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The hypothesis that local hypoxia and chlorophyll concentration are spatially tethered to local, sediment-driven nutrient release was examined in a small, nutrient-impacted estuary in the Southern Gulf of St. Lawrence, Canada. Sediment reactor core samples were taken at 10 locations between 0.25 and 100% of the estuary area in spring and fall (2019) and used to estimate nitrogen and phosphate flux. Sediment organic matter, carbonate, percent nitrogen, percent carbon, δ13C, and δ15N were measured from the reactor core stations. Oxygen was recorded continually using oxygen loggers while chlorophyll and salinity were measured bi-weekly. A hydrodynamic model was used to determine water renewal time at each station. The most severe eutrophication effects were in the upper one-fifth of the estuary. There were strong local relationships between sediment biogeochemistry, hypoxia, and chlorophyll metrics but not with water renewal time. Internal nutrient loading represented 65% and 69% of total N loading, and 98% and 89% of total P loading to the estuary in June and September, respectively. Sediment nitrogen flux was highly predictable from a range of local sediment variables that reflect either nutrient content, or organic carbon enrichment in general. Percent nitrogen and percent carbon were highly correlated but sediment P flux was poorly predicted from sediment parameters examined. The highest correlations were with percent nitrogen and percent carbon. These results indicate that incorporating internal nutrient loading into nutrient monitoring programs is a critical next step to improve predictive capacity for eutrophication endpoints and to mitigate nutrient effects.

Eutrophication is a main threat to continental aquatic ecosystems. Prevention and amelioration actions have been taken under the assumption of a stable climate, which needs reconsideration. Here, we show that reduced precipitation can bring a lake ecosystem to a more productive regime even with a decline in nutrient external load. By analyzing time series of several decades in the largest lake of the Iberian Peninsula, we found autocorrelated changes in the variance of state variables (i.e., chlorophyll and oxygen) indicative of a transient situation towards a new ecosystem regime. Indeed, exceptional planktonic diatom blooms have occurred during the last few years, and the sediment record shows a shift in phytoplankton composition and an increase in nutrient retention. Reduced precipitation almost doubled the water residence time in the lake, enhancing the relevance of internal processes. This study demonstrates that ecological quality targets for aquatic ecosystems must be tailored to the changing climatic conditions for appropriate stewardship.

Submerged macrophytes are crucial for the restoration of shallow eutrophic lake but they are diminished in coverage or lost with eutrophication. Their recovery after nutrient loading reduction depends on water and sediment nutrient levels. We studied the combined impacts of sediment fertility (low/high nitrogen and phosphorus content) and water nutrient concentrations (low/high nitrogen and phosphorus addition) on Vallisneria denseserrulata in a mesocosm experiment. We hypothesized that both the elevated external nutrient addition and high sediment nutrient contents would inhibit plant growth. We found that an increase in nutrient concentrations resulted in a significant increase in algal biomass. Furthermore, high external nutrient addition significantly reduced both the relative growth rate (RGR) and the density of V. denseserrulata growing in the nutrient-rich sediment, while in the nutrient-poor sediment treatment, RGR was not affected but the plant density decreased. Interestingly, low nutrient addition appeared to be more conducive to growth and reproduction of V. denseserrulata in the nutrient-rich sediment than in the nutrient-poor sediment. Our findings emphasize the importance of reducing external nutrient inputs is of key higher importance when restoring shallow eutrophic lakes, while the plants may benefit of the nutrient-rich sediment occurring in such lakes after eutrophication.

Lakes play a crucial role in the nutrient cycling of Earth, despite covering only a small fraction of the planet’s surface. Their interactions with their surrounding catchment areas significantly impact ecosystems and regulatory services. The connection between a lake and its catchment, especially the drainage ratio (catchment area to lake surface area), shapes the characteristics of lakes and their response to catchment processes. Within the catchment area, geological, land cover, and land use factors influence the composition of stream water that flows into the lake. These factors play a role in transporting various substances, both organic and inorganic, to the streams. Lakes act as dynamic filters, altering the chemical composition of water that flows through them. This study aims to investigate how a large, shallow lake impacts the quality of the river water as it passes through. It builds on an analysis of nutrient (carbon, nitrogen, phosphorus, silicon) fluxes into Lake Võrtsjärv, using six years of monthly monitoring data from five main inflows and the outflow. The research explores how catchment characteristics and hydrology affect nutrient concentrations and loadings into the lake, as well as the retention or release of substances by the lake. Findings reveal that catchment characteristics, such as land use and forest cover, significantly influence water quality parameters. Different inflows showed variations in water quality, and annual variations were observed, largely correlated with precipitation and discharge. Võrtsjärv plays a critical role in retaining or releasing nutrients, with varying impacts depending on the water budget of the lake. In years with a positive water balance, the lake retains all nutrients, whereas in dry years only inflowing N and P loads exceed their outflow. Overall, this study underscores the importance of lakes as integral components of catchment ecosystems, shedding light on their complex interactions with the environment and the implications for water quality. It emphasizes the need for careful consideration of land use and hydrological factors in managing and preserving these vital aquatic systems.

