Internal Phosphorus Loading Research Articles

Reducing nutrient increases diatom biomass in a subtropical eutrophic lake, China–Do the ammonium concentration and nitrate to ammonium ratio play a role?

Response of aquatic organisms to eutrophication have been well reported, while less studies are available for the recovery of eutrophic lakes following a reduction in the external loading, especially for systems where nitrogen is reduced but the phosphorus concentration is maintained high due to internal loading. Diatoms are nitrate (NO3-N) opportunists but can also use ammonium (NH4-N). They may, therefore, be more sensitive to nitrogen reduction than other algae that typically prefer NH4-N. We document the variations of nutrients and diatoms in subtropical, eutrophic Lake Taihu over 28 yr during which a reduction of the external loading resulted from lake management. According to the results of change point analysis, data on environmental variables were divided into two periods (P1: 1992–2006; P2: 2007–2019) with two different seasons (WS: Winter-Spring; SA: Summer-Autumn), respectively. Compared with P1-WS, the concentration of NH4-N decreased significantly whereas NO3-N showed no significant change in P2-WS. In contrast, NH4-N concentrations were low and showed no significant changes in P1-SA and P2-SA and NO3-N decreased significantly in the latter period. Accordingly, NO3-N: NH4-N mass ratios in P1-SA and P2-WS were all significantly higher than those in P2-SA and P1-WS, respectively. The biomass of WS diatom increased significantly and the timing of the peak biomass shifted from P1-SA to P2-WS since 2007. The SEM analysis showed that NO3-N was retained as a statistically significant predictor for diatom biomass in P1-SA and significant effects of windspeed, zooplankton and NH4-N on diatom biomass in P2-WS. Windspeed and zooplankton have further changed the biomass of diatoms in the case of declining inorganic nitrogen. We conclude that the magnitude of vernal suppression or stimulation of diatom assemblages has increased, concomitant with the variations of NH4-N and NO3-N: NH4-N mass ratios. Diatoms response to NH4-N or NO3-N is apparently changing in response to water temperature in this eutrophic shallow lake. Thus, parallel reductions in external nitrogen loading, along with variations in dominant inorganic nitrogen, will stimulate the growth of diatom and therefore increase the total biomass of phytoplankton in still high internal phosphorus loading, which is should be regarded as a good sign of restoration measures.

Annual water residence time effects on thermal structure: A potential lake restoration measure?

Innovative methods to combat internal loading issues in eutrophic lakes are urgently needed to speed recovery and restore systems within legislative deadlines. In stratifying lakes, internal phosphorus loading is particularly problematic during the summer stratified period when anoxia persists in the hypolimnion, promoting phosphorus release from the sediment. A novel method to inhibit stratification by reducing residence times is proposed as a way of controlling the length of the hypolimnetic anoxic period, thus reducing the loading of nutrients from the sediments into the water column. However, residence time effects on stratification length in natural lakes are not well understood. We used a systematic modelling approach to investigate the viability of changes to annual water residence time in affecting lake stratification and thermal dynamics in Elterwater, a small stratifying eutrophic lake in the northwest of England. We found that reducing annual water residence times shortened and weakened summer stratification. Based on finer-scale dynamics of lake heat fluxes and water column stability we propose seasonal or sub-seasonal management of water residence time is needed for the method to be most effective at reducing stratification as a means of controlling internal nutrient loading.

