External Phosphorus Loading Research Articles

Effects of internal phosphorus loadings and food-web structure on the recovery of a deep lake from eutrophication

Abstract We used monitoring data from Lake Lugano (Switzerland and Italy) to assess key ecosystem responses to three decades of nutrient management (1983–2014). We investigated whether reductions in external phosphorus loadings (Lext) caused declines in lake phosphorus concentrations (P) and phytoplankton biomass (Chl a), as assumed by the predictive models that underpinned the management plan. Additionally, we examined the hypothesis that deep lakes respond quickly to Lext reductions. During the study period, nutrient management reduced Lext by approximately a half. However, the effects of such reduction on P and Chl a were complex. Far from the scenarios predicted by classic nutrient-management approaches, the responses of P and Chl a did not only reflect changes in Lext, but also variation in internal P loadings (Lint) and food-web structure. In turn, Lint varied depending on basin morphometry and climatic effects, whereas food-web structure varied due to apparently stochastic events of colonization and near-extinction of key species. Our results highlight the complexity of the trajectory of deep-lake ecosystems undergoing nutrient management. From an applied standpoint, they also suggest that [i] the recovery of warm monomictic lakes may be slower than expected due to the development of Lint, and that [ii] classic P and Chl a models based on Lext may be useful in nutrient management programs only if their predictions are used as starting points within adaptive frameworks.

Restoring lakes through external phosphorus load reduction: the case of Lake Pusiano (Southern Alps)

We estimated the external phosphorus load (EPL) between 1960 and 2015 in Lake Pusiano, a mid-sized subalpine lake that reached its maximum trophic state in the mid-1980s. Using historical data we also estimated the internal phosphorus load (IPL) between 1972 and 2015. EPL reached its maximum in the mid-1980s with phosphorus (P) values ~21 t yr−1 compared to the current value close to 6 t yr−1. IPL was one order of magnitude less and ranged between 0.25 t yr−1 (2015) and 3.6 (1985) t yr−1. The strong reduction of the P load determined a marked decrease of both P (from 200 to 23 μg L−1) and chlorophyll a (from 18 to 8 μg L−1) in-lake concentrations. The process of eutrophication and subsequent recovery, however, showed hysteresis between P load and in-lake P concentrations. In recent years, in particular, the P concentrations at winter overturn seem to be independent from the P load, related to modifications in the hydrological management of the lake that favored P flush-out in October–November, when the EPL is maximal. This process led to a marked decrease of the total P concentrations at winter overturn (23 μg L−1), which recently resulted in lower-than-target concentrations (30 μg L−1) established by the Lombardy Region. The lake, nevertheless, suffers from the presence of the toxic cyanobacterium Planktothrix rubescens, and eradicating this species and further improving water quality will likely require an additional abatement of the EPL.

Internal loading of phosphate in Lake Erie Central Basin.

After significant reductions in external phosphorus (P) loads, and subsequent water quality improvements in the early 1980s, the water quality of Lake Erie has declined considerably over the past decade. The frequency and magnitude of harmful algal blooms (primarily in the western basin) and the extent of hypoxic bottom waters in the central basin have increased. The decline in ecosystem health, despite meeting goals for external P loads, has sparked a renewed effort to understand P cycling in the lake. We use pore-water P concentration profiles and sediment cores incubation experiments to quantify the P flux from Lake Erie central basin sediments. In addition, the oxygen isotopes of phosphate were investigated to assess the isotopic signature of sedimentary phosphate inputs relative to the isotopic signature of phosphate in lake water. Extrapolating the total P sediment flux based on the pore-water profiles to the whole area of the central basin ranged from 300 to 1250metric tons per year and using the flux based on core incubation experiments an annual flux of roughly 2400metric tons of P is calculated. These estimates amount to 8-20% of the total external input of P to Lake Erie. The isotopic signature of phosphate in the extractable fraction of the sediments (~18‰) can explain the non-equilibrium isotope values of dissolved phosphate in the deep water of the central basin of Lake Erie, and this is consistent with sediments as an important internal source of P in the Lake.

Spatial distributions of external and internal phosphorus loads in Lake Erie and their impacts on phytoplankton and water quality

Re-eutrophication in Lake Erie has led to new programs to reduce external phosphorus loads, and it is important to understand the interrelated dynamics of external and internal phosphorus loads. In addition to developing phosphorus load response curves for algal biomass in the western basin and hypoxia in the central basin, we used a two-dimensional (vertical-longitudinal) hydrodynamic and ecological model to show that both external and internal phosphorus loads were distributed homogeneously in the water column in Lake Erie's western basin. In the stratified central and eastern basins phosphorus released by organic matter decay and crustacean zooplankton excretion was concentrated in the upper water column, contributing 100–119% of the phytoplankton phosphorus demand, while phosphorus released by dreissenids and from anoxic sediments was distributed primarily in the hypolimnion during the growing season. Simulated reductions in external phosphorus loads decreased individual phytoplankton groups most at times when they were normally most abundant, e.g., Microcystis decreased the most during September. Phosphorus was limiting over the simulation periods, but water temperature and light conditions also played critical roles in phytoplankton succession. While water column phosphorus responded quickly to external phosphorus reduction, pulses of phosphorus (riverine input or sediment resuspension) occurring immediately before the Microcystis bloom period could allow it to bloom despite long-term external phosphorus load reduction. Studies are warranted to assess the contribution of seasonal dynamics in phosphorus loading (including sediment resuspension) to Microcystis bloom development.

