DIN) And Phosphorus Research Articles

Typhoon-induced stormwater drives nutrient dynamics and triggers phytoplankton blooms in Laizhou Bay, China

In this study, we investigated the hydrological and ecological impacts of heavy rainfall caused by the storm Rumbia and Typhoon Lekima on Laizhou Bay (LZB) through land‒sea synchronous field surveys, online remote sensors, and simulated enclosure experiments. Within two weeks of Rumbia, approximately 9% and 16% of the annual riverine total nitrogen (TN) and total phosphorus (TP) fluxes, respectively, were transported to the LZB and the proportions were 17% and 35%, respectively, for Lekima. The land use on the watersheds increased the rates of land-derived nutrient loading and altered their biogeochemical forms. Consequently, the average concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the LZB increased by 2.6 and 1.0 times post-Rumbia and by 3.5 and 1.3 times post-Lekima, respectively. Relatively lower salinity and temperature, sudden increases in DIN, and strengthened coastal currents stimulated the growth of highly adaptable and small diatoms, resulting in the first diatom blooms. Subsequently, a bloom of Noctiluca scintillans formed.

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.

Long-term changes in the concentrations of nutrients in the marginal seas (Yellow Sea, East China Sea, and East/Japan Sea) neighboring the Korean Peninsula

We examined the changes in the concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the surface waters of the Yellow Sea (YS), the southern sea (SS) of Korea, and the East/Japan Sea (ES) from 1995 to 2021. These marginal seas neighboring the Korean Peninsula maintained nutrient concentrations approximately an order of magnitude higher than those in the Kuroshio waters, indicating extraordinarily large terrestrial source inputs. Generally, the DIN concentration in the YS increased gradually due to the accumulation of terrestrial inputs, while the nutrient concentrations in the ES declined gradually mainly due to enhanced water stratification. The SS showed the maximum DIN concentrations around 2005, associated with freshwater influence. In Korean coastal waters within ~10 km from the coastline, nutrient concentrations declined sharply during this period due to decreased terrestrial nutrient inputs. The rapid changes in the nutrient levels in these seas may significantly alter biological production.

Trends in nutrients in the Changjiang River

Better documentation and understanding of long-term temporal dynamics of nutrients in watersheds are necessary to support effective water quality management. We examined the hypothesis that the recent management of fertilizer use and pollution control in the Changjiang River Basin could govern the fluxes of nutrients from the river to the sea. Results based on historical data since 1962 and surveys in recent years show that concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the mid- and downstream reaches were higher than those in the upper reaches due to intensive anthropogenic activities, while dissolved silicate (DSi) was distributed evenly from the up- to downstream reaches. Fluxes of DIN and DIP increased rapidly, and DSi declined during the 1962–1980 and 1980–2000. After the 2000s, concentrations and fluxes of DIN and DSi remained almost unchanged; those of DIP remained stable until the 2010s and slightly decreased afterward. The decline in fertilizer use explains 45 % of the variance in the decline of DIP flux, followed by pollution control, groundwater and water discharge. As a result, the molar ratio of DIN:DIP, DSi:DIP and ammonia:nitrate varied largely during 1962–2020, and the excess DIN relative to DIP and DSi lead to increased limitations of silicon and phosphorus. A turning point probably occurred for nutrient fluxes in the Changjiang River in the 2010s, with the pattern of DIN from continuous increase to stability and DIP from increase to decrease. This decline in phosphorus in the Changjiang River has many similarities with the rivers worldwide. The continued basin nutrient management is likely to have a major influence on river nutrient delivery and therefore may control coastal nutrient budget and ecosystem stability.

Nutrient and Silicon Isotope Dynamics in the Laptev Sea and Implications for Nutrient Availability in the Transpolar Drift

