Eutrophication Of Coastal Waters Research Articles

Mesoscale mosaics of interannual variations in surface temperature, chlorophyll a concentration, and their relation in a coastal fishing ground

AbstractTo test the potential of high‐resolution satellite image analysis for assessing and predicting the mesoscale (<10 km in this study) effects of climate and environmental change on temperature and primary productivity in fishing grounds, we conducted satellite image analysis around an island in a coastal strait west of Japan from 2018 to 2023. We observed a distinct north–south gradient in sea surface temperature (SST) and chlorophyll a concentration (CHL) over approximately 20 km of the transect, which was likely affected by the current system. The model configuration suggests that the frequency of southward currents during winter–spring can control the magnitude of spring phytoplankton blooms. In the study region, an increase in SST at a rate of 0.06–0.13°C y−1 occurred during the study period, accompanied by a decrease in CHL. The north–south gradient in the rate of change suggests that the variation in the temperature and flow rate of the Kuroshio Current into the study area was due to these abrupt changes. The relationship between the annual mean SST and CHL was also spatially heterogeneous, showing a higher sensitivity of CHL to SST in the southwest of the island than in the north. In addition to the intrusion of warm and oligotrophic Kuroshio waters, the spread of less saline and more eutrophic coastal waters likely influenced this spatial heterogeneity. The satellite image analysis in the present study successfully revealed mesoscale mosaics of environmental conditions in coastal fishery grounds.

Optical water characterization and atmospheric correction assessment of estuarine and coastal waters around the AERONET-OC Bahia Blanca

The site AERONET-OC Bahía Blanca (BB-AERONET-OC) is located at the mouth of the Bahía Blanca Estuary, Argentina (Southwestern Atlantic Ocean), a coastal system defined by its high suspended loads and relatively low colored dissolved organic matter. The typically high turbidity of these waters makes the BB-AERONET-OC distinctive within the AERONET-OC network stations, providing exceptional opportunities not only for the validation of atmospheric correction algorithms but also for the development of regional algorithms for coastal complex waters. A SeaWiFS Photometer Revision for Incident Surface Measurements (SeaPRISM) instrument was deployed in January 2020 in the upper rail of a Mareograph Tower, a 15 m tall structure, located 10 miles away from the coast in optically deep waters. In this work we used the remote sensing reflectance (Rrs) derived from the BB-AERONET-OC measurements along with in situ hyperspectral radiometric data to classify optical water types (OWTs). We assigned each Rrs(λ) spectra to one of the five OWTs defined by Tran et al., and OWTs were further characterized with the concentrations of optically significant components (chlorophyll-a and suspended particulate matter) and inherent optical properties (absorptions of phytoplankton, non-algal particles, and dissolved organic matter), retrieved from water samples obtained simultaneously with radiometric spectra. Based on a match-up exercise with in situ data, different schemes of atmospheric correction methods were applied to Sentinel-3 Ocean and Land Colour Instrument (OLCI) images. The operational product OLCI Level 2 European Space Agency (ESA) standard (hereafter referred to as “Standard Neural Network (NN)”) proves to be the most suitable atmospheric correction algorithm, which was then used to describe spatial patterns and temporal variability of the different OWTs in the region. The BB-AERONET-OC site is located in a sharp transition between estuarine and coastal waters that present contrasting optical conditions: OWT 4 dominates over time (73.72% of the observations), followed by OWT 3 (24.74%) and OWT 5 (1.53%). OWTs 4 and 5 are associated with turbid waters of the Bahía Blanca Estuary, especially OWT 5, which typifies the very turbid waters from the inner estuary, with the particulate load dominated by mineral sediments and detritus. OWT 3, in turn, depicts the eutrophic coastal waters of the inner shelf. The variability of OWTs and the relative contribution of organic and inorganic compounds to the suspended material would be mostly related with the prevalence of northwest winds in the area, which would drive the export of estuarine sediments to the shelf.