High-resolution modeling for development of nearshore ecosystem objectives in eastern Lake Erie

Abstract We develop a high-resolution (600 m) three-dimensional water quality model for Lake Erie capable of resolving predominant physical processes to study nutrient dynamics, with a particular emphasis on the northern nearshore region of Lake Erie's eastern basin. The lake model output in conjunction with the Cladophora growth model (CGM) is used to predict Cladophora growth. The models were validated using extensive nearshore water quality, Cladophora biomass and tissue phosphorus measurements collected during April–September of 2013. Together, the models were used to evaluate the response of nearshore phosphorus concentrations and Cladophora growth due to changes in external phosphorus loading. The water quality model described here was able to resolve the nearshore dominant physical processes that occur within the Cladophora habitat zone (0–8 m depth) and revealed that phosphorus concentrations in the nearshore are governed by a combination of local inputs from the Grand River and exchange with the hypolimnion. Frequent upwelling events driven by favorable winds of 5–10 days period dominated nearshore–offshore exchanges, and in some years (e.g., 2013) can potentially add significant phosphorus (as soluble reactive phosphorus; SRP) to the nearshore during the spring; however, in other years (e.g., 2008), their effects are less important. The model demonstrates that the offshore supply of phosphorus via upwelling is of ecological significance with respect to Cladophora and that both lake-wide and local action may be required to address nuisance blooms of Cladophora in Lake Erie.

Modeling the response of phytoplankton to reduced external nutrient load in a subtropical Chinese reservoir using DYRESM-CAEDYM

Cui Y, Zhu G, Li H, Luo L, Cheng X, Jin Y, and Trolle D. Modeling the response of phytoplankton to reduced external nutrient load in a subtropical Chinese reservoir using DYRESM-CAEDYM. Lake Reserv Manage. 32: 146--157.The DYRESM-CAEDYM model was used to quantify the potential effects of reductions in external nutrient loads to the Shahe Reservoir in China, with an emphasis on effects on phytoplankton biomass. DYRESM-CAEDYM was calibrated using field data for a 2-year period and validated against a separate 1-year period. Simulated values for water temperature, dissolved oxygen, total nitrogen, total phosphorus, and chlorophyll a showed good agreement with field measurements. The model tended to underestimate peak values of inorganic nutrients such as ammonium, nitrate, and phosphate, with phosphate levels close to detection limits in both observed and simulated values. The model successfully captured seasonal phytoplankton succession, indicating spring peaks of diatom biomass and summer peaks of cyanophytes. Scenarios in which different reductions in external loading were applied suggest that simultaneous reductions in both external nitrogen and external phosphorus loading is more effective in controlling phytoplankton biomass than reducing input of either nutrient alone. Diatoms responded more strongly to load reductions in phosphorus input than nitrogen load, whereas cyanophytes showed the opposite pattern. A combined 25% reduction in the external loading of both nutrients could significantly decrease phytoplankton biomass in the reservoir below the 7 μg/L chlorophyll a required by drinking water standards, and the study indicated that DYRESM-CAEDYM can be a useful management tool for eutrophication control.

Negligible effect of hypolimnetic oxygenation on the trophic state of Lake Jyväsjärvi, Finland

Hypolimnetic oxygenation by pumping oxygen-rich surface water to the hypolimnion (HLO) is a commonly used tool for the restoration of nutrient-loaded dimictic lakes. However, in recent years its effectiveness has been questioned. In this case study we evaluated monitoring data covering a period of 23-years to show that, although experimental cessation of HLO drastically changed the lake's temperature and dissolved oxygen regimes, it did not significantly affect its trophic status. Thus, we recommend that the limited financial resources available are better directed towards further lowering the lake's external phosphorus load than continuing HLO.