Internal phosphorus loading and its driving factors in the dry period of Brazilian semiarid reservoirs

This study investigated the relationships between physical, limnological and climatic drivers with the internal total phosphorus (TP) loading produced over the dry period in 30 water supply reservoirs of the Brazilian semiarid. Improvements in the understanding of sedimentary TP fluxes in reservoirs of dryland regions are pressing as they usually have serious water quality related issues, remaining mostly eutrophic especially under frequent drought events. Gross daily fluxes and net seasonal average release rates were calculated from mass balance and regression equations considering water and sediment TP concentrations, anoxic duration, water temperature and fish contribution. Additionally, the ratio of wind speed to reservoir volume was proposed as a new surrogate and then applied as explanatory variable to predictive models. The results indicated TP release rates higher than reported for non-semiarid lakes/reservoirs with average gross fluxes ranging from 17.64 to 35.99 mg m−2 day−1. This may be attributed to the enriched sediments (1029.49 ± 552.49 mg kg−1) allied with warmer water temperature, high trophic state, and prolonged anoxic periods (average duration of about 60 days). The average release rates were negatively correlated with water transparency and water depth, and positively correlated with Chl-a, wind speed and trophic state. The release rates increased across the trophic gradient (p < 0.05), about 10-fold higher under hypertrophic conditions than in oligotrophic ones. As anoxia is linked with eutrophication, phosphorus release is more likely in eutrophic ecosystems. Regarding the new surrogate, a strong predictive ability for TP release (R2: 0.26–0.93) was observed. Similarly, the proposed models presented a physically consistent behavior with a stabilizing releasing pattern suggesting the achievement of equilibrium in nutrient exchange between sediment-water interface. This research advanced by combining and proposing methods to assess and quantify sedimentary fluxes in data-scarce regions balancing accuracy and transferability, in order to be replicable to other dryland environments globally.

Adsorption of humic acids to lake sediments: Compositional fractionation, inhibitory effect of phosphate, and implications for lake eutrophication

Humic acid (HA) and phosphate interactions play a vital role in the biogeochemical cycle of carbon and nutrients and thus the trophic state of a lake. The adsorption behavior of HAs to sediments in the absence and presence of phosphate was investigated in this study. Three types of HAs were used, AHA from algae-dominated lake sediments, MHA from macrophyte-dominated lake sediments, and a reference HA (RHA) with terrestrial sources. The adsorption capacity of lake sediments was highest for AHA, which can be explained by that AHA contained more carboxyl-containing molecules, proteinaceous compounds and polysaccharides that were preferentially adsorbed by minerals. Phosphate showed a stronger inhibitory effect on MHA adsorption than on AHA adsorption, suggesting that AHA can more effectively replace phosphate adsorbed to sediments. Our findings show that the functional groups of organic compounds control not only their fractionation and burial but also their ability to replace phosphate in sediments. We propose a novel mechanism to explain the legacy effect of lake eutrophication. That is, as lakes shift from a macrophyte-dominated state to more eutrophic, algae-dominated state, increasing algae-derived organic compounds can promote the release of phosphate from sediments, forming a positive feedback loop that sustains internal phosphorus loading and hence lake eutrophication.

Decreased Internal Phosphorus Loading From Eutrophic Sediment After Artificial Light Supplement: Preliminary Evidence From a Microcosm Experiment

Nutrient loading reduction is an essential approach for controlling eutrophication. In addition to external nutrient loading, internal phosphorus (P) loading is usually considered one of the most important factors determining nutrient levels in water. The underwater light climate plays a pivotal role. However, few studies have been reported on the effect of light intensity on P release, and the existing studies have not drawn a definitive conclusion on whether underwater light has a positive or negative effect on P release. To confirm the effect of underwater light on internal P release, a 1-month microcosm experiment was carried out (18 November–15 December 2020) under three light intensities. The P release flux (FP) was significantly higher in the control (no light) group than in the low light group during the first 2 weeks (p = 0.03). No difference among treatments occurred for accumulative FP in the past 4 weeks, although it tended to be higher in the control than in both the low and high light groups (p &amp;gt; 0.05). Spearman rank correlations showed that photosynthetic photon flux density (PPFD) was positively correlated with DO, pH, phytoplankton chlorophyll a (Chl aPhyt) and benthic algae chlorophyll a (Chl aBent). DO was positively correlated with Chl aPhyt and Chl aBent. The results indicate that light may promote the growth of phytoplankton and benthic algae, both of which may increase DO and pH. DO inhibits P release from sediment and pH promotes P release from sediment. Lake-scale studies are needed to fully evaluate its effect under natural conditions, as many other factors (e.g., wind or fish disturbance) in natural lakes could regulate P release from sediment.