AbstractRealistic prediction of the near‐future response of Arctic Ocean primary productivity to ongoing warming and sea ice loss requires a mechanistic understanding of the processes controlling nutrient bioavailability. To evaluate continental nutrient inputs, biological utilization, and the influence of mixing and winter processes in the Laptev Sea, the major source region of the Transpolar Drift (TPD), we compare observed with preformed concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP), silicic acid (DSi), and silicon isotope compositions of DSi (δ30SiDSi) obtained for two summers (2013 and 2014) and one winter (2012). In summer, preformed nutrient concentrations persisted in the surface layer of the southeastern Laptev Sea, while diatom‐dominated utilization caused intense northward drawdown and a pronounced shift in δ30SiDSi from +0.91 to +3.82‰. The modeled Si isotope fractionation suggests that DSi in the northern Laptev Sea originated from the Lena River and was supplied during the spring freshet, while riverine DSi in the southeastern Laptev Sea was continuously supplied during the summer. Primary productivity fueled by river‐borne nutrients was enhanced by admixture of DIN‐ and DIP‐rich Atlantic‐sourced waters to the surface, either by convective mixing during the previous winter or by occasional storm‐induced stratification breakdowns in late summer. Substantial enrichments of DSi (+240%) and DIP (+90%) beneath the Lena River plume were caused by sea ice‐driven redistribution and remineralization. Predicted weaker stratification on the outer Laptev Shelf will enhance DSi utilization and removal through greater vertical DIN supply, which will limit DSi export and reduce diatom‐dominated primary productivity in the TPD.

Effects of nutrient-rich submarine groundwater discharge on marine aquaculture: A case in Lianjiang, East China Sea

Submarine groundwater discharge (SGD) and associated nutrient have long been received insufficient attention in the aquaculture areas of Lianjiang, East China Sea. In this study, we used 224Ra mass balance model to evaluate the importance of SGD in the aquaculture areas of Lianjiang in different seasons. The results showed that SGD fluxes were 0.14 ± 0.070 m3 m−2 d−1 in July and 0.077 ± 0.040 m3 m−2 d−1 in October 2019, which corresponded to approximately 22% and 32% of the river discharge, respectively. Nevertheless, the dissolved inorganic nitrogen (DIN) and phosphorus (DIP) fluxes contributed by SGD over total nutrient inputs were approximately 58% and 73%, respectively in July, 11% and 33%, respectively in October 2019. The ratio of DIN to DIP in seawater was slightly higher than that derived from SGD in July and significantly lower than that derived from the river and SGD in October, which was prone to profound modulated by shellfish (e.g., clam Ruditapes philippinarum and Sinonovacula constricta) and nori (e.g., Pyropia haitanensis) that selectively acquire nitrogen and phosphorous, suggesting that SGD could affect the nutrient structure and aquaculture activities in Lianjiang. Moreover, the new primary production via SGD yielded (2.47 ± 1.32) × 103 mg C m−2 d−1 in July and (3.52 ± 1.76) × 102 mg C m−2 d−1 in October 2019, which were approximately 8–25% and 10–31% of the production in the entire study area. These meaningful discoveries indicated that SGD and associated nutrient are the predominant regulator of nutrient and aquaculture structure under significant seasonal differences.

Blooms of Alexandrium catenella in Coastal Waters of Chilean Patagonia: Is Subantarctic Surface Water Involved?

At the southern tip of South America, evidence of shellfish toxicity has been recorded in the accounts of early explorers and shipwreck survivors since the late 16th Century. Blooms of the toxic dinoflagellate Alexandrium catenella were described in the western Magellan Strait in the early 1970s and have since shown a northward progression through Chilean Patagonia, culminating in a catastrophic toxic event around Chiloé Island in 2016. This shift has taken place through coastal areas of extremely sparse human population density, and anthropogenically driven eutrophication is therefore unlikely to be significantly involved, at least in the south. However, human activities – such as salmon cultivation – may play a role in the intensification of blooms in the more densely populated areas of northern Patagonia. In the fjords and channels of Chilean Patagonia, phytoplankton assemblages are shaped by complex interactions between freshwater (FW) run-off and intrusions of subantarctic surface water (SASW). In the context of blooms of A. catenella, we review the properties of SASW – transformed in coastal waters into modified subantarctic water (MSAW). FW input is characterized by very low concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP), but relatively high concentrations of silicic acid (DSi); DIN and DIP are instead supplied predominantly by SASW which is severely deficient in DSi. These waters therefore show strong vertical gradients in DIN, DIP and DSi, but also potentially in dissolved trace metals and CO2. Large scale shifts in the relative inputs of SASW or FW can modify these vertical gradients, potentially forcing competitive changes in phytoplankton assemblages with latitude, with implications for growth and toxicity of A. catenella and other harmful species. The northward shift of blooms of A. catenella could be associated with anomalies in the Southern Annular Mode (SAM) that modify the influence of MSAW through variations in FW input to coastal waters. The historical presence of blooms in southern Patagonia and Tierra del Fuego, combined with the strongly contrasting conditions with latitude and depth, mean that southern Chile represents an ideal natural laboratory to study climatic and oceanographic influences on dynamics of A. catenella populations.