Increasing carbon and nutrient burial rates in mangroves coincided with coastal aquaculture development and water eutrophication in NE Hainan, China

Mangroves sequester and store large area-specific quantities of blue carbon (C) and essential nutrients such as nitrogen (N) and phosphorous (P). Quantifying C and nutrient burial rates in mangroves across a centennial time span and relating these rates to mangrove habitat is fundamental for elucidating the role of mangroves in carbon and nutrient budgets and their responses to environmental changes. However, relevant data are very limited in China. In this study, we used the radionuclides (210Pb and 137Cs) to determine chronologies and C, N and P burial rates in two mangrove forests located at different geomorphologic settings in NE Hainan Island, China. We found that the temporal patterns of C, N and P burial rates since 1900 fitted a quadratic function with a notable increase after 1960s in both mangroves, which coincided with the rapid development of coastal aquaculture since 1960s in NE Hainan and the subsequent coastal water eutrophication in this area. Sediment accretion rate (SAR) and mass accumulation rate (MAR) stayed relatively steady in the open-coastal mangroves, while they increased exponentially in the estuarine mangroves since 1900. The estuarine mangroves had significantly higher SAR and C, N and P burial rates than the open-coastal mangroves. C, N and P burial rates averaged at 141.52 g m−2 a−1, 6.27 g m−2 a−1 and 1.14 g m−2 a−1, respectively in the estuarine core, and these rates averaged at 61.71 g m−2 a−1, 3.71 g m−2 a−1 and 0.43 g m−2 a−1, respectively in the open-coastal core. The results suggest that estuarine mangroves may be more capable of surviving accelerating sea level rise under climate change and play a greater role in C accumulation and nutrient filtering under anthropogenic nutrient enrichment than marine-dominated mangroves. Blue C burial may be enhanced by coastal water eutrophication, but such a relationship needs to be tested in further studies.

A Review of Seagrass Bed Pollution

Due to climate change and human activities, seagrass is in crisis as the coverage of seagrass declines at an accelerated rate globally. In this paper, the severe challenges of seagrass ecosystem were briefly reviewed, including adverse effects of natural factors and human activities on seagrass beds. The research status of pollutants and pollution in seagrass bed ecosystem was reviewed, the future research directions in related fields were proposed as well. The eutrophication in coastal waters and discharge of pollutants such as sulfide, heavy metals, organic matter and microplastics caused by human activities are important reasons for seagrass loss. In addition, environmental stressors lead to reduced immunity and decreased resistance of seagrass to various pathogens, leading to seagrass wasting diseases. Future studies concerning the influence of novel pollutants, i.e., plastic waste on non-native algae, microorganisms and seagrasses, as well as their interrelationships, will be of vital importance. In addition, researches on seagrass wasting diseases and their pathogens should be much accounted in China, to fill in gaps in related fields and improve the response ability to emergent seagrass diseases. In conclusion, this review was proposed to arouse the concern about the seagrass bed pollution, and provide possible enlightening information for the protection and restoration of this significant ecosystem.

Sampling frequency, load estimation and the disproportionate effect of storms on solute mass flux in rivers

Riverine discharge (Q) and dissolved concentrations (C) dictate solute mass export from watersheds. Commonly Q is tracked at a much higher frequency than C for most major solutes, leading to the necessity of load estimation algorithms which are often based on sparse data. The result is that the disproportionate effects of short-duration events (e.g., storms) on solute mass fluxes are poorly known. Here we use novel lab-in-the-field instrumentation to compare high temporal-resolution (∼30 min to 7 h) datasets of major ion chemistry collected over a year of continuous monitoring in three watersheds ranging over four orders of magnitude in drainage area. In these diverse settings, we quantify the errors associated with common load estimation algorithms and reduced sampling frequencies. When sample frequencies are coarsened, the mass flux of solutes which are diluted by storm events (i.e., Ca2+, Mg2+, Na+, Cl− and SO42−) are systematically overestimated, while nutrients which become mobilized by these events (K+ and NO3−) are underestimated. This is most pronounced in the largest river, and strongly tied to the increasing likelihood that storm events are missed as sampling frequencies decrease. Coarsening our high-resolution data to monthly sampling frequency yields an average overestimate of 8 % for Na+ and an average underestimate of 32.5 % for K+ across the three watersheds, illustrating clear implications for estuary and coastal water eutrophication, chemical weathering budgets, and agricultural land management practices. SynopsisA new ‘lab-in-the-field’ technology produces continuous high-frequency records of the full suite of major ions in rivers. These data highlight the disproportionate effect of large storms on catchment solute exports and the error associated with temporally coarse monitoring.