Our study aimed to analyze the effects of chronic nutrient loading on the capacity of headwater streams to retain phosphorus and ammonium pulses of different duration. For this purpose, we selected nine headwater streams located across a gradient of increasing agricultural land use and eutrophication. In each stream, we performed sequential plateau additions with increasing nutrient concentrations in summer 2015 and instantaneous slug additions in summer 2016 under similar hydrological conditions. We modelled kinetic uptake curves from the slug additions via the Tracer Additions for Spiraling Curve Characterization method and calculated ambient uptake parameters. Ambient uptake rates generally increased (1.4–20.8 μg m⁻² s⁻¹ for NH₄–N and 0.3–10.3 μg m⁻² s⁻¹ for SRP, respectively), while ambient uptake velocities decreased from oligotrophic to polytrophic streams (1.8–14.0 mm min⁻¹ for NH₄–N and 1.6–9.9 mm min⁻¹ for SRP, respectively). However, correlations between ambient uptake parameters and background concentrations were weak. Concentration-dependent uptake rates followed either a linear or a Michaelis–Menten saturation model, regardless of the degree of nutrient loading. Uptake rate curves showed counter-clockwise hysteresis in oligotrophic streams and clockwise hysteresis in streams of higher trophic states, indicating a reduced significance of hyporheic uptake with increasing nutrient loading. Comparisons of slug and plateau additions revealed that oligotrophic streams were most efficient in uptake during short nutrient pulses, while eutrophic streams profited from longer pulse duration. The results indicate that nutrient uptake is increasingly transport-controlled in polluted streams where increased biofilm thickness and clogging of sediments restrict nutrient transport to reactive sites.

Lake Trafford, a 600-ha subtropical lake in southwestern Florida, has suffered from over 50 years of cultural eutrophication, resulting in the invasion of Hydrilla verticillata and organic sediment accumulation due to herbicide treatments. This study aimed to assess the effects of dredging on nutrient dynamics. A pre-dredging nutrient budget, developed using land use models and climatic data, estimated nutrient loads of 190 kg d−1 for total nitrogen (TN) and 18.6 kg d−1 for total phosphorus (TP), with total maximum daily loads (TMDLs) of 70.4 kg d−1 for TN and 4.15 kg d−1 for TP. Post-dredging analysis, using detailed spatiotemporal data, showed higher nutrient loads of 274.3 kg d−1 for TN and 24.2 kg d−1 for TP. While dredging reduced legacy nutrient accumulation, it led to increased nutrient influx from groundwater, caused by the exposure of organic sediment, as evidenced by increased lake water electrical conductivity. These findings demonstrate the importance of conducting thorough pre-dredging assessments to mitigate unintended consequences, offering practical insights for managing nutrient loads and improving restoration strategies in eutrophic lakes.

Sediment porewater nutrients often occur at concentrations that are orders of magnitude higher than nutrients in overlying waters, and accordingly may subsidise growth of benthic macroalgal mats in estuarine ecosystems. The relative contribution of porewater nutrients is expected to be particularly important for macroalgae entrained in intertidal mudflat sediments, where access to water column nutrients is tidally constrained. In this study, filamentous Gracilaria chilensis thalli were simultaneously exposed to sediment and overlying water nutrient sources, labelled using ¹⁵N tracers (¹⁵NH₄⁺ or ¹⁵NO₃⁻) during a 5-day experiment. Dissolved inorganic N (DIN) uptake from porewater and overlying water accounted for 33 and 52%, respectively, of the N estimated as necessary to support the growth of G. chilensis, despite the two-fold lower DIN concentration of the overlying water and its periodic availability (8 h day⁻¹). Of the total N assimilated by the plants, ∼ 15% could not be accounted for, supporting the acquisition of other N forms in order to meet demand. We also found that regardless of background NH₄⁺:NO₃⁻ ratios (i.e. 1:3 in overlying water and 12:1 in porewater), plants accumulated ¹⁵NH₄⁺ significantly more readily than ¹⁵NO₃⁻, indicating a preference for NH₄⁺. This ability to utilise multiple sources and species of N relatively rapidly may partly explain the competitive success of entrained macroalgae relative to non-entrained species and historically abundant seagrass beds in these environments. These results underscore the significance of both internal nutrient loading and external inputs as important in sustaining opportunistic macroalgal blooms in shallow estuaries.

