Changes In Nutrient Loading Research Articles

Scenario Analysis of Socio-Economic and Climate Related Changes in Nutrient Load and Retention for the Pregolya River Catchment (South-Eastern Baltic): The View at the Beginning of 21<sup>st</sup> Century

The study analysed the transboundary Pregolya River Catchment, covering both the Polish and Russian parts, using the HYPE hydrological module and FyrisNP emission-retention model. The results revealed significant spatial variations in nutrient retention. The data assessment indicates that, at the start of the 21st century, the nutrient load from the Polish part of the catchment is significantly greater than that from the Russian part. Model simulations based on climatic projections for the years 2041–2060, but with current nutrient loads, showed a significant level of uncertainty in the changes of nutrient export to the Baltic Sea. The range for total nitrogen was –10 % to +27 %, and for total phosphorus it was –29 % to –10 %. Model simulations based on different socio-economic scenarios, but current climate conditions, showed that if present 5-year trends are maintained, nutrient export will only increase slightly (3 % for total nitrogen and total phosphorus). If the plans for socio-economic growth of Polish and Russian local governments are implemented, including the expansion of agriculture in Kaliningrad Oblast, there will be a significant increase in nutrient export (78 % for total nitrogen and 55 % for total phosphorus).

Increased nutrient retention and cyanobacterial blooms in a future coastal zone

Climate change and coastal management directly affect the magnitude of nutrient loads and the functioning of the coastal zones. In this novel approach, a coastal-zone-model was used to study future retention capacity of nitrogen and phosphorus loads and changes in cyanobacterial blooms in an archipelago in a eutrophied coastal sea. By conducting projections under the combined impact of changing climate and nutrient loads we demonstrate that i) future filter efficiency of the archipelago increases compared to present day regardless of the nutrient load projection, ii) the cyanobacteria biomass increased 5-fold in a worst-case nutrient load projection and iii) resulting in an increased spread of bottom anoxia and hypoxia. However, the phytoplankton biomass and hypoxia/anoxia will remain stable if the planned mitigation is successfully implemented. This study highlights the importance of nutrient load management and the need to understand more about the complex interactions taking place in the coastal systems.

Governance pathway for coastal eutrophication based on regime shifts in diatom–dinoflagellate composition of the Bohai and Baltic Seas

Regime shifts in the diatom–dinoflagellate composition have occurred in the Baltic Sea (BS) and Bohai Sea (BHS) under eutrophication and have affected the entire coastal ecosystem, damaging the regulatory, provisioning, cultural, and supporting service functions of marine ecosystems. Therefore, finding a solution to restore the balance of phytoplankton community composition and mitigate eutrophication is of utmost importance. In this study, the Driver (per capita gross domestic product)-Pressure (terrestrial inputs)-State (seawater environmental parameters)-Impact (proportions of diatoms and dinoflagellates)-Response (eutrophication governance projects) framework served as a guide for our analysis of the causal relationship among various environmental components in the coastal system. The relevant data in BS and BHS spanning from the 1950s to the 2010s were collected and used to construct a diatom–dinoflagellate composition single index, which allowed us to identify the shifts in regimes (mutation points and phases) of the diatom–dinoflagellate composition and environmental factors using sequential t-test analysis. We also identified key environmental factors that moderated the diatom–dinoflagellate composition using redundancy analysis and analyzed the partial effects of the main environmental factors on the diatom–dinoflagellate composition using a generalized additive model. Finally, the regulation of the eutrophication governance investment on diatom–dinoflagellate composition was investigated. We found that (1) BS is a “time machine,” with coastal eutrophication governance and regime shift of diatom–dinoflagellate composition and environmental factors two decades earlier than that in BHS; (2) in BS, the key moderation factor of diatom proportion is SiO3-Si and those of dinoflagellates are sea surface salinity and N:P ratio; in BHS, the key moderation factors of diatom proportion are PO4-P and Si:N ratio and those of dinoflagellate are dissolved inorganic nitrogen and N:P and Si:P ratios; (3) it is projected that BHS will enter its recovery phase from eutrophication after mid-2020s. In summary, the N/P/Si stoichiometric relationships should be given greater consideration, with the exception of the “dose-response” relationship in both sea areas. Our results indicate an urgent need for an improved mechanistic understanding of how phytoplankton biodiversity changes in response to changes in nutrient load and how we should ultimately deal with the challenges that arise.