Regional Models for Sentinel-2/MSI Imagery of Chlorophyll a and TSS, Obtained for Oligotrophic Issyk-Kul Lake Using High-Resolution LIF LiDAR Data

The development of regional satellite bio-optical models for natural waters with high temporal and spatial variability, such as inland seas, reservoirs, and coastal ocean waters, requires the implementation of an intermediate measuring link in the chain, “water sampling—bio-optical models”, and this link must have certain intermediate characteristics. The most crucial of them are the high-precision measurements of the main water quality parameters, such as the concentration of chlorophyll a (Chl a), colored dissolved organic matter (CDOM), and total suspended sediments (TSS) in the upper water layer, together with a high operational rate and the ability to cover a large water area in a short time, which corresponds to a satellite overpass. A possible solution is to utilize laser-induced fluorescence (LIF) of water constituents measured by a marine LiDAR in situ with a high sampling rate from a high-speed vessel. This allows obtaining a large ground-truth dataset of the main water quality parameters simultaneously with the satellite overpass within the time interval determined by NASA protocols. This method was successfully applied to the oligotrophic Issyk-Kul Lake in Kyrgyzstan, where we obtained more than 4000 and 1000 matchups for the Chl a and TSS, respectively. New preliminary regional bio-optical models were developed on the basis of a one-day survey and tested for archive Sentinel-2A data for 2022. This approach can be applied for regular monitoring and further correction in accordance with seasonal variability. The obtained results, together with previously published similar studies for eutrophic coastal and productive inland waters, emphasize the applicability of the presented method for the development or adjustment of regional bio-optical models for water bodies of a wide trophic range.

Quantification and valuation of the potential of shellfish ecosystem services in mitigating coastal eutrophication

Eutrophication of coastal waters due to anthropogenic nutrient enrichment is a significant problem in European seas and worldwide. Bivalve shellfish remove nutrients from the water through filtration of suspended particles. This ‘bioextraction’ capacity is a key regulating ecosystem service that contributes to eutrophication control, but the extent and value of this ecosystem service has not been well established. This study aims to assess the potential of shellfish ecosystem services in mitigating coastal eutrophication in European coastal waters. The estimation of the amount of nutrients removed by different bivalve species under specific culture practices and locations is relevant due to the variety, geographical distribution, and scale at which shellfish species are farmed in Europe.The approach used here consisted of (a) estimation of nutrient loading to European regional seas, and source apportionment where possible; (b) evaluation of nutrient removal by key species of bivalve shellfish; (c) analysis of the role of shellfish in top-down control of eutrophication as a complement to the established bottom-up approach of emissions reduction.The nitrogen removal capacity of bivalves was assessed using two complementary approaches: (i) elemental analysis of N concentration in soft tissues and shell; and (ii) modelling the physiology of shellfish at the typical farm. In both cases, the results were upscaled to the shellfish aquaculture production in Western Europe.Our results show that European shellfish aquaculture can help reduce negative water quality impacts of excess nutrients (nitrogen and phosphorus) in coastal communities. Different shellfish species have different removal rates, which can also be influenced by environmental conditions at the distinct locations and culture practices. The species responsible for the largest N removal (3356–3491 tonnes y−1) was the Mediterranean mussel (accounting for 53–70% of the total) which is also the farmed species with highest production in Europe. Our study shows that the EU annual current production of over half a million metric tonnes of bivalves removes between 4.8 and 6.5 kilotonnes of N per year. The annual cost of removing the same amount of nutrients using other measures would be between 15.9 and 21.6 billion €. This ecosystem service has not been used in Europe as part of a nutrient management framework. The results of this study aim to provide the basis for strategic guidelines to include shellfish aquaculture in watershed-scale nutrient management policies in the EU, following the same principles as the on-going programmes in the United States.

Seasonal nutrients variation, eutrophication pattern, and Chlorophyll a response adjacent to Guangdong coastal water, China