Major efforts have been made world-wide to improve the ecological quality of shallow lakes by reducing external nutrient loading. These have often resulted in lower in-lake total phosphorus (TP) and decreased chlorophyll a levels in surface water, reduced phytoplankton biomass and higher Secchi depth. Internal loading delays recovery, but in north temperate lakes a new equilibrium with respect to TP often is reached after <10-15 years. In comparison, the response time to reduced nitrogen (N) loading is typically <5 years. Also increased top-down control may be important. Fish biomass often declines, and the percentage of piscivores, the zooplankton:phytoplankton biomass ratio, the contribution of Daphnia to zooplankton biomass and the cladoceran size all tend to increase. This holds for both small and relatively large lakes, for example, the largest lake in Denmark (40 km²), shallow Lake Arresø, has responded relatively rapidly to a ca. 76% loading reduction arising from nutrient reduction and top-down control. Some lakes, however, have proven resistant to loading reductions. To accelerate recovery several physico-chemical and biological restoration methods have been developed for north temperate lakes and used with varying degrees of success. Biological measures, such as selective removal of planktivorous fish, stocking of piscivorous fish and implantation or protection of submerged plants, often are cheap versus traditional physico-chemical methods and are therefore attractive. However, their long-term effectiveness is uncertain. It is argued that additional measures beyond loading reduction are less cost-efficient and often not needed in very large lakes. Although fewer data are available on tropical lakes these seem to respond to external loading reductions, an example being Lake Paranoá, Brazil (38 km²). However, differences in biological interactions between cold temperate versus warm temperate-subtropical-tropical lakes make transfer of existing biological restoration methods to warm lakes difficult. Warm lakes often have prolonged growth seasons with a higher risk of long-lasting algal blooms and dense floating plant communities, smaller fish, higher aggregation of fish in vegetation (leading to loss of zooplankton refuge), more annual fish cohorts, more omnivorous feeding by fish and less specialist piscivory. The trophic structures of warm lakes vary markedly, depending on precipitation, continental or coastal regions locations, lake age and temperature. Unfortunately, little is known about trophic dynamics and the role of fish in warm lakes. Since many warm lakes suffer from eutrophication, new insights are needed into trophic interactions and potential lake restoration methods, especially since eutrophication is expected to increase in the future owing to economic development and global warming.

Atmospheric deposition (AD) is a less understood and quantified source of nutrient loading to waterbodies. AD occurs via settling (large particulates), contact (smaller particulates and gaseous matter), and precipitation (rain, snow) transport pathways. Utah Lake is a shallow eutrophic freshwater lake located in central Utah, USA, with geophysical characteristics that make it particularly susceptible to AD-related nutrient loading. Studies have shown AD to be a significant contributor to the lake’s nutrient budget. This study analyzes nutrient samples from nine locations around the lake and four precipitation gauges over a 6-year study period using three different methods to estimate AD from the precipitation transport pathway. The methods used are simple averaging, Thiessen polygons, and inverse distance weighting, which we use to spatially interpolate point sample data to estimate nutrient lake loads. We hold that the inverse distance weighting method produces the most accurate results. We quantify, present, and compare nutrient loads and nutrient loading rates for total phosphorus (TP), total inorganic nitrogen (TIN), and ortho phosphate (OP) from precipitation events. We compute loading rates for the calendar year (Mg/yr) from each of the three analysis methods along with monthly loading rates where Mg is 106 g. Our estimated annual precipitation AD loads for TP, OP, and TIN are 120.96 Mg/yr (132.97 tons/yr), 60.87 Mg/yr (67.1 tons/yr), and 435 Mg/yr (479.5 tons/yr), respectively. We compare these results with published data on total AD nutrient loads and show that AD from precipitation is a significant nutrient source for Utah Lake, contributing between 25% and 40% of the total AD nutrient load to the lake.

This review summarized the past and current studies on forest nutrient export and existing watershed water quality models that are capable of predicting nutrient loadings from forest-dominated watersheds. Emphasis was given to the watershed models used under cold climate conditions and their capacities and limitations in assessing the impacts of forest best management practices (BMPs) and climate change scenarios on nutrient loadings at a watershed scale. The nutrient export rates in forest-dominated watersheds were found to vary significantly controlled by local climate and landscape conditions. Some watershed water quality models can estimate nutrient loadings from forests either with a simplified forest growth function or without a forest growth component. No existing watershed water quality models have explicit representation forest BMP functions. Combining or coupling with a forest growth model is required for a realistic simulation of nutrient dynamics and assessing the impact of forest BMPs in a forest-dominated watershed. The review also considered the suitability of models for exploring the potential effects of climate change on hydrologic and nutrient processes relevant to forest management. Discussions on the challenges and limitations of forested watershed water quality models and recommendations for future development were made following the review. The findings of this study can provide valuable references for water quality modeling studies in forest-dominated watersheds under cold climate conditions.