Alum Addition Triggers Hypoxia in an Engineered Pit Lake.

Here, we examine the geobiological response to a whole-lake alum (aluminum sulfate) treatment (2016) of Base Mine Lake (BML), the first pilot-scale pit lake established in the Alberta oil sands region. The rationale for trialing this management amendment was based on its successful use to reduce internal phosphorus loading to eutrophying lakes. Modest increases in water cap epilimnetic oxygen concentrations, associated with increased Secchi depths and chlorophyll-a concentrations, were co-incident with anoxic waters immediately above the fluid fine tailings (FFT) layer post alum. Decreased water cap nitrate and detectable sulfide concentrations, as well as increased hypolimnetic phospholipid fatty acid abundances, signaled greater anaerobic heterotrophic activity. Shifts in microbial community to groups associated with greater organic carbon degradation (i.e., SAR11-LD12 subclade) and the SRB group Desulfuromonodales emerged post alum and the loss of specialist groups associated with carbon-limited, ammonia-rich restricted niches (i.e., MBAE14) also occurred. Alum treatment resulted in additional oxygen consumption associated with increased autochthonous carbon production, watercap anoxia and sulfide generation, which further exacerbate oxygen consumption associated with on-going FFT mobilized reductants. The results illustrate the importance of understanding the broader biogeochemical implications of adaptive management interventions to avoid unanticipated outcomes that pose greater risks and improve tailings reclamation for oil sands operations and, more broadly, the global mining sector.

Assessment of phosphorus loading dynamics in a tropical reservoir with high seasonal water level changes

Nutrient accumulation in man-made reservoirs has been documented worldwide. Therefore, quantifying phosphorus loading and understanding its temporal dynamics in reservoirs is mandatory for sustainable water management. In this study, the Vollenweider's complete-mix phosphorus budget model was adapted to account for high water level variations, which are a common feature in tropical reservoirs, and for internal phosphorus loads. First- and zero-order kinetics were adopted to simulate phosphorus settling and release from the sediment layer, respectively, considering variable area of phosphorus release according to the height of the anoxic layer. The modeling approach was applied for a 52-months period to a 31-years-old reservoir located in the semiarid region of Brazil with 7.7 hm3 storage capacity. The simulations were supported by hydrological, meteorological and water quality data, as well as analyses of phosphorus partitioning of the reservoir bed sediment. The external phosphorus load was estimated from a relationship adjusted between inflow and phosphorus concentration, revealing an u-shaped pattern. Sedimentary phosphorus linked to iron and aluminum (PFeAl) increased over time and along the reservoir. Such measurements were used to estimate the internal phosphorus load, i.e., the yield from the bed sediments to the water column. The adaptations proposed to the model's structure improved its capacity to simulate phosphorus concentration in the water column, from “not satisfactory” to “good”. We estimate that the internal phosphorus load currently accounts for 44% of the total load. It prevailed during the wet period, when reservoir stratification and hypolimnetic hypoxia were more notable, resulting in higher phosphorus concentration in the water column due to the combined effects of internal and external loadings. However, if the reservoir were 70 years older, the internal load would reach 83% of the total and the reservoir would become a source instead of a sink of phosphorus.