Testing the hydroponic performance of the edible halophyte Halimione portulacoides, a potential extractive species for coastal Integrated Multi-Trophic Aquaculture

Sea purslane Halimione portulacoides (L.) Aellen is a candidate extractive species for coastal Integrated Multi-Trophic Aquaculture (IMTA) to recycle the dissolved inorganic nitrogen (DIN) and phosphorus (DIP) wasted by excretive species. To test its suitability, saline aquaculture effluents were simulated in the laboratory using a hydroponics approach to cultivate the plants. Nutrient extraction efficiency, growth performance and nutritional profile were assessed under a range of DIN and DIP concentrations representing three different aquaculture intensification regimes and using Hoagland's solution as a control. Over a 10-week period, hydroponic units under non-limited N and P conditions displayed daily extraction rates between 1.5 and 2.8 mg DIN-N L−1 day−1 and 0.1–0.2 mg DIP-P L−1 day−1 and yielded between 63.0 and 73.0 g m−2 day−1 of H. portulacoides biomass. Relatively to biomass produced, H. portulacoides extracted between 2.6 and 4.2 mg DIN-N g−1 and 0.1–0.4 mg DIP-P g−1. The treatment with low-input of DIN and DIP (6.4 mg N L−1 and 0.7 mg P L−1) induced some degree of nutrient limitation, as suggested by the extremely high extraction efficiencies of DIN extraction (99%) in parallel with lower productivity. The nutritional profile of H. portulacoides leaves is comparable to that of other edible halophytes and leafy greens and could be a low-sodium alternative to salt in its lyophilized form. From the present study, we conclude that the edible halophyte H. portulacoides can be highly productive in hydroponics using saline water irrigation with non-limiting concentrations of DIN and DIP and is, therefore, a suitable extractive species for coastal IMTA in brackish waters.

Vertical distribution of a harmful red-tide dinoflagellate, Karenia mikimotoi, at the decline stage of blooms

The red-tide dinoflagellate Karenia mikimotoi can vertically migrate ~20 m in a day. This property enables K. mikimotoi to acquire nutrients over a wide range of depths and to receive optimal irradiance. We conducted day and night observations of diurnal vertical migrations (DVMs) during the declining stage of blooms and daily-weekly investigations of the relationship between environmental factors and a K. mikimotoi bloom in four different bays of western Japan. Moreover, we performed correlation analysis between DVM parameters, such as migratory distance and environmental conditions, using data from the present study and previous studies, which DVMs were observed at the developmental or stationary stage of K. mikimotoi bloom. Observations of evident DVMs of K. mikimotoi were commonly absent, although the depths where K. mikimotoi accumulated throughout the day differed among observations. Migratory distance showed a positive correlation with the number of days it took for the bloom to decline and a negative correlation with the vertical stratification. Precipitation was low, irradiance was high, and dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations were inadequate for cell growth 1–2 weeks before the DVM observations. Moreover, average DIP concentration at the depth of the highest K. mikimotoi cell concentration in the water column and the growth rate estimated from DIN and DIP concentrations during the DVM observations showed positive correlations with migratory distance. During DVM observation, the amount of light received was inadequate for growth when K. mikimotoi cells remained in deep waters, and photoinhibition occurred when they remained in surface waters. The results suggest that adverse conditions, such as nutrient deficiency and strong stratification, may prevent K. mikimotoi from achieving the benefits of DVM behavior and thereby promote the collapse of a bloom.

Impact of finfish aquaculture on biogeochemical processes in coastal ecosystems and elemental sulfur as a relevant proxy for assessing farming condition.

We conducted experiments to investigate the effects of finfish aquaculture and to propose appropriate proxies for assessing their environmental impact. Due to enhanced fish feed input, sulfate reduction (SR) and the resulting metabolic products (H2S, NH4+, PO43-) were significantly greater at the farm than at the control site. Benthic release of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) from farm sediment accounted for 52-837% and 926-1048%, respectively, of the potential DIN and DIP demand for phytoplankton production. The results suggest that excess organic loading in fish farms induces deleterious eutrophication and algal blooms in coastal ecosystems via benthic-pelagic coupling. Direct SR measurement provided the most useful information of all the parameters on organic contamination in fish farms. However, given its abundance, relatively lower chemical reactivity and relative ease of analysis, elemental sulfur was regarded as the most appropriate proxy for assessing the environmental impacts of finfish aquaculture.