Nutrients were the key biogenic elements for the primary production in coastal water, and the increase of nutrient concentration led to eutrophication and frequent occurrence of harmful algal blooms. However, the seasonal nutrients variation, eutrophication pattern, and Chlorophyll a (Chl-a) response adjacent to Guangdong coastal water were still scarcely. In this study, to clarity the seasonal nutrients variation, eutrophication pattern, and Chl-a, response adjacent to coastal water, the spatiotemporal dissolved inorganic nitrogen (DIN) and phosphorus (DIP) patterns and Chl-a were explored by field observation using 52 stations in the coastal waters of Guangdong Province during the dry (April and May), wet (July and August) and normal (October and November) seasons in 2020. The results showed that the variability of Chl-a, DIN and DIP were significantly different in seasons (P<0.01), and the mean concentrations of Chl-a, DIN and DIP were 11.97 ± 28.12 μg/L, 25.84 ± 35.72 μmol/L and 0.59 ± 0.71 μmol/L.Among them, the mean value of Chl-a increased significantly from 9.99 ± 9.84 μg/L in the dry season to 18.28 ± 38.07 μg/L in the wet season, and then decreased significantly to 7.65 ± 27.64 μg/L in the normal season.The mean DIN value decreased significantly from 30.68 ± 43.58 μmol/L in the dry season to 21.91 ± 35.45 μmol/L in the wet season, and then increased to 24.91 ± 26.12 μmol/L in the normal season. the mean DIP value decreased from 0.58 ± 0.73 μmol/L in the dry season to 0.48 ± 0.65 μmol/L in the wet season and then increased significantly to 0.70 ± 0.73μmol/L in the normal season. In addition, the DIN and DIP concentrations at most monitoring stations met the Grade II national seawater quality standards, and only a few monitoring stations fail to meet the Grade IV national seawater quality standard. The DIN/DIP ratios ranged from 2.05 to 259.47, with an average of 43.77 ± 41.01, far exceeding the Redfield ratio, indicating the presence of P limitation in the nearshore waters of Guangdong Province. Besides, the EI values in the coastal waters of Guangdong Province are higher at 0.00 and 82.51, with an average of 4.16 ± 10.90. DIN and DIP were significantly and positively correlated with COD in each season (P<0.05). Moreover, DIN/DIP showed significantly positive correlations with Chl-a in all seasons (P<0.01), indicating that high Chl-a concentrations could be sustained by the nutrients supply in marine ecosystems. Therefore, it is necessary to strengthen the integrated management of land and sea and effectively mitigate regional estuarine and coastal water eutrophication and harmful algal blooms.

Meadow trophic status regulates the nitrogen filter function of tropical seagrasses in seasonally eutrophic coastal waters

AbstractThe nutrient filter function is an important ecosystem service of seagrass meadows that mitigates the consequences of coastal eutrophication. In northeast Hainan in China, large seagrass areas were lost due to chronic eutrophication induced by untreated pond aquaculture effluents. However, in adjacent areas, seagrasses could survive due to seasonal exposure (i.e., not chronic) to eutrophication only. In a way, the conditions in these areas represent a transitional environment which allows investigating the effect of eutrophication on seagrass performance and their nitrogen uptake capacity. We tested how a 4‐week in situ nutrient enrichment affected inorganic nitrogen uptake rates of a multispecies seagrass meadow in eutrophic and non‐eutrophic seasons, in light and in darkness. All species maintained nitrogen uptake in the dark and preferred ammonium over nitrate. In the eutrophic season, the seagrass leaf biomass and growth were lower resulting in a lower nitrogen filter capacity. Among the species present, Cymodocea rotundata and Cymodocea serrulata covered 48% and 45%, respectively, of their daily nitrogen demand for leaf growth through leaf uptake from the water column, while it was only 30% for Thalassia hemprichii, the last remaining species in meadows degraded by eutrophication, as deduced from previous studies. It indicates that a multispecies seagrass meadow has a higher nitrogen filter capacity than a monospecific T. hemprichii meadow. By reducing seagrass diversity and, hence, the nitrogen filter function, eutrophication triggers a self‐reinforcing process. Once the nitrogen filtering capacity of a seagrass meadow is exhausted, further eutrophication and seagrass loss are expected.

Effects of phosphorus species and zinc stress on growth and physiology of the marine diatom Thalassiosira weissflogii

Human activities, including industrial and agricultural production, as well as domestic sewage discharge, have led to heavy metal pollution and eutrophication in coastal waters. This has caused a deficiency of dissolved inorganic phosphorus (DIP), but an excess dissolved organic phosphorus (DOP) and high concentrations of zinc. However, the impact of high zinc stress and different phosphorus species on primary producers remains unclear. This study examined the impact of different phosphorus species (DIP and DOP) and high zinc stress (1.74 mg L−1) on the growth and physiology of the marine diatom Thalassiosira weissflogii. The results showed that compared to the low zinc treatment (5 μg L−1), high zinc stress significantly decreased the net growth of T. weissflogii, but the decline was weaker in the DOP group than in the DIP group. Based on changes in photosynthetic parameters and nutrient concentrations, the study suggests that the growth inhibition of T. weissflogii under high zinc stress was likely due to an increase in cell death caused by zinc toxicity, rather than a decrease in cell growth caused by photosynthesis damage. Nonetheless, T. weissflogii was able to reduce zinc toxicity by antioxidant reactions through enhancing activities of superoxide dismutase and catalase and by cationic complexation through enhancing extracellular polymeric substances, particularly when DOP served as the phosphorus source. Furthermore, DOP had a unique detoxification mechanism by producing marine humic acid, which is conducive to complexing metal cations. These results provide valuable insights into the response of phytoplankton to environmental changes in coastal oceans, particularly the effects of high zinc stress and different phosphorus species on primary producers.