Linking watershed nutrient loading to estuary water quality with generalized additive models.

Evaluating estuary water quality responses to reductions (or increases) in nutrient loading attributed to on the ground management actions can be challenging due to the strong influence of environmental drivers on nutrient loads and non-linear relationships. This study applied generalized additive models to calculate watershed nutrient loads and assess responses in estuary water quality to seasonally-adjusted freshwater inflow and flow-adjusted nutrient loads in Lavaca Bay, Texas. Lavaca Bay is a secondary embayment on the Texas coast displaying early potential for eutrophication and water quality degradation. Use of flow-adjusted nutrient loads allowed the study to evaluate the response in water quality to changes in nutrient loads driven by anthropogenic sources. Cross-validation indicated that, despite data constraints, semiparametric models performed well at nutrient load prediction. Based on these models, delivered annual nutrient loads varied substantially from year to year. In contrast, minimal changes in flow-normalized loads indicate that nutrient loadings were driven by natural variation in precipitation and runoff as opposed to changes in management of nonpoint sources. Models indicated no evidence of long-term changes in dissolved oxygen or chlorophyll-a within Lavaca Bay. However, site specific long-term increases in both organic and inorganic nitrogen are concerning for their potential to fuel eutrophication. Further analysis found freshwater inflow had strong influences on nutrient and chlorophyll-a concentrations but there was no evidence that changes in watershed nutrient loading explained additional variation in dissolved oxygen and limited evidence that watershed nutrient loadings explained chlorophyll-a concentrations. In addition to providing a baseline assessment of watershed nutrient loading and water quality responses in the Lavaca Bay watershed, this study provides methodological support for the use of semiparametric models in load regression models and estuary assessments.

Nutrient uptake in tropical rivers receiving wastewater treatment plant discharge: High mass removal but low nutrient uptake efficiencies

Wastewater treatment plants (WWTPs) have eliminated many problems related to sewage inputs to aquatic systems. However, the treated effluents still affect river and stream ecosystems. Yet, very limited information is available about the fate of treated effluents in tropical receiving water bodies. Here, we examined the longitudinal changes in ammonium (NH4-N), nitrate (NO3-N), and soluble reactive phosphorus (SRP) concentrations in reaches downstream from chronic WWTP inputs in three receiving rivers in São Paulo state (Southeastern Brazil). The studied WWTPs were not designed or operated to have specific tertiary treatment for nutrient removal at the time of the study. We conducted a total of five sampling campaigns between 2019 and 2021. We applied the nutrient spiraling approach to these high-magnitude nutrient additions to examine net nutrient uptake. Changes in nutrient loads along the reaches were also examined. Overall, nutrient concentrations and loads were considerably increased by WWTP discharges. Net uptake lengths (SW-net) for NO3-N, NH4-N, and SRP ranged from 1.2 to 13.6, 1.5–infinity (uptake coefficient < 0), and 1.2–7.5 km, respectively. Net uptake velocities (Vf-net) varied between 0.7–2.5, −0.4 to 4.1, and 0.2–10.2 mm/min, respectively. Areal net uptake rates (Unet) ranged from 0.78 to 10.7, −20.08–45.8, and 0.18–4.4 g/m2/min for NO3-N, NH4-N, and SRP, respectively. High export (long SW-net) and limited net uptake along the reaches (low Vf-net) indicated that nutrients were transported downstream for long distances without efficient removal (nutrient availability higher than demand) despite the high mass removal (high Unet). These results suggest that the export of nutrients is substantial, potentially creating water quality impairments to downstream ecosystems. Our study highlighted that WWTPs with limited nutrient removal have considerable effects on water chemistry, nutrient cycling and loading in tropical receiving freshwaters.