Simulated impacts of climate change on Lake Simcoe water quality

ABSTRACT Lake Simcoe has undergone eutrophication and hypoxia since the 1960s. Climate change, leading to enhanced summer thermal stratification, has been identified as a key stressor. In this study, we modeled the impacts of climate change on hydrodynamics and biogeochemistry in Lake Simcoe by applying a 1-dimensional (vertical) model forced with A2 and B1 scenario outputs from a global climate model over 2000–2100. The model was calibrated in 2008 and validated in 2009, with maximum root mean square error (RMSE) of modelled temperature between 1.5 and 3.0 °C and dissolved oxygen RMSE between 0.5 and 2.5 mg L−1. Phytoplankton chlorophyll a was simulated with RMSE between 1.25 µg L−1 (large diatoms) and ∼0.5 µg L−1 (other groups). Interannual variability in spring water temperature and length of stratification were related to changes in the North Atlantic and Artic Oscillation indices, respectively. Under A2 and B1 forcing, the duration of stratification will increase by 45 and 38 days in summer between spring and fall turnover, respectively. The extended stratified period leads to a reduction in hypolimnetic dissolved oxygen from 3–7 to <3 mg L−1, thereby reducing the quality of cold-water fish habitat and increasing internal phosphorus loading from the benthos. These internal loads, combined with increased water temperatures, lead to increased cyanobacteria concentrations, beginning around 2070.

Molecular weight driving bioavailability and intrinsic degradation mechanisms of dissolved organic phosphorus in lake sediment

The sediment dissolved organic phosphorus (DOP) for the “internal phosphorus (P) loading” has raised intensive concern, but its bioavailability and intrinsic degradation mechanism have not been fully elucidated. In this work, multi-techniques were combined to construct the response of sediments DOP's bioavailability to molecular weight (MW) based on ten lakes of China, thereby elucidating the intrinsic degradation mechanism of sediment DOP. A high percentage (74.5% on average) and significantly positive correlations with respect to different MWs were observed, highlighting the importance of DOP to dissolved P in sediments. DOP is mainly composed of a low MW (LMW) portion (63.8%) and the substances are primarily derived from microbial sources. Bioavailable DOP species were closely related to MW, with labile monoester P and diester P decreased with decreasing MW. Analysis of environmental processes showed that microbial utilization capacity and the characteristics of dissolved organic matter (DOM) with different MWs were the dominant drivers in determining the bioavailability of DOP. That is, microorganisms exhibit high DOM utilization capacity in LMW portion, promoting the degradation and transformation of bioavailable DOP species. Furthermore, the increased humic and fulvic-like substances by microbial degradation might in turn inhibit the enzymatic hydrolysis of LMW-DOP. This pattern explains why the contents of LMW-DOP are very high, but it contains less bioavailable DOP. By studying the bioavailability of sediment DOPs with different MWs, it is found that, under natural conditions, labile monoester and diester P in LMW-DOP have a high tendency to degrade than those in HMW-DOP. The results further show that, microbial utilization and DOM characteristics, as well as their linkage with DOP's bioavailability and degradability, have important implications for assessing DOP's degradation potential. The insights from this study might shed light on more effective strategies for mitigating the risks of "internal P loading".

The nexus among long-term changes in lake primary productivity, deep-water anoxia, and internal phosphorus loading, explored through analysis of a 15,000-year varved sediment record