Photosynthetic characteristics of Pyropia yezoensis (Ueda) Hwang & Choi measured using Diving-PAM in the Jindo-Haenam region on the southwestern coast of the Korean Peninsula

Jeollanam-do in the Republic of Korea is home to 94% of the nation’s seaweed aquaculture production, and the Jindo-Haenam region on the coast of Jeollanam-do is mostly occupied by Pyropia farms that are experiencing nutrient deficiencies and reduced seaweed growth. To understand the causes of these problems, we examined the relationships between chemical parameters and the photosynthetic efficiency of Pyropia yezoensis by analyzing its morphological characteristics, C and N concentrations, C and N stable isotope ratios, and photosynthetic pigments in the Jindo-Haenam region. The dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations in seawater and N in P. yezoensis thalli were 4.01–4.45 μM, 0.16–0.38 μM, and 36.9–57.0 mg DW g−1, respectively, generally meeting the minimum demand for the growth of Pyropia. The maximum electron transfer rate was 9.6 ± 1.5 (4.5–15.0) μmol electrons m−2 s−1, with most measured values being low, except at some survey stations; St. 10 and 11 also had low photosynthetic efficiencies of 0.03–0.05. Survey stations in the southwestern portion of the study area had nutrient levels below the minimum demand for Pyropia growth due to intensive P. yezoensis farming, and the photosynthetic efficiency of the algae was also low in this area. Therefore, it is necessary to restructure the farm facilities and reduce the area of Pyropia farming in the Jindo-Haenam region because the productivity of Pyropia farms is decreasing due to production in excess of the nutrient availability.

Feedback Regulation between Aquatic Microorganisms and the Bloom-Forming Cyanobacterium Microcystis aeruginosa.

The frequency and intensity of cyanobacterial blooms are increasing worldwide. Interactions between toxic cyanobacteria and aquatic microorganisms need to be critically evaluated to understand microbial drivers and modulators of the blooms. In this study, we applied 16S/18S rRNA gene sequencing and metabolomics analyses to measure the microbial community composition and metabolic responses of the cyanobacterium Microcystis aeruginosa in a coculture system receiving dissolved inorganic nitrogen and phosphorus (DIP) close to representative concentrations in Lake Taihu, China. M. aeruginosa secreted alkaline phosphatase using a DIP source produced by moribund and decaying microorganisms when the P source was insufficient. During this process, M. aeruginosa accumulated several intermediates in energy metabolism pathways to provide energy for sustained high growth rates and increased intracellular sugars to enhance its competitive capacity and ability to defend itself against microbial attack. It also produced a variety of toxic substances, including microcystins, to inhibit metabolite formation via energy metabolism pathways of aquatic microorganisms, leading to a negative effect on bacterial and eukaryotic microbial richness and diversity. Overall, compared with the monoculture system, the growth of M. aeruginosa was accelerated in coculture, while the growth of some cooccurring microorganisms was inhibited, with the diversity and richness of eukaryotic microorganisms being more negatively impacted than those of prokaryotic microorganisms. These findings provide valuable information for clarifying how M. aeruginosa can potentially modulate its associations with other microorganisms, with ramifications for its dominance in aquatic ecosystems.IMPORTANCE We measured the microbial community composition and metabolic responses of Microcystis aeruginosa in a microcosm coculture system receiving dissolved inorganic nitrogen and phosphorus (DIP) close to the average concentrations in Lake Taihu. In the coculture system, DIP is depleted and the growth and production of aquatic microorganisms can be stressed by a lack of DIP availability. M. aeruginosa could accelerate its growth via interactions with specific cooccurring microorganisms and the accumulation of several intermediates in energy metabolism-related pathways. Furthermore, M. aeruginosa can decrease the carbohydrate metabolism of cooccurring aquatic microorganisms and thus disrupt microbial activities in the coculture. This also had a negative effect on bacterial and eukaryotic microbial richness and diversity. Microcystin was capable of decreasing the biomass of total phytoplankton in aquatic microcosms. Overall, compared to the monoculture, the growth of total aquatic microorganisms is inhibited, with the diversity and richness of eukaryotic microorganisms being more negatively impacted than those of prokaryotic microorganisms. The only exception is M. aeruginosa in the coculture system, whose growth was accelerated.