Harmful algal bloom aerosols and human health.

Harmful algal blooms (HABs) are increasing across many locations globally. Toxins from HABs can be incorporated into aerosols and transported inland, where subsequent exposure and inhalation can induce adverse health effects. However, the relationship between HAB aerosols and health outcomes remains unclear despite the potential for population-level exposures. In this review, we synthesized the current state of knowledge and identified evidence gaps in the relationship between HAB aerosols and human health. Aerosols from Karenia brevis, Ostreopsis sp., and cyanobacteria were linked with respiratory outcomes. However, most works did not directly measure aerosol or toxin concentrations and instead relied on proxy metrics of exposure, such as cell concentrations in nearby waterbodies. Furthermore, the number of studies with epidemiological designs was limited. Significant uncertainties remain regarding the health effects of other HAB species; threshold dose and the dose-response relationship; effects of concurrent exposures to mixtures of toxins and other aerosol sources, such as microplastics and metals; the impact of long-term exposures; and disparities in exposures and associated health effects across potentially vulnerable subpopulations. Additional studies employing multifaceted exposure assessment methods and leveraging large health databases could address such gaps and improve our understanding of the public health burden of HABs.

Influence of cultural eutrophication, climate, and landscape connectivity on 3 Kawartha lakes (Ontario, Canada) since the early 1800s

Laird KR, Li S, Gushulak CAC, Moir KE, Wang Y, Leavitt PR, Cumming BF. 2023. Influence of cultural eutrophication, climate, and landscape connectivity on 3 Kawartha lakes (Ontario, Canada) since the early 1800s . Lake Reserv Manage. 39:120–140. Paleolimnological analyses of 3 lakes within the Trent-Severn Waterway (TSW) were examined to evaluate the role of regional land-use practices (forestry and agriculture), climate change, and landscape position on cultural eutrophication and lake response over the past ∼200 yr. The lakes were selected to represent lake position along the chain of lakes of the TSW; these 3 lakes vary in lake-water conditions from a lower nutrient headwater lake (Cameron Lake) to more nutrient-rich sites (Pigeon and Stony lakes) downstream. Diatom-inferred total phosphorus (TP) concentrations estimate that all lakes had TP levels of ∼15 µg/L in the 1700s, which increased to ∼20–30 µg/L in the 1800s and 1900s. Significant changes in diatom and Cladocera assemblages circa the 1830s are consistent with increased water levels and elevated phosphorus conditions associated with the intensification of forest harvest, agriculture, and development of the TSW. Diatom-inferred TP levels varied among lakes since the early 1900s, declining in Cameron Lake, increasing in Pigeon Lake, and stabilizing in Stony Lake, reflective of landscape position, degree of water inflow from the northern forested regions, and responsiveness of basins to point-source nutrient reductions and climate change. Timing of changes in TP, and changes in planktonic community composition varied among lakes. Lake production (as pigments) increased coherently in 2 lakes over the past ∼20 yr, and modern algal composition (diatoms and pigments) are lake specific and unique in comparison to the last ∼200 yr, likely reflecting combined effects of climate change and changes in nutrient loading.