Increased cultural eutrophication since the 20th century, caused by phosphorus (P) enrichment, has become a major problem worldwide. In deep, stratified lakes, eutrophication-induced hypolimnetic anoxia often stimulates the release of labile P from the sediment into the water column. This positive feedback, termed internal P loading, maintains or even accelerates eutrophication. However, most studies on internal P loading have focused on recent times. Little is known about whether such positive feedbacks caused by labile P release from sediments also played a role under natural conditions with little or no human impact. We investigated a high-resolution 15,000-year sediment record of paleoproduction, anoxia, and five sedimentary P fractions from a small, deep lake, Soppensee, on the Swiss Central Plateau. We estimated long-term qualitative internal P loading by comparing the Holocene record of diatom-inferred epilimnetic total P (DI-TP) concentrations with labile P fraction (Fe P) concentrations in sediments under changing trophic state, redox, and lake mixing regimes. Intensified P cycling from sediments into the water column (enhanced internal P loading) apparently occurred as a positive feedback to natural eutrophication with persistent bottom-water anoxia during the early to mid-Holocene (~9000–6000 cal BP). However, this positive feedback was not inferred for other eutrophic phases. Fe-rich layers formed during seasonal mixing of the lake in the late Holocene (~2000–200 cal BP) and magnetite-type minerals produced by magnetotactic bacteria (MTB) internal P loading during anoxic phases in the mid- to late Holocene (~6000–2000 cal BP) appeared to prevent internal P loading. MTB presence resulted in high concentrations of potentially labile Fe P in sediments. Our study demonstrates the potential contribution of internal P loading during long-term natural eutrophication of deep stratified lakes and has wide implications for lake management and restoration. Our results highlight the importance of the coupled geochemical cycles of P and Fe in the long-term trophic state evolution of stratified, ferruginous, low-sulfate-water lakes, conditions that have been reported to serve as analogs for the Archaean Ocean. • First qualitative estimates of internal P loadings in a deep stratified lake during the Holocene. • Evidence of pre-anthropogenic feedbacks between productivity, anoxia and P cycling. • Microorganisms modulate the Holocene geochemical cycle of P under meromictic conditions. • Postglacial forest and climate changes affect lake productivity, anoxia and P cycling.

Phosphorus internal loading and sediment diagenesis in a large eutrophic lake (Lake Chaohu, China)

Sediment phosphorus (P) release and retention are important in controlling whole-system P dynamics and budget in eutrophic lakes. Here we combine short- (seasonal) and long-term (years to decades) studies to quantify the internal P loading and P release potential in the sediments of Lake Chaohu and explore their controlling mechanisms. In the west region of the lake, short-term P diffusive fluxes ranged from 0.2 mg/m2·d−1 to 6.69 mg/m2·d−1 (averaged 2.76 mg/m2·d−1) and long-term net P release ranged from 2.25 mg/m2·d−1 to 8.94 mg/m2·d−1 (averaged 5.34 mg/m2·d−1); in the east region, short-term P diffusive fluxes varied from 0.73 mg/m2·d−1 to 1.76 mg/m2·d−1 (averaged 1.05 mg/m2·d−1) and long-term P release ranged from 0.13 mg/m2·d−1 to 4.15 mg/m2·d−1 (averaged 1.3 mg/m2·d−1). Both short- and long-term P releases were in the same order of magnitudes as the external P inputs (3.56 mg/m2·d−1). Comparison of the long-term and short-term sediment P release indicates that while the high summer P release in the east might only represent a snapshot value, the sediments in the west contribute to large P release for years or even decades, impeding water quality recovery under lake management. Mobilization of surface sediment legacy P accounted for 81% of short-term P release. The long-term release was dominated by remobilization of iron bond P (BD-P) (average 52.1%) at all sites, while Aluminium-bound P (NaOH-rP) exhibited partly reactive and potentially mobile, releasing P to the water column in most sites in the west. Our study demonstrates the importance of sediments as P sources in lake Chaohu. The combination of short- and long-term P release studies can help understand the roles of sediments in regulating the water quality and eutrophication.

Control of internal phosphorus loading using coagulants and clays in water and the sediment of a semiarid reservoir susceptible to resuspension

Control of internal phosphorus loading using coagulants and clays in water and the sediment of a semiarid reservoir susceptible to resuspension