Land-derived inorganic nutrient loading to coastal waters and potential implications for nearshore plankton dynamics

Algoa Bay represents a complex suite of aquatic ecosystems that support high biodiversity and provide significant socio-economic benefits. Since land-ocean interactions are central to shaping the structure and functioning of these ecosystems, the aim of this study was to quantify land-derived inorganic nutrient loads entering the coastal waters of Algoa Bay. Dissolved inorganic nitrogen (DIN) and phosphorus (DIP) data together with flow discharge volumes were sourced from long-term monitoring programmes and existing literature to determine annual and seasonal nutrient loads. The annual loads of DIN and DIP entering the coastal waters of Algoa Bay were estimated at 8.7 × 105 and 1.4 × 105 kg, respectively. Seasonal peaks were observed in winter for DIN and in spring for DIP. Wastewater discharges were the dominant source of inorganic nutrients to the nearshore environment, representing 71.3% and 62.0% of total DIN and DIP loads, respectively. The DIN and DIP inputs entering estuarine waters of Algoa Bay accounted for 21.9% and 38.0% of total loads, respectively. Inshore zooplankton biomass in Algoa Bay has increased significantly (p < 0.01) since 2013 concomitant with elevated DIN inputs from WWTPs. Additionally, anthropogenic nutrient loading to the estuarine and coastal waters of Algoa Bay has facilitated, in part, the increased observations of eutrophic symptoms, including harmful algal blooms (e.g. Heterosigma akashiwo and Lingulodinium polyedra) and hypoxia (<2 mg l−1). Effective ecosystem-based management is key to preventing the exacerbation of these undesirable disturbances, and thus maintaining ecosystem functionality.

Modeling investigation of the nutrient and phytoplankton variability in the Chesapeake Bay outflow plume

The Chesapeake Bay outflow plume (CBOP) is the mixing zone between Chesapeake Bay and less eutrophic continental shelf waters. Variations in phytoplankton distribution in the CBOP are critical to the fish nursery habitat quality and ecosystem health; thus, an existing hydrodynamic-biogeochemical model for the bay and the adjacent coastal ocean was applied to understand the nutrient and phytoplankton variability in the plume and the dominant environmental drivers. The simulated nutrient and chlorophyll a distribution agreed well with field data and real-time satellite imagery. Based on the model calculation, the net dissolved inorganic nitrogen (DIN) and phosphorus (DIP) flux at the bay mouth was seaward and landward during 2003–2012, respectively. The CBOP was mostly nitrogen-limited because of the relatively low estuarine DIN export. The highest simulated phytoplankton biomass generally occurred in spring in the near field of the plume. Streamflow variations could regulate the estuarine residence time, and thus modulate nutrient export and phytoplankton biomass in the plume area; in comparison, changing nutrient loading with fixed streamflow had a less extensive impact, especially in the offshore and far-field regions. Correlation analyses and numerical experiments revealed that southerly winds on the shelf were effective in promoting the offshore plume expansion and phytoplankton accumulation. Climate change including precipitation and wind pattern shifts is likely to complicate the driving mechanisms of phytoplankton variability in the plume region.

From Ecological Stoichiometry to Biochemical Composition: Variation in N and P Supply Alters Key Biosynthetic Rates in Marine Phytoplankton.

One of the major challenges in ecological stoichiometry is to establish how environmental changes in resource availability may affect both the biochemical composition of organisms and the species composition of communities. This is a pressing issue in many coastal waters, where anthropogenic activities have caused large changes in riverine nutrient inputs. Here we investigate variation in the biochemical composition and synthesis of amino acids, fatty acids (FA), and carbohydrates in mixed phytoplankton communities sampled from the North Sea. The communities were cultured in chemostats supplied with different concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) to establish four different types of resource limitations. Diatoms dominated under N-limited, N+P limited and P-limited conditions. Cyanobacteria became dominant in one of the N-limited chemostats and green algae dominated in the one P-limited chemostat and under light-limited conditions. Changes in nutrient availability directly affected amino acid content, which was lowest under N and N+P limitation, higher under P-limitation and highest when light was the limiting factor. Storage carbohydrate content showed the opposite trend and storage FA content seemed to be co-dependent on community composition. The synthesis of essential amino acids was affected under N and N+P limitation, as the transformation from non-essential to essential amino acids decreased at DIN:DIP ≤ 6. The simple community structure and clearly identifiable nutrient limitations confirm and clarify previous field findings in the North Sea. Our results show that different phytoplankton groups are capable of adapting their key biosynthetic rates and hence their biochemical composition to different degrees when experiencing shifts in nutrient availability. This will have implications for phytoplankton growth, community structure, and the nutritional quality of phytoplankton as food for higher trophic levels.