Evaluation of Biogeochemical Changes in Channelized and Restored Portions of a Subtropical Floodplain

Floodplains are critically important ecosystems that provide a whole suite of ecosystem services, including nutrient and carbon sequestration, flood mitigation, water storage, and critical wildlife habitat. However, human modification of rivers and floodplains through channelization, artificial levee construction, reductions in the active floodplain area, and water management can significantly reduce the ecosystem function of river–floodplain systems. In this study, we evaluated the changes in the nutrient loading of the Kissimmee River floodplain during the restoration of the river–floodplain system. In addition to time-series loading analysis, we also evaluated soil nutrient concentrations across the lower portion of the Kissimmee River floodplain. During the 44-year nutrient loading time-series, the floodplain remained a nutrient exporter with changes in nutrient loading generally corresponding to both water quality (i.e., point source reductions) and hydrologic restoration activities in the watershed and Kissimmee River floodplain. During the study period, inputs of total phosphorus and total nitrogen loads from upstream either significantly increased or remained the same. In addition to external sources of nutrients, internal sources of nutrients from floodplain soils can also contribute to the total nutrient export from the system. These internal sources could be organic via the decomposition of organic matter or geologic from the original excavation of the canal and/or restoration backfilling. Soil nutrient concentrations vary between vegetative communities and landscape position and could be a significant source of phosphorus to the downstream system, which is plagued by eutrophic conditions. Therefore, as floodplain function in the Kissimmee River continues to be restored and managed, additional effort may be needed to address nutrient inputs and internal legacy nutrients.

Changing water quality and thermocline depth along an aquaculture gradient in six tropical crater lakes

Understanding how lakes respond to changes in nutrient loading along a productivity gradient can help identify key drivers of aquatic change, thereby allowing appropriate mitigation strategies to be developed. Physical, chemical and biological water column measurements combined with long-term water monitoring data for six closely located crater lakes, in Southeast Asia, were compared to assess the response of lakes along a productivity gradient equating to a transect of increasing aquaculture intensity. Increasing chlorophyll a (phytoplankton biomass) in the upper waters appeared to modify the thermocline depth and light availability causing a shift from a deep chlorophyll maximum at low aquaculture intensity to the emergence of algal dead zones lower in the water column with high aquaculture intensity. High phosphorus loading and light limitation from enhanced algal biomass, associated with high aquaculture intensity, exacerbated nitrogen drawdown, leading to the prevalence of potentially nitrogen-fixing cyanobacteria. Seasonal overturn during the cooler season resulted in low dissolved oxygen concentrations in the epilimnion, potential harmful algal blooms, a reduction in the habitable depth for fish and ultimately increased mortality amongst farmed fish.

Distribution of dinoflagellate cysts in the modern sediments from the coastal aquaculture area and its adjacent oceanic shelf in southeast China

Changes in nutrient loading due to coastal aquaculture and the targeted expansion of monitoring programs over the past few decades have amply demonstrated a marked increase in harmful algal blooms (HABs). In this study, viable dinoflagellate cyst composition and assemblages were investigated by evaluating and classifying their distribution patterns in surface sediments of the Fujian aquaculture coastal region of southeast China. The surface sediment at 5 stations during the spring and summer of 2019 to classify the dinoflagellate cysts present and identified 36 species representing six known and three uncertain taxa groups. The highest cyst was recorded at coastal station P11 in the spring (744.7 cysts g−1 DW), while the lowest cyst was observed at coastal station P5 in spring (50.9 cysts g−1 DW). The number and abundance of the heterotrophic species were greater than the autotrophic species, with the highest species volume belonging to the protoperidinioid group. We also detected potentially harmful algae bloom cysts that hold the potential to form their initial blooms in the sediments. It is important to note that six dinoflagellate cysts were found in the waters along the Fujian coast, as compared to historical data. Although the abundance of these cysts was not high, their presence indicates a potentially grave risk for toxic red tides. More alarmingly, these results suggest that seedbeds may trigger future blooms and outbreaks of toxic species in the present study area.

Time after time: detecting annual patterns in stream bacterial biofilm communities.