Simultaneous adsorption of phosphate and zinc by lanthanum modified zeolite

Lanthanum modified zeolite (LMZ) has been proven to be an effective inactivation agent for lake internal phosphorus load control. However, its performance in the co-sequestration of P and heavy metal is still unclear. Thus, simultaneous adsorption of phosphate and zinc by LMZ was investigated in this study, and its co-inactivation performance in sediment was also verified. Separate adsorption capacities of LMZ for phosphate and zinc were 2.31 and 0.367 mmol/g, respectively. In co-adsorption scenario, removal rate of zinc (0.15 mmol/L) increased from 73.53 to 84.60% when co-existing phosphate increased from 0.097 to 0.48 mmol/L (pH 6.5), while zinc had an inapparent impact on phosphate adsorption. Furthermore, an increase of background NaCl concentration in the system inhibited zinc removal by 25%, indicating that part of zinc was physically adsorbed by the zeolite fraction of LMZ. XPS and Raman spectra suggested that phosphate in the simultaneous adsorption system was innerspherically adsorbed by lanthanum oxide component of LMZ, while zinc was adsorbed by forming both outer-sphere complex and inner-sphere complex. Part of zinc could chelate with the adsorbed phosphate, forming phosphate bridged ternary complex and enhancing zinc removal. Finally, a lab-scale inactivation experiment showed that simultaneous stabilization of P and zinc in sediment could be achieved by dosing 5% weight percentage LMZ, making it a promising inactivation agent in the future. • Simultaneous adsorption of phosphate and zinc by lanthanum modified zeolite (LMZ) was investigated. • Zinc adsorption was enhanced by co-existing phosphate, while phosphate adsorption was not influenced by zinc. • LMZ is a promising agent for the simultaneous inactivation of P and zinc in sediment.

The spatial variations of water quality and effects of water landscape in Baiyangdian Lake, North China.

Baiyangdian Lake (BYD), a large shallow lake in North China, has complex water landscape patterns that are underlies spatial variations in water quality. In this study, we collected 61 water samples from three water landscapes (reed littoral zones, fish ponds, and open water) and analyzed them for water quality parameters, such as dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP). Water landscape distribution (determined using remote sensing imagery) was then used to assess correlations between water quality parameters and water landscape proportion in differently scaled buffer zones. There was substantial variation across all subareas, with TN and TP concentrations ranging from 0.90 to 4.10mg/L and 0.06 to 0.18mg/L, respectively, in class IV of water quality as a whole. Spatial variations in water quality were mainly caused by water landscape distribution and external nutrient inputs. There were negative correlations between DOC, TN, and TP concentrations and the area proportion of reed littoral zones in the 300 and 500m buffers. In contrast, DOC, TN, and TP concentrations were significantly positively correlated with the area proportion of fish ponds in the 100m buffer. Furthermore, compared with reed littoral zones, a lower ratio of nitrogen to phosphorus and a higher proportion of dissolved organic nitrogen and tyrosine-like proteins were found in fish ponds. These effects were mainly attributed to the development of internal sediment loadings due to nutrient exchange across the sediment-water interface. Therefore, dredging-based sediment removal from fish ponds should be considered to suppress internal phosphorus loading and accelerate recovery of the BYD ecosystem.

A New Framework for Modelling Fine Sediment Transport in Rivers Includes Flocculation to Inform Reservoir Management in Wildfire Impacted Watersheds

Fine-grained cohesive sediment is the primary vector for nutrient and contaminant redistribution through aquatic systems and is a critical indicator of land disturbance. A critical limitation of most existing sediment transport models is that they assume that the transport characteristics of fine sediment can be described using the same approaches that are used for coarse-grained non-cohesive sediment, thereby ignoring the tendency of fine sediment to flocculate. Here, a modelling framework to simulate flow and fine sediment transport in the Crowsnest River, the Castle River, the Oldman River and the Oldman Reservoir after the 2003 Lost Creek wildfire in Alberta, Canada was developed and validated. It is the first to include explicit description of fine sediment deposition/erosion processes as a function of bed shear stress and the flocculation process. This framework integrates four existing numerical models: MOBED, RIVFLOC, RMA2 and RMA4 using river geometry, flow, fine suspended sediment characteristics and bathymetry data. Sediment concentration and particle size distributions computed by RIVFLOC were used as the upstream boundary condition for the reservoir dispersion model RMA4. The predicted particle size distributions and mass of fine river sediment deposited within various sections of the reservoir indicate that most of the fine sediment generated by the upstream disturbance deposits in the reservoir. Deposition patterns of sediment from wildfire-impacted landscapes were different than those from unburned landscapes because of differences in settling behaviour. These differences may lead to zones of relatively increased internal loading of phosphorus to reservoir water columns, thereby increasing the potential for algae proliferation. In light of the growing threats to water resources globally from wildfire, the generic framework described herein can be used to model propagation of fine river sediment and associated nutrients or contaminants to reservoirs under different flow conditions and land use scenarios. The framework is thereby a valuable tool to support decision making for water resources management and catchment planning.