Coupling and decoupling of marine stramenopiles and cyanobacteria in eutrophic coastal waters of Korea

The spatiotemporal distribution of MASTs (MArine STramenopiles), mostly affiliated with heterotrophic protists, and their interactions with Synechococcales were investigated in an anthropogenically polluted bay of the East Sea using 18S rRNA and 16S rRNA gene sequences. The bay was characterized by strong stratification between the surface and bottom layers and cold and nutrient-rich water intrusion in summer, whereas the bay water was well mixed in winter. MAST-3, MAST-6, MAST-7, and MAST-9 were the major MAST clades, whereas the dominance of MAST-9 declined from >80 % in summer to <10 % in winter and the diversity of MAST communities increased in winter. Co-occurrence network analysis via the sparse partial least squares revealed that MAST-3 had a Synechococcales-specific interaction during the study periods but prey-specific interactions with other MAST clades were not detected. Temperature and salinity markedly influenced the relative abundance of major MAST clades. The relative abundance of MAST-3 increased at temperatures above 20 °C and salinities above 33 ‰, however, the abundance of MAST-9 decreased under the same conditions. Analysis of the metabolic functions of cyanobacteria using the FAPROTAX (Functional Annotation of Prokaryotic Taxa) indicated that the response of photosynthetic cyanobacteria to NH4+ and PO43− was significant in summer but these functions were not tightly coupled with the abundance of Synechococcales. Similarly, strong associations of MAST-3 with high temperature/salinity and Synechococcales were indicative of coupled cascading during bottom-up processes. However, other major MAST clades were likely decoupled with Synechococcales and environmental conditions in which cyanobacteria can thrive. Therefore, our results demonstrated that MAST communities can couple or decouple with environmental variables and potential prey depending on MAST clades. Collectively, our findings provide novel insights into the role of MAST communities in microbial food webs in eutrophic coastal waters.

Evaluating the Potential of Mangrove Phytoremediation for Mitigating Coastal Water Eutrophication in Macao SAR: A Field and Mesocosm Study

Eutrophication due to anthropogenic nutrient inputs is a serious issue in many coastal and marine environments. Mangrove plants form unique intertidal forests at the edge of the land and sea, forming multifunctional ecosystems that provide an array of services, such as the phytoremediation of pollutants. The purpose of this study was to evaluate the levels of nutrients (PO43−, NO2−, NO3−, NH4+) in the coastal waters around Macao SAR, in areas with and without mangroves, in order to assess their phytoremediation potential. The work was reinforced through a mesocosm experiment with various treatments with and without mangroves. The results of the field investigations indicated a high degree of eutrophication in the coastal waters of Macao, with average values of 0.30 mg/L, 0.09 mg/L, 23.8 mg/L, and 0.36 mg/L of phosphate, nitrite, nitrate, and ammonium, respectively. There were no apparent significant differences in the levels of nutrients in areas with and without mangroves, which was most likely caused by the conditions during sampling as well as the density of the mangrove forest. The mesocosm experiments, however, revealed a clearer effect of the presence of mangroves (and sediments) in the degradation of nutrients. Therefore, it is highly recommended to plant more mangroves to help mitigate coastal water eutrophication in the area as a phytoremediation approach.

Multiple Timescale Variations in Water Transparency in the Eastern China Seas over the Period 1997–2019