SummaryTo quantify the major environmental drivers of stream bacterial population dynamics, we modelled temporal differences in stream bacterial communities to quantify community shifts, including those relating to cyclical seasonal variation and more sporadic bloom events. We applied Illumina MiSeq 16S rRNA bacterial gene sequencing of 892 stream biofilm samples, collected monthly for 36‐months from six streams. The streams were located a maximum of 118 km apart and drained three different catchment types (forest, urban and rural land uses). We identified repeatable seasonal patterns among bacterial taxa, allowing their separation into three ecological groupings, those following linear, bloom/trough and repeated, seasonal trends. Various physicochemical parameters (light, water and air temperature, pH, dissolved oxygen, nutrients) were linked to temporal community changes. Our models indicate that bloom events and seasonal episodes modify biofilm bacterial populations, suggesting that distinct microbial taxa thrive during these events including non‐cyanobacterial community members. These models could aid in determining how temporal environmental changes affect community assembly and guide the selection of appropriate statistical models to capture future community responses to environmental change.

The Response of North Sea Ecosystem Functional Groups to Warming and Changes in Fishing

Achieving Good Environmental Status (GES) requires managing ecosystems subject to a variety of pressures such as climate change, eutrophication, and fishing. However, ecosystem models are generally much better at representing top-down impacts from fishing than bottom-up impacts due to warming or changes in nutrient loading. Bottom-up processes often have to be parameterised with little data or worse still taken as a system input rather than being represented explicitly. In this study we use an end-to-end ecosystem model (StrathE2E2) for the North Sea with 18 broad functional groups, five resource pools, and representations of feeding, metabolism, reproduction, active migrations, advection, and mixing. Environmental driving data include temperature, irradiance, hydrodynamics, and nutrient inputs from rivers, atmosphere, and ocean boundaries, so the model is designed to evaluate rigorously top-down and bottom-up impacts and is ideal for looking at possible changes in energy flows and “big picture” ecosystem function. In this study we considered the impacts of warming (2 and 4°C) and various levels of fishing, by demersal and pelagic fleets, on the structure and function of the foodweb. A key aim is to demonstrate whether monitoring of broad ecosystem groups could assist in deciding whether GES was being achieved. We found that warming raised primary productivity and increased the size (total biomass) of the ecosystem. Warming raised metabolic demands on omnivorous zooplankton and reduced their abundance, thus favouring benthivorous and piscivorous demersal fish at the expense of planktivorous pelagic fish but otherwise had modest effects on energy pathways and top predators, whereas changes in fishing patterns could materially alter foodweb function and the relative outcomes for top predators. We suggest that GES should be defined in terms of an unfished state and that abundances of broad groupings and the balance between them can help to assess whether indicator outcomes were consistent with GES. Our findings underwrite the need for an ecosystem approach for the management of human activities supported by relevant monitoring. We also highlight the need to improve our basic understanding of bottom-up processes, improve their representation within models, and ensure that our ecosystem models can capture growth limitation by nitrogen and other elements, and not just food/energy uptake.

Land Use Change to Reduce Freshwater Nitrogen and Phosphorus will Be Effective Even with Projected Climate Change

Recent studies have demonstrated that projected climate change will likely enhance nitrogen (N) and phosphorus (P) loss from farms and farmland, with the potential to worsen freshwater eutrophication. Here, we investigate the relative importance of the climate and land use drivers of nutrient loss in nine study catchments in Europe and a neighboring country (Turkey), ranging in area from 50 to 12,000 km2. The aim was to quantify whether planned large-scale, land use change aimed at N and P loss reduction would be effective given projected climate change. To this end, catchment-scale biophysical models were applied within a common framework to quantify the integrated effects of projected changes in climate, land use (including wastewater inputs), N deposition, and water use on river and lake water quantity and quality for the mid-21st century. The proposed land use changes were derived from catchment stakeholder workshops, and the assessment quantified changes in mean annual N and P concentrations and loads. At most of the sites, the projected effects of climate change alone on nutrient concentrations and loads were small, whilst land use changes had a larger effect and were of sufficient magnitude that, overall, a move to more environmentally focused farming achieved a reduction in N and P concentrations and loads despite projected climate change. However, at Beyşehir lake in Turkey, increased temperatures and lower precipitation reduced water flows considerably, making climate change, rather than more intensive nutrient usage, the greatest threat to the freshwater ecosystem. Individual site responses did however vary and were dependent on the balance of diffuse and point source inputs. Simulated lake chlorophyll-a changes were not generally proportional to changes in nutrient loading. Further work is required to accurately simulate the flow and water quality extremes and determine how reductions in freshwater N and P translate into an aquatic ecosystem response.