Low Benthic Oxygen and High Internal Phosphorus-Loading are Strongly Associated With the Invasive Macrophyte Nitellopsis obtusa (starry stonewort) in a Large, Polymictic Lake

Lake Scugog is an important headwater to the Trent Severn-Waterway in Ontario, Canada. In recent years, notable ecosystem-level changes have occurred coinciding with the emergence of the non-native invasive charophyte Nitellopsis obtusa. Despite N. obtusa arriving in North America in the early 1970s, studies documenting the impact of N. obtusa on invaded ecosystems are scarce. Given the increasing dominance of N. obtusa in inland waters of the Great Lakes basin, we investigated the ecosystem-level impacts of N. obtusa in Lake Scugog over a 3-year period. We show for the first time a strong association between N. obtusa occurrence and biomass with benthic anoxia in this shallow, polymictic lake. Benthic dissolved oxygen concentrations were significantly lower (p-value &amp;lt; 0.001) at sites with N. obtusa compared to sites without N. obtusa. Additionally, N. obtusa biomass was a negative predictor of near-bed oxygen concentration (R2 = 0.59, p-value &amp;lt; 0.001). Knowing that anoxia can promote the internal loading of phosphorus, we measured soluble reactive phosphorus (SRP) in the pore-water of sediments at each site, and found N. obtusa biomass explained 90% of sediment pore-water SRP (R2 = 0.90, p-value &amp;lt; 0.001). These notable associations between N. obtusa and key lake elements indicates that N. obtusa may be acting as an ecosystem engineer in invaded lakes by altering the biogeochemical fate of oxygen and phosphorus.

Accelerated sediment phosphorus release in Lake Erie's central basin during seasonal anoxia

AbstractEutrophication remains a serious threat to Lake Erie and has accelerated over past decades due to human activity in the watershed. Internal phosphorus (P) loading from lake sediment contributes to eutrophication, but our understanding of this process in Lake Erie is more uncertain than for its riverine P inputs. Past study has focused on incubating sediment cores in oxic or anoxic conditions, meaning we know little about sediment flux during state transitions. We used 56 controlled sediment core incubation experiments to quantify rates and onset of P release in Lake Erie's central basin as a function of depositional environment, season (spring, summer, and fall), temperature, and dissolved oxygen (DO) concentration. P flux under oxic or hypoxic (&gt; 0 to ≤ 2 mg L−1 DO) conditions was slow (0.31–0.50 mg m−2 d−1) compared to anoxic P flux (5.19–30.7 mg m−2 d−1). The transition between slow and fast flux occurred within 24 h of anoxia (0 mg L−1 DO). Oxic or anoxic P flux was generally similar across seasons and incubation temperatures (8°C and 14°C). In 14°C incubated cores anoxic P flux onset was earliest in fall, when sediments had already been exposed to anoxic conditions in the lake. Re‐oxygenation of experimental cores that temporarily developed anoxia reversed the direction of P flux, but P release resumed at similar rates once the water returned to anoxia. Understanding the effects of hypolimnion oxygen conditions on internal P loading allows us to better constrain nutrients sources and implications for P budget management.