AbstractThe detection of water transparency (Zsd) changes over multiple timescales is an important task that has wide implications from water quality to climate change research. However, to date, the understanding of Zsd variations and their driving factors in the Eastern China Seas is fairly limited. Using the Data Interpolating Empirical Orthogonal Functions (DINEOF) method and a semi‐analytic algorithm, monthly cloud‐free Zsd products over the period 1997–2019 are retrieved from the European Space Agency Ocean Color Climate Change Initiative datasets. Significant Zsd variations on seasonal, interannual, and long‐term timescales, as well as their spatial discrepancies, are quantified in the Eastern China Seas. The first EOF mode of Zsd accounts for 57.46% of the total variance, which is dominated by a seasonal cycle in response to seasonal changes in oceanic and atmospheric factors (e.g., surface winds, sea surface temperature and ocean stratification). The second and third EOF modes are likely associated with seasonal phytoplankton blooms and ocean circulations, respectively. Factors that affect the interannual variation in Zsd, including sea surface temperature, wind speed, river discharge, and the Oceanic Niño Index, are quantitatively evaluated. The long‐term trends suggest different potential driving forces for the Zsd patterns at a regional scale, such as climate‐driven increased sea surface temperatures, weakened surface winds, reduced sediment discharge, and eutrophication of coastal waters. Overall, this study presents the first comprehensive investigation of Zsd variations in the Eastern China Seas at multiple timescales and an analysis of their underlying physical and environmental drivers.

Proteomic and toxicological analysis of the response of dinoflagellate Alexandrium catenella to changes in NaNO3 concentration

Dinoflagellates of the genus Alexandrium cause Harmful Algal Blooms (HABs) in coastal waters worldwide, damaging marine environments, aquaculture, and human health. They synthesize potent neurotoxic alkaloids known as PSTs (i.e., Paralytic Shellfish Toxins), the etiological agents of PSP (i.e., Paralytic Shellfish Poisoning). In recent decades, the eutrophication of coastal waters with inorganic nitrogen (e.g., nitrate, nitrite, and ammonia) has increased the frequency and scale of HABs. PSTs concentrations within Alexandrium cells can increase by up to 76% after a nitrogen enrichment event; however, the mechanisms that underlie their biosynthesis in dinoflagellates remains unclear. This study combines mass spectrometry, bioinformatics, and toxicology and investigates the expression profiles of PSTs in Alexandrium catenella grown in 0.4, 0.9 and 1.3 mM NaNO3. Pathway analysis of protein expression revealed that tRNA amino acylation, glycolysis, TCA cycle and pigment biosynthesis were upregulated in 0.4 mM and downregulated in 1.3 mM NaNO3 compared to those grown in 0.9 mM NaNO3. Conversely, ATP synthesis, photosynthesis and arginine biosynthesis were downregulated in 0.4 mM and upregulated in 1.3 mM NaNO3. Additionally, the expression of proteins involved in PST biosynthesis (sxtA, sxtG, sxtV, sxtW and sxtZ) and overall PST production like STX, NEO, C1, C2, GTX1-6 and dcGTX2 was higher at lower nitrate concentrations. Therefore, increased nitrogen concentrations increase protein synthesis, photosynthesis, and energy metabolism and decrease enzyme expression in PST biosynthesis and production. This research provides new clues about how the changes in the nitrate concentration can modulate different metabolic pathways and the expression of PST biosynthesis in toxigenic dinoflagellates.

Simulating Hypoxia in a New England Estuary: WASP8 Advanced Eutrophication Module (Narragansett Bay, RI, USA).

Anthropogenic sources of nutrients cause eutrophication in coastal waters. Narraganset Bay (USA), the largest estuary in New England, has large seasonal zones of hypoxia. In response, management strategies have been implemented to reduce nutrient loadings. In this study, a mechanistic, mass balance fate and transport modeling framework was developed and applied to Narragansett Bay to improve our understanding of the processes governing hypoxia. Discrete and continuous observations were used for model comparison and evaluation. Simulations captured the general trends and patterns in dissolved oxygen (DO) with depth and space. Simulations were unable to capture the wide diurnal range of observed continuous DO and phytoplankton concentrations, potentially suggesting the need for improved understanding of processes at this time scale. Mechanistic modelling scenarios were performed to investigate how different sources of nutrients affect DO. Results suggest tributary sources of nitrogen affected upper layers of DO, while sediment oxygen demand and nutrient fluxes affected deeper waters. This work highlights the importance of understanding and simulating the legacy effects of historic nutrient loading to estuary systems to understand the magnitude and timing of long-term recovery due to reductions in nutrient loadings.

