinity induced depletion of the organic osmolyte pool has been suggested to set a critical boundary in osmoconforming marine invertebrates. Whether inorganic ions also play a persistent role during low-salinity acclimation processes is currently inconclusive. We investigated the salinity tolerance of six marine invertebrate species following a four-week acclimation period around their low-salinity tolerance threshold. The species investigated were Asterias rubens, Mytilus edulis, Littorina littorea, Diadumene lineata, Strongylocentrotus droebachiensis and Psammechinus milliaris. To obtain complete osmolyte budgets of seawater, body fluids and tissues we quantified total osmolality (via osmometer), organic osmolytes (methylamine and free amino acids) via 1H-NMR spectroscopy and inorganic osmolytes (anions and cations) via flame photometry and a novel protocol using ion-chromatography. We further determined the fitness proxies survival, growth and tissue water content. Our data show the importance of the organic and inorganic osmolyte pool during low-salinity acclimation. It also shows the importance of specific compounds in some species. This data can be used in future osmolyte and salinity tolerance research. This type of data is essential to establish reliable physiological limits of species in order to estimate consequences of future salinity changes with ongoing climate change. It can be used to assess the salinity tolerance capacity and to obtain a better understanding of the basic mechanisms that are utilized in a wide range of species. The established cellular inorganic and organic osmolyte profiles can build a foundation for applied cellular physiological research, for example for designing suitable buffers for in vitro assays as these buffers need to incorporate complex organic and inorganic osmolyte changes. Knowledge about cellular and whole-organism biochemistry and physiology is absolutely crucial for characterizing the functions of genes that are under selection by climate change stressors. A quantitative knowledge of cellular osmolyte systems is key to understand the evolution of euryhalinity and to characterize targets of selection during rapid adaptation to ongoing desalination

Inorganic ion concentrations in bodyfluids of marine invertebrates exposed to low salinity in the laboratory

Laboratory experiments were conducted in the climate chambers at GEOMAR Helmholtz Centre for Ocean Research Kiel in the time between March and November 2018. Experiments were designed to study the effect of long-term (1 month) exposure to low salinity in osmoconforming invertebrates. The study organisms (Asterias rubens, Mytilus edulis, Littorina littorea, Diadumene lineata, Strongylocentrotus droebachiensis and Psammechinus milliaris) were collected in Kiel Fjord, Eckernförder Bight or the Kattegat from spring to autumn 2018. Organisms were acclimated to climate chamber conditions for 1 week (under habitat salinity, 14˚C, constant aeration) and then subjected to salinity acclimation for 1-2 weeks until the final salinity treatment level was reached. Then different salinity treatments were maintained for 4 weeks. Water physiochemistry (temperature, salinity, pH, nitrite, nitrate, phosphate) was recorded frequently. After the experiment, samples were taken from seawater and body fluids to measure total osmolality (mosmol/kg) with an osmomat and inorganic ions (mmol/l). No body fluid samples were taken from Diadumene lineata as organisms were too small and volumes too low. Anions were measured with a novel protocol via ion chromatography, cations were measured via flame photometry

Survival, growth and water content of marine invertebrates exposed to low salinity in the laboratory

Laboratory experiments were conducted in the climate chambers at GEOMAR Helmholtz Centre for Ocean Research Kiel in the time between March and November 2018. Experiments were designed to study the effect of long-term (1 month) exposure to low salinity in osmoconforming invertebrates. The study organisms (Asterias rubens, Mytilus edulis, Littorina littorea, Diadumene lineata, Strongylocentrotus droebachiensis and Psammechinus milliaris) were collected in Kiel Fjord, Eckernförder Bight or the Kattegat from spring to autumn 2018. Organisms were acclimated to climate chamber conditions for 1 week (under habitat salinity, 14˚C, constant aeration) and then subjected to salinity acclimation for 1-2 weeks until the final salinity treatment level was reached. Then different salinity treatments were maintained for 4 weeks. Water physiochemistry (temperature, salinity, pH, nitrite, nitrate, phosphate) was recorded frequently. Throughout the experiment survival was recorded. Before and after the experiment organism weight and, where feasible, size was measured. Weight data was also used to calculate tissue water content. This dataset comprises the physiological fitness parameters for each species at the respective salinity treatment. Given are data for survival, growth and tissue water content

Submarine Groundwater Discharge Releases CO2 to a Coral Reef

Submarine groundwater discharge (SGD) flows into coral reefs. In volcanically active areas; the incoming groundwater is typically CO2-rich which can alter the carbon balance and views on how coral reefs function at prevailing high CO2. We quantified dynamic hydrothermal SGD and CO2 fluxes to a Philippine coral reef over a spring-neap tidal cycle. SGD rates; with mean of 35 cm d−1 and 5−95% range of 0−147.8 cm d−1 . The groundwater-CO2 fluxes (266 mmol m d−1; range: 0−1111 mmol m2 d−1) were up to ∼300-fold larger than evasion of CO2 to the atmosphere. The reef seawater pCO2 (493 μatm; range: 421−680 μatm) remained above atmospheric values and spanned the upper end of the range of atmospheric levels (400−500 μatm) expected for the next century. Because of the hydrothermal SGD; the reef has prevailing above-atmospheric CO2 and is a source to the atmosphere and nearby waters

Submarine Groundwater Discharge Impacts on Coastal Nutrient Biogeochemistry

Submarine groundwater discharge (SGD) links terrestrial and marine systems, but has often been overlooked in coastal nutrient budgets because it is difficult to quantify. In this Review, we examine SGD nutrient fluxes in over 200 locations globally, explain their impact on biogeochemistry and discuss broader management implications. SGD nutrient fluxes exceed river inputs in ~60% of study sites, with median total SGD fluxes of 6.0 mmol m−2 per day for dissolved inorganic nitrogen, 0.1 mmol m−2 per day for dissolved inorganic phosphorus and 6.5 mmol m−2 per day for dissolved silicate. SGD nitrogen input (mostly in the form of ammonium and dissolved organic nitrogen) often mitigates nitrogen limitation in coastal waters, since SGD tends to have high nitrogen concentrations relative to phosphorus (76% of studies showed N:P values above the Redfield ratio). It is notable that most investigations do not distinguish saline and fresh SGD, although they have different properties. Saline SGD is a ubiquitous, diffuse pathway releasing mostly recycled nutrients to global coastal waters, whereas fresh SGD is occasionally a local, point source of new nutrients. SGD-derived nutrient fluxes must be considered in water quality management plans, as these inputs can promote eutrophication if not properly managed