From local measures to regional impacts: Modelling changes in nutrient loads to the Baltic Sea

Study RegionOur study region is the Baltic Sea Drainage Basin (BSDB), which covers an area of 1.8 Mio km2 distributed over 14 countries in northern Europe. Study FocusWe use a large-scale hydrological and nutrient transport model (E-HYPE) to model basin-wide impacts of measure scenarios on the Baltic Sea, where eutrophication is a critical issue for the marine ecosystem. We constructed measure scenarios based on stakeholder acceptance, established in workshops in different regions around the Baltic. These measures include local stream reach to catchment scale measures aiming to reduce nutrient transport into the stream network (buffer strips, stormwater ponds) and measures aiming to reduce regional nutrient source releases (fertiliser leaching rates, rural household emissions). New Hydrological Insights for the RegionNutrient load reductions are often needed to reduce eutrophication and improve overall surface water quality in fresh-water and enclosed bays and seas, where dilution is limited and load emissions have long residence times. To reduce riverine nutrient loads, remediation measures are necessary, e.g. establishment of buffer strips or improved wastewater treatment. Such measures are, however, typically not designed to target nutrient load reductions at sea outlets, but rather focus on local improvements. Here, we show that measures, notwithstanding other meliorating ecosystem benefits, must include reductions in load emissions across large basin areas to impact integrated net loads into coastal sea basins, because reduction measures that only target nutrient delivery to the stream network typically cannot be implemented in a significant enough proportion of the total drainage area of large coastal river basins. Our impact scenarios show BSDB-wide nutrient reductions of up to 9 % for nitrogen and phosphorus compared to a reference scenario, if load emissions are reduced in the scenario assumptions.

Multi-Year Simulation of Western Lake Erie Hydrodynamics and Biogeochemistry to Evaluate Nutrient Management Scenarios

During the 1970s, harmful cyanobacteria (HFCB) were common occurrences in western Lake Erie. Remediation strategies reduced total P loads and bloom frequency; however, HFCB have reoccurred since the mid-1990s under increased system stress from climate change. Given these concurrent changes in nutrient loading and climate forcing, there is a need to develop management tools to investigate historical changes in the lake and predict future water quality. Herein, we applied coupled one-dimensional hydrodynamic and biogeochemical models (GLM–AED) to reproduce water quality conditions of western Lake Erie from 1979 through 2015, thereby removing the obstacle of setting and scaling initial conditions in management scenarios. The physical forcing was derived from surface buoys, airports, and land-based stations. Nutrient loads were reconstructed from historical monitoring data. The root-mean-square errors between simulations and observations for water levels (0.36 m), surface water temperature (2.5 °C), and concentrations of total P (0.01 mg L−1), PO4 (0.01 mg L−1), NH4 (0.03 mg L−1), NO3 (0.68 mg L−1), total chlorophyll a (18.74 μg L−1), chlorophytes (3.94 μg L−1), cyanobacteria (12.44 μg L−1), diatoms (3.17 μg L−1), and cryptophytes (3.18 μg L−1) were minimized using model-independent parameter estimation, and were within literature ranges from single year three-dimensional simulations. A sensitivity analysis shows that 40% reductions of total P and dissolved reactive P loads would have been necessary to bring blooms under the mild threshold (9600 MTA cyanobacteria biomass) during recent years (2005–2015), consistent with the Annex 4 recommendation. However, these would not likely be achieved by applying best management practices in the Maumee River watershed.