Assessment of Trophic Responses of a Reservoir to Seasonal and Annual Variations in Monsoon

The variations in water quality parameters and trophic status of a multipurpose reservoir in response to changing intensity of monsoon rain was investigated by applying a trophic state index deviation (TSID) analysis and an empirical regression model to the data collected in two periods from 2014 to 2017. The reservoir in general maintained mesotrophic conditions, and Carlson’s trophic state index (TSIc) was affected most by TSITP. Nutrient concentrations, particularly phosphorus, did not show strong correlations with precipitation, particularly in the period with weak monsoon, and a significant increase in total phosphorus (TP) was observed in Spring 2015, indicating the possibility of internal phosphorus loading under decreased depth and stability of water body due to a lack of precipitation. TSIChl was higher than TSISD in most data in period 1 when a negligible increase in precipitation was observed in the monsoon season while a significant fraction in period 2 showed the opposite trend. Phytoplankton growth was not limited by nutrient limitation although nutrient ratios (N/P) of most samples were significantly higher than 20, indicating phosphorus-limited condition. TSID and regression analysis indicated that phytoplankton growth was limited by zooplankton grazing in the Spring, and that cell concentrations and community structure in the monsoon and post-monsoon season were controlled by the changing intensity of the monsoon, as evidenced by the positive and negative relationships between community size and cyanobacterial population with the amount of precipitation in the Summer, respectively. The possibility of contribution from internal loading and an increase in cyanobacterial population associated with weak monsoon, in addition to potential for nutrient enrichment in the post-monsoon season, implies a need for the application of more stringent water quality management in the reservoir that can handle all potential scenarios of eutrophication.

Simulating Nutrients and Phytoplankton Dynamics in Lakes: Model Development and Applications

Due to eutrophication, many lakes require periodic management and restoration, which becomes unpredictable due to internal nutrient loading. To provide better lake management and restoration strategies, a deterministic, one-dimensional water quality model MINLAKE2020 was modified from daily MINLAKE2012 by incorporating chlorophyll-a, nutrients, and biochemical oxygen demand models into the regional year-around temperature and dissolved oxygen (DO) model. MINLAKE2020 was applied to six lakes (varying depth and trophic status) in Minnesota focusing on studying the internal nutrient dynamics. The average root-mean-square errors (RMSEs) of simulated water temperature and DO in six lakes are 1.51 °C and 2.33 mg/L, respectively, when compared with profile data over 2–4 years. The average RMSE of DO simulation decreased by 24.2% when compared to the MINLAKE2012 model. The internal nutrient dynamics was studied by analyzing time series of phosphorus, chlorophyll-a, and DO over several years and by performing a sensitivity analysis of model parameters. A long-term simulation (20 years) of Lake Elmo shows that the simulated phosphorus release from sediment under the anoxic condition results in surface phosphorus increase, which matches with the observed trends. An average internal phosphorus loading increase of 92.3 kg/year increased the average daily phosphorus concentration by 0.0087 mg/L.

Estimates of internal loading of phosphorus in a western US reservoir using 3 methods

Burnet SH, Wilhelm FM. 2021. Estimates of internal loading of phosphorus in a western US reservoir using 3 methods. Lake Reserv Manage. 37:261–274. We used 3 methods to estimate internal loading of total and dissolved phosphorus (TP/DP) in 2 years in a stratified eutrophic western US reservoir with high internal loading of phosphorus. We calculated the internal load from (i) net increases of TP/DP from reservoir water column samples, (ii) gross TP mass balances, and (iii) gross TP/DP loading estimated from laboratory-incubated sediment cores and the anoxic reservoir area. We also present a cost comparison and spreadsheet to allow managers to estimate costs for each method. The highest internal loads were estimated in 2015 from the net in situ samples (999 mg/m2/yr) and the gross annual mass balance (850 mg/m2/yr). Gross estimates from sediment cores were 1.2× higher in 2015 than in 2014 due to differences in anoxic area resulting from a drought and record reservoir drawdown. Costs were highest for the mass balance method and lowest for the sediment core method. Given the small monetary difference between the sediment core and in situ methods, we suggest managers obtain in situ data to estimate internal loading because this yields a net estimate most reflective of whole-lake dynamics. Our data provide an important benchmark of internal loading in a human-made reservoir given the general paucity of such estimates in this large geographic region.