Decadal Changes in Water Quality and Net Productivity of a Shallow Danish Estuary Following Significant Nutrient Reductions

We utilized an extensive data set (1977–2013) from a water quality monitoring program to investigate the recovery of a Danish estuary following large reductions in total phosphorus (TP) and total nitrogen (TN) loading. Monthly rates of net transport and biogeochemical transformation of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) were computed in two basins of the estuary using a box model approach, and oxygen-based rates of net ecosystem production (NEP) were determined. Since 1990, nutrient loading was reduced by 58 % for nitrogen and 80 % for phosphorus, causing significant decreases in DIN (60 %) and DIP (85 %) concentrations. Reductions in nutrient loadings and concentrations reduced annual chlorophyll levels by 50 % in the inner estuary and improved Secchi depth by approximately 1 m during the same period, particularly in the summer period. In the outer, deeper region of the estuary trends in water quality was less evident. Improvements in the inner estuary were strongly coupled to declines in DIN. Thresholds of DIN and DIP concentrations limiting phytoplankton growth indicated that both regions of the estuary were nitrogen limited. NEP rates indicated the development of more net autotrophic conditions over time that were likely associated with higher benthic primary production stimulated by improved light conditions. Box model computations revealed a modest reduction in summer net production of DIP over time, despite the persistence of elevated fluxes for several years after external loads were reduced. Since the mid-1990s, nutrient loading and transformation were stable while nutrient concentrations continued to decline and water quality improved in the inner estuary. The oligotrophication trajectory involved an initial fast transformation and modest retention of nutrients followed by a gradual decline in the rate of improvement towards a new stable condition.

On the Role of Temperature and Salinity Data Assimilation to Constrain a Coupled Physical–Biogeochemical Model in the Baltic Sea

AbstractA three-dimensional variational data assimilation (3DVAR) method is implemented in a coupled physical–biogeochemical (CPB) model in the Baltic Sea. This study carries out a 10-yr assimilation experiment with satellite sea surface temperature (SST) and observed in situ temperature (T) and salinity (S) profiles. The impact of the assimilation is assessed with the focus on how the biogeochemical model responds to the improved hydrodynamics. The assimilation of temperature and salinity data yields considerable improvements in the physical model. On a basin scale, the mean bias of SST, T, S, and mixed layer depth (MLD) is decreased by 0.18°C (57%), 0.31°C (49%), 0.34 psu (43%), and 1.8 m (43%), respectively. More importantly, the biogeochemical simulation is improved in response to the physical data assimilation. Compared with in situ observations, the mean biases of chlorophyll a (Chl), dissolved inorganic nitrogen (DIN) and phosphorus (DIP) are decreased by 0.09 mg m−3 (15.5%), 0.19 mmol m−3 (9%), and 0.15 mmol m−3 (23%). Physical data assimilation also improves the simulated variability of Chl, DIN, and DIP and their correlations with observation. Compared with satellite observations, the mean bias of surface chlorophyll is reduced by 0.10–0.32 mg m−3 especially in the Skagerrak–Kattegat area and Bornholm basin. The decrease of total Chl change is caused by different mechanisms for winter and summer. While the deepened mixed layer acts as a dilution factor in winter, strengthened stratification agrees well with the decrease of chlorophyll in summer. In the vertical, relatively large changes of DIN and DIP occur below 60 m, which corresponds to the mean permanent halocline depth (~60–80 m) of the Baltic Sea.