Integrated ecosystem impacts of climate change and eutrophication on main Baltic fishery resources

The Baltic Sea is a heavily impacted ecosystem with multiple pressures acting simultaneously. In order to quantify ecosystem impacts of integrated climate change and eutrophication pressures under constant high fishing pressure, and to support decision-making and policies in generating environmental and economic sustainable systems, the Baltic Atlantis holistic and mechanistic ecosystem model was applied. The overall aim was to run scenarios of separate and integrated impacts of climate and riverine nutrient load changes, taking into account the interactions of the full food web in the entire Baltic Sea. This was done to identify which of those two pressures will likely dominate the future of the Baltic Sea ecosystem, and to test effects of different riverine nutrient forcing sources as well as the Baltic Atlantis functions in relation to hydrographic spawning thresholds. By integrating the hydrography, the biology covering all trophic levels of the food web, and multiple pressures, i.e. eutrophication, climate change and fishery, we were able to evaluate relative impacts of 3 climate scenarios and 3 nutrient load scenarios, using two sources of nutrient forcing and predict likely trends in ecosystem effects. With focus on major fish stocks, our model, with its assumptions, indicated that nutrient loads are the main driver of the changes in the ecosystem as long as the hydrographic thresholds for spawning are not reached. If the thresholds are reached for the Baltic cod, climate change impact will become most important. Furthermore, higher nutrient loads resulted in cod decrease, and increase in sprat and herring. This effect is amplified by stronger climate change. Overall, it is of crucial importance for the future of the Baltic Sea fisheries and stocks that potential impacts are considered both separate and integrated in a dynamic ecosystem-based management approach.

Modelling framework for flow, sediments and nutrient loads in a large transboundary river watershed: A climate change impact assessment of the Nemunas River watershed

This study provides an in-depth analysis of the Nemunas River watershed, situated in the Baltic Sea basin, and possible future changes to the stream flow, hydrologic regime, sediment (SS), Total Nitrogen (TN) and Total Phosphorus (TP) load from the river to the Curonian Lagoon under different climate change scenarios using high-resolution modelling. The sub-regions of the watershed, represented by sub-basins of the Nemunas River tributaries and the main river branch, were modelled using the Soil and Water Assessment Tool (SWAT). The model setup was performed using the developed customizable MATLAB scripts for an advance Hydrologic Response Unit configuration and a hillslope delineation procedure. The modelling framework was used to assess the climate changes in the watershed under two future scenarios: RCP4.5 and RCP8.5, using the projected changes to precipitation, temperature, and carbon dioxide concentrations for the near-term (up to 2050) and long-term period (up to 2100) compared to the baseline period (1995–2010). The projections show that the annual long-term trends for flow, depending on the scenario, are small or not existing. Even if there is no visible change in trends over the years, inter-annual changes on flow, sediment and nutrient load will occur. The findings of the study suggest that the conditions of the RCP4.5 are likely to be more “wet”, whereas RCP8.5 will likely be “drier”. Most shifts will occur in the winter season, especially in January and February. A decrease in snow cover across the watershed, together with the greater frequency of soil freeze-thaw cycles can weaken the nutrient retention of soils and increase nitrogen and phosphorus losses. Coupled with the increased flows in winter, the projected nutrient load changes during winter season indicate a two-fold increase in sediment, up to 42% and 62% in TN and TP load to the Nemunas River and subsequently, to the Curonian lagoon.

Physiological Responses of the Copepods Acartia tonsa and Eurytemora carolleeae to Changes in the Nitrogen:Phosphorus Quality of Their Food