Unraveling seasonal and interannual nutrient variability shows exceptionally high human impact in eutrophic coastal waters

AbstractNutrient import into eutrophic estuarine‐coastal zones is modulated by river discharge and anthropogenic activities (agriculture, industry, sewage, etc.). Comparing anthropogenic vs. hydrological influences on nutrient loading is important for ecosystem managers but has been a long‐standing challenge. As an attempt to fill this gap, a retrospective study was conducted using multiyear (2012–2022) data in one of the most eutrophic coastal seas in China, the Jiangsu coastal waters. Of the two main pollutants on the Jiangsu coast, dissolved inorganic nitrogen (DIN) is extremely overloaded relative to phosphate (PO4). Their spatial distribution shows clear terrestrial sources as concentrations decrease offshore. Although in each survey DIN and PO4 are enriched in low‐salinity waters, their spatially averaged concentrations are out of phase with river discharge on the seasonal and interannual scales. A box model was applied to decompose contributions of river discharge and nutrient import by anthropogenic activities. Results show that the human impacts on the interannual nutrient variability exceed those of hydrological factors on the Jiangsu coast. Anthropogenic nutrient loading decreases during 2012–2020, paralleled with the reduced fertilizer use and other nonpoint sources. Sporadic nutrient fluxes from point sources are likely responsible for the aseasonal nutrient‐rich patches. Our study demonstrates that water quality in the Jiangsu coastal waters is highly affected by anthropogenic activities. Findings in this study have broad implications for local monitoring strategies and restoration efforts and provide insight into assessing human stress from field data in global eutrophic ecosystems.

Unravelling Nutrients and Carbon Interactions in an Urban Coastal Water during Algal Bloom Period in Zhanjiang Bay, China

Nutrients and carbon play important roles in algal bloom and development. However, nutrients and carbon interactions in the period of the spring algal bloom are not well understood. The aim of this study is to explore the nutrients and carbon interactions in the period of the spring algal bloom covering an urban Jinsha Bay (JSB) coastal water in Zhanjiang Bay (South China Sea) using in situ multidiscipline observation. The results showed that the average concentration of total nitrogen (TN), total phosphorus (TP), and dissolved silicon (DSi) was 97.79 ± 26.31 μmol/L, 12.84 ± 4.48 μmol/L, and 16.29 ± 4.00 μmol/L in coastal water, respectively. Moreover, the average concentration of total dissolved carbon (TDC), dissolved inorganic carbon (DIC) and organic carbon (DOC) in JSB was 2187.43 ± 195.92 μmol/L, 1516.25 ± 133.24 μmol/L, and 671.13 ± 150.81 μmol/L, respectively. Furthermore, the main dominant species were Phaeocystis globosa and Nitzschia closterium during the spring algal bloom. Additionally, the correlation analysis showed salinity (S) was significantly negatively correlated with nutrients, indicating that nutrients derived from land-based sources sustained spring algal bloom development. However, as the major fraction of TDC, DIC was significantly positively correlated with S, which was mainly derived from marine sources. Besides, the algal density showed a significant positive correlation with temperature (T) (p &lt; 0.001) and dissolved oxygen (DO) (p &lt; 0.001), but a significant negative correlation with DIC (p &lt; 0.05), suggesting that spring algal blooms may be simulated by water T increase, and then large amounts of DIC and nutrients were adsorbed, accompanying DO release through photosynthesis in coastal water. This study revealed nutrients and carbon interactions in the spring algal bloom of urban eutrophic coastal water, which has implications for understanding the nutrients and carbon biogeochemical cycle and algal bloom mitigation under climate change and anthropogenic pressures in the future.

Effects of riverine nutrient inputs on the sinking fluxes of microbial particles in the St. Lawrence Estuary

Discharges of nitrogenous nutrients associated with agriculture and wastewater treatment promote eutrophication in coastal waters around the world. Since the Lower St. Lawrence Estuary shows early signs of eutrophication, with the presence of hypoxic and acidified deep waters, a mesocosm experiment was conducted to determine the impact of variations of nitrogenous nutrients from riverine inputs to this estuarine environment during the spring–summer phytoplankton growth season. Four treatments were applied to recreate conditions mimicking 1) upwelled waters at the head of the Lower Estuary (“control”), 2) a mix of upwelled and riverine water (”+N”), 3) the same mix of upwelled and riverine waters with a greater proportion of urea in the total nitrogenous pool (”+Urea”), and 4) waters with a 50% decrease in nitrogenous riverine inputs (”–N”). The composition and vertical fluxes of microbial communities were characterized under these four scenarios. Nitrogenous nutrients were consumed within three days in all treatments. A higher concentration of nitrogenous nutrients led to increases in phytoplankton carbon biomass. The centric diatom Skeletonema costatum dominated in all treatments. The microbial community primarily consumed nitrate in all treatments, but equally preferred nitrate and urea in the +Urea treatment. Mean particle sinking velocities were not statistically different between the two most nitrogen-enriched treatments and the control, but the higher dissolved silicon:nitrate uptake ratio in the –N treatment could have favored silicification of the diatoms, leading to increased sinking speeds. Indeed, the total biogenic carbon sinking flux in the –N was on average 51% higher than in the other treatments (∼391 mg C m−2 d−1 vs ∼253–262 mg C m−2 d−1). These results indicate that efficient control measures concerning the St. Lawrence watershed should consider concentrations and ratios of nutrients that potentially promote phytoplankton growth in the Lower Estuary and consequently increase the amount of carbon exported to and decomposed in hypoxic and acidified deep layers.