Effects of nitrate and phosphate supply on chromophoric and fluorescent dissolved organic matter in the Eastern Tropical North Atlantic: a mesocosm study

Abstract. In open-ocean regions, as is the Eastern Tropical North Atlantic (ETNA), pelagic production is the main source of dissolved organic matter (DOM) and is affected by dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations. Changes in pelagic production under nutrient amendments were shown to also modify DOM quantity and quality. However, little information is available about the effects of nutrient variability on chromophoric (CDOM) and fluorescent (FDOM) DOM dynamics. Here we present results from two mesocosm experiments ("Varied P" and "Varied N") conducted with a natural plankton community from the ETNA, where the effects of DIP and DIN supply on DOM optical properties were studied. CDOM accumulated proportionally to phytoplankton biomass during the experiments. Spectral slope (S) decreased over time indicating accumulation of high molecular weight DOM. In Varied N, an additional CDOM portion, as a result of bacterial DOM reworking, was determined. It increased the CDOM fraction in DOC proportionally to the supplied DIN. The humic-like FDOM component (Comp.1) was produced by bacteria proportionally to DIN supply. The protein-like FDOM component (Comp.2) was released irrespectively to phytoplankton or bacterial biomass, but depended on DIP and DIN concentrations. Under high DIN supply, Comp.2 was removed by bacterial reworking, leading to an accumulation of humic-like Comp.1. No influence of nutrient availability on amino acid-like FDOM component in peptide form (Comp.3) was observed. Comp.3 potentially acted as an intermediate product during formation or degradation of Comp.2. Our findings suggest that changes in nutrient concentrations may lead to substantial responses in the quantity and quality of optically active DOM and, therefore, might bias results of the applied in situ optical techniques for an estimation of DOC concentrations in open-ocean regions.

Salt marsh tidal exchange increases residence time of silica in estuaries

We present flux measurements of dissolved silicon (DSi), biogenic Si (BSi), and dissolved inorganic nitrogen (DIN) and phosphorus (DIP) on a temperate salt marsh in Narragansett Bay, Rhode Island, over neap, mid, and spring tide cycles during the spring and the summer seasons to determine the stoichiometry of marsh tidal export and the potential effect on Si availability in the receiving estuary. During the spring, when incoming waters were depleted in DSi (average 1.7 µmol L−1), the marsh was a net sink of BSi (132 mol h−1) and a source of DSi (31 mol h−1) to the estuary. During this period, the DIN : DSi ratio of ebbing water was more than five times lower than that of flood water. Together, these data indicate that marsh nutrient fluxes “outwell” nutrients in ratios that support spring diatom growth in Narragansett Bay. However, during the entire sampling period the marsh served as a net sink for both DSi and BSi, importing, on average, 14 mol DSi h−1 and 86 mol BSi h−1. We hypothesize that the net import and retention of Si by salt marshes provides a previously overlooked ecosystem service of increasing the residence time of Si in estuarine systems. In the absence of salt marshes, we calculate that 5.1 × 104 kmol of Si would be exported from the estuary during the growing season, decreasing the availability of Si in the system and having direct repercussions for phytoplankton species composition in nearby estuarine waters.

Seasonal dissolved inorganic nitrogen and phosphorus budgets for two sub-tropical estuaries in south Florida, USA

Abstract. Interactions among geomorphology, circulation, and biogeochemical cycling determine estuary responses to external nutrient loading. In order to better manage watershed nutrient inputs, the goal of this study was to develop seasonal dissolved inorganic nitrogen (DIN) and phosphorus (DIP) budgets for the two estuaries in south Florida, the Caloosahatchee River estuary (CRE) and the St. Lucie Estuary (SLE), from 2002 to 2008. The Land–Ocean Interactions in the Coastal Zone (LOICZ) approach was used to generate water, salt, and DIN and DIP budgets. Results suggested that internal DIN production increases with increased DIN loading to the CRE in the wet season. There were hydrodynamic effects as water column concentrations and ecosystem nutrient processing stabilized in both estuaries as flushing time increased to &gt;10 d. The CRE demonstrated heterotrophy (net ecosystem metabolism or NEM &lt; 0.0) across all wet and dry season budgets. While the SLE was sensitive to DIN loading, system autotrophy (NEM &gt; 0.0) increased significantly with external DIP loading. This included DIP consumption and a bloom of a cyanobacterium (Microcystis aeruginosa) following hurricane-induced discharge to the SLE in 2005. Additionally, while denitrification provided a microbially-mediated N loss pathway for the CRE, this potential was not evident for the SLE where N2 fixation was favored. Disparities between total and inorganic loading ratios suggested that the role of dissolved organic nitrogen (DON) should be assessed for both estuaries. Nutrient budgets indicated that net internal production or consumption of DIN and DIP fluctuated with inter- and intra-annual variations in freshwater inflow, hydrodynamic flushing, and primary production. The results of this study should be included in watershed management plans in order to maintain favorable conditions of external loading relative to internal material cycling in both dry and wet seasons.