Two contrasting estuarine copepods, Acartia tonsa and Eurytemora carolleeae, the former a broadcast spawner and the latter a brood spawner, were fed a constant carbon-based diatom diet, but which had a variable N:P content, and the elemental composition (C, N, P) of tissue and eggs, as well as changes in the rates of grazing, excretion, egg production and viability were measured. To achieve the varied diet, the diatom Thalassiosira pseudonana was grown in continuous culture at a constant growth rate with varying P supply. Both copepods altered their chemical composition in response to the varied prey, but to different degrees. Grazing (clearance) rates increased for A. tonsa but not for E. carolleeae as prey N:P increased. Variable NH4+ excretion rates were observed between copepod species, while excretion of PO43− declined as prey N:P increased. Egg production by E. carolleeae was highest when eating high N:P prey, while that of A. tonsa showed the opposite pattern. Egg viability by A. tonsa was always greater than that of E. carolleeae. These results suggest that anthropogenically changing nutrient loads may affect the nutritional quality of food for copepods, in turn affecting their elemental stoichiometry and their reproductive success, having implications for food webs.

Hot moments and hotspots of cyanobacteria hyperblooms in the Curonian Lagoon (SE Baltic Sea) revealed via remote sensing-based retrospective analysis

A temporally and spatially detailed historical (1985–2018) analysis of cyanobacteria blooms was performed in the Curonian Lagoon (Lithuania, Russia), the largest coastal lagoon in the Baltic Sea. Satellite data allowed the mapping of cyanobacteria surface accumulations, so-called “scums”, and of chlorophyll-a concentration. The 34-year time series shows a tendency towards later occurrence (October–November) of the cyanobacteria scum presence, whereas the period of its onset (June–July) remains relatively constant. The periods when scums are present, “hot moments”, have been consistently increasing in duration since 2008. The differences in the starting, ending and annual duration of cyanobacteria blooms have been significantly altered by hydro-meteorological conditions (river discharge, water temperature, and wind conditions) and their year-round patterns. The most important environmental factors that determined the temporal changes of the scum presence and area were the standing stock of cyanobacteria and the ambient wind conditions. The “hotspots”, the areas where the blooms most likely occur, were distributed in the south-southwestern and central parts of the lagoon. The least affected areas were the northern part, which is connected to the coastal waters of the Baltic Sea, and the Nemunas River delta region. The longstanding, well-established spatial patterns of cyanobacteria blooms were linked to hydrodynamic features, namely water renewal time and current patterns, and to potential nutrient sources that included muddy sediments and the locations of colonies of piscivorous birds. Our findings confirmed that the annual and seasonal variations of cyanobacteria blooms and their regulation are a complex issue due to interactions between multiple factors over spatially and temporally broad scales. Despite great progress in the prevention and control of eutrophication and cyanobacteria blooms, the lagoon is still considered to be in a poor ecological status. This work provides a new and missing understanding on the spatial and temporal extent of cyanobacteria blooms and the factors that govern them. Such an understanding can help in planning management strategies, forecasting the magnitude and severity of blooms under changing nutrient loads and potential climate scenarios.

The long-term response of lake nutrient and chlorophyll concentrations to changes in nutrient loading in Ireland's largest lake, Lough Neagh

The long-term response of chlorophyll-a (chl-a) to changing lake water nutrient concentrations and increasing water temperature was investigated in Lough Neagh, a large, hypereutrophic lake in Northern Ireland. Trends in external and internal nutrient loading and their relation to lake nutrient concentrations were also established. Lake water concentrations of total P (TP) have increased since the 1990s but were not correlated with catchment inputs, which showed no trend. The characteristics of internal loading of P have changed since the mid-1990s, with an earlier and larger mass of P released from the sediments each summer. Catchment inputs of total oxidised N (NOx) decreased from the peak value of 10,186T/yr in 1995 to 5,396T/yr in 2011, coinciding with a reduction in lake water concentrations. External inputs and lake concentrations of NOx were highly correlated (R=0.88). Water temperature increased approximately 1C and was a predictor of variation in chl-a from 1974 to 2012. After the peak chl-a concentration in 1993, dissolved inorganic N (DIN) also became an important predictor, accounting for almost half of the 44% variance explained by a hierarchical partition model. Decreasing log (DIN:TP) ratios suggest that N limitation of chl-a has become more important in the lake recently.