Evaluation of Eutrophication in Jiaozhou Bay via Water Color Parameters Determination with UAV-Borne Hyperspectral Imagery

The continued increase in greenhouse gas emissions as a result of unprecedented eutrophication has resulted in a rising trend of red tides in the sea, which may be responsible for ecological degradation in the surrounding environment. Studies rarely investigate the accurate concentration of seawater eutrophication substances in offshore aquaculture areas, which may lead to the exacerbated pollution of inshore aquaculture. We examined whether offshore seawater quality monitoring can be effectively performed through unmanned aerial vehicles’ (UAVs) airborne hyperspectral remote sensing technique at Jiaozhou Bay, a water body associated with eutrophication. We used the UAV airborne hyperspectral imager to detect and measure typical marine aquaculture areas in Jiaozhou Bay and selected the key parameters of seawater quality, chlorophyll-a (Chl-a) concentrations, and total suspended matter (TSM) concentrations as indicators of seawater eutrophication. The hyperspectral inversion model of the Jiaozhou Bay seawater (JZBZ) was established with the optimal sensitive band of parameters. Results showed that in comparison with the traditional inversion model, the inversion R2 of the Chl-a was 0.712, the RPD was 3.72, and the inversion R2 of the TSM concentration was 0.756 while the RPD was 5.83. We found that this model is more suitable for the retrieval of water color parameters in Jiaozhou Bay. Additionally, by actual measurement, it can be seen that the concentration ranges of Chl-a in the observation area are 0.380–1.74 mg/m3, and the concentration range of TSM is 12.6–131 mg/L. The results of this study indicate that the Jiaozhou Bay Water Quality Translation Model (JZBM), based on the UAV airborne hyperspectral imager, performs well in the inversion of the concentration of chlorophyll and suspended particulate matter in offshore water, which advances our understanding with a new method to assess the degree of eutrophication in coastal waters.

Mesopredator‐mediated trophic cascade can break persistent phytoplankton blooms in coastal waters

Managing eutrophied systems using only nutrient decreases to impose bottom–up control can be economically and ecologically challenging. Top–down controls through increased consumption have sometimes effectively controlled phytoplankton blooms. However, mechanistic insights, especially on possible trophic cascades, are less understood in brackish, species‐poor coastal waters, where large cladocera are absent. In this study, we set‐up large mesocosms for three consecutive years during the growing season. One set of mesocosms contained mesopredators (gobies and shrimps), whereas the other mesocosms had no such mesopredator present. The results were standardized to monitoring data from the ecosystem to track possible differences between treatments and the system. We found that mesopredator mesocosms showed lower turbidity, phytoplankton biomass and nutrients compared to no‐mesopredator mesocosms, and compared to the ecosystem. This decrease allowed macrophytes to colonize water depths only sparsely colonized in the ecosystem. Rotifer biomass increased in mesopredator mesocosms compared to the ecosystem and to the no‐mesopredator mesocosms. Likewise, copepod biomass that potentially grazes upon rotifers and other microzooplankton decreased in mesopredator mesocosms. No‐mesopredator mesocosms were colonized by an omnivorous mesograzer Gammarus tigrinus, potentially creating additional pressure on macrophytes and increasing grazing‐mediated nutrient release. Zooplankton was not able to control the non‐nutrient limited phytoplankton. We propose a new mechanism, where a higher mesopredator density will increase grazing on phytoplankton by promoting microzooplankton capable of grazing on picophytoplankton. This proposed mechanism would contrast with freshwater systems, where a decrease of zooplanktivorous fish would promote larger phytoplankton grazer like cladocera. Biomanipulation in such species‐poor eutrophic coastal waters may be more successful, due to less trophic pathways, that can cause complex top–down controls like in other systems. Stocking eutrophic coastal waters with gobies and shrimps may be an alternative biomanipulative approach rather than selectively removing large piscivorous or omnivorous fish from eutrophic coastal waters.