Geo-engineering experiments in two urban ponds to control eutrophication

Many urban ponds experience detrimental algal blooms as the result of eutrophication. During a two year field experiment, the efficacy of five in situ treatments to mitigate eutrophication effects in urban ponds was studied. The treatments targeted the sediment phosphorus release and were intended to switch the ponds from a turbid phytoplankton-dominated state to a clear-water state with a low phytoplankton biomass. Two eutrophic urban ponds were each divided into six compartments (300–400 m2; 210–700 m3). In each pond the following treatments were tested: dredging in combination with biomanipulation (involving fish biomass control and the introduction of macrophytes) with and without the addition of the flocculant polyaluminiumchloride, interception and reduction of sediment phosphorus release with lanthanum-modified bentonite (Phoslock®) in combination with biomanipulation with and without polyaluminiumchloride; biomanipulation alone; and a control. Trial results support the hypothesis that the combination of biomanipulation and measures targeting the sediment phosphorus release can be effective in reducing the phytoplankton biomass and establishing and maintaining a clear-water state, provided the external phosphorus loading is limited. During the experimental period dredging combined with biomanipulation showed mean chlorophyll-a concentrations of 5.3 and 6.2 μg L−1, compared to 268.9 and 52.4 μg L−1 in the control compartments. Lanthanum-modified bentonite can be an effective alternative to dredging and in combination with biomanipulation it showed mean chlorophyll-a concentrations of 5.9 and 7.6 μg L−1. Biomanipulation alone did not establish a clear-water state or only during a limited period. As the two experimental sites differed in their reaction to the treatments, it is important to choose the most promising treatment depending on site specific characteristics. In recovering the water quality status of urban ponds, continuing attention is required to the concurrent reduction of external phosphorus loading and to maintaining an appropriate fish community.

Shelf-to-basin iron shuttling enhances vivianite formation in deep Baltic Sea sediments

Coastal hypoxia is a growing and persistent problem largely attributable to enhanced terrestrial nutrient (i.e., nitrogen and phosphorus) loading. Recent studies suggest phosphorus removal through burial of iron (II) phosphates, putatively vivianite, plays an important role in nutrient cycling in the Baltic Sea — the world's largest anthropogenic dead zone — yet the dynamics of iron (II) phosphate formation are poorly constrained. To address this, a reactive-transport model was used to reconstruct the diagenetic and depositional history of sediments in the Fårö basin, a deep anoxic and sulphidic region of the Baltic Sea where iron (II) phosphates have been observed. Simulations demonstrate that transport of iron from shelf sediments to deep basins enhances vivianite formation while sulphide concentrations are low, but that pyrite forms preferentially over vivianite when sulphate reduction intensifies due to elevated organic loading. Episodic reoxygenation events, associated with major inflows of oxic waters, encourage the retention of iron oxyhydroxides and iron-bound phosphorus in sediments, increasing vivianite precipitation as a result. Results suggest that artificial reoxygenation of the Baltic Sea bottom waters could sequester up to 3% of the annual external phosphorus loads as iron (II) phosphates, but this is negligible when compared to potential internal phosphorus loads due to dissolution of iron oxyhydroxides when low oxygen conditions prevail. Thus, enhancing vivianite formation through artificial reoxygenation of deep waters is not a viable engineering solution to eutrophication in the Baltic Sea. Finally, simulations suggest that regions with limited sulphate reduction and hypoxic intervals, such as eutrophic estuaries, could act as important phosphorus sinks by sequestering vivianite. This could potentially alleviate eutrophication in shelf and slope environments.

Fighting internal phosphorus loading: An evaluation of the large scale application of gradual Fe-addition to a shallow peat lake

Abstract Abatement of external phosphorus (P) loading and biomanipulation are measures that are often applied with the aim to restore a macrophyte dominated clearwater state in turbid, anthropogenically eutrophied lakes. The recovery of such lakes, however, is often hampered by ‘internal eutrophication’, as a result of the release of historically accumulated P from the sediment into the water column. One way to combat this internal P loading is by adding iron (Fe) into the lake, which naturally binds to phosphate. Although studied in the laboratory or mesocosms, the effects of iron addition on a whole-lake scale are largely unknown. In this study we therefore compiled lake monitoring data to evaluate the effect of a gradual dose of 33 g Fe m −2 on the water quality and biotic communities (phytoplankton, zooplankton and macrophytes) of Lake Terra Nova. During and after the Fe-addition, we also carried out assays to evaluate the effect of the Fe-addition on sediment P-release rates. Lake Terra Nova is a eutrophied, shallow peaty lake that has been subjected to biomanipulation measures for 10 years. The large scale addition of iron during 2010 and 2011 resulted in a substantial reduction of dissolved P, suspended matter (SM), phytoplankton biomass and relative Cyanobacterial biomass, whereas macrophytes reappeared. Fe-addition also resulted in strongly reduced sediment P-mobilisation rates. Nevertheless, reductions of water column TP in response to the Fe-addition were short-lived. This discrepancy between the observed TP-levels in the water column and sediment P-release rates could indicate that sustained loading with external P and interactions of chemical compounds with Fe may jeopardize long term restoration success by depleting the water column reservoir of reactive Fe. Ecological processes can in addition enhance this process by shunting P from the sediment to the water column.

Métodos de control de las floraciones de cianobacterias en aguas continentales

Blooms of phytoplankton organisms, especially cyanobacteria represent an important economic and ecological problem in the management of water and aquatic ecosystems. The biomass increment, besides causing aesthetic problems, such as the appearance of foams and odors, alters the taste of drinking water and causes deoxygenation and water chemistry changes when it decomposes compromising the survival of aquatic organisms. However, blooms of cyanobacteria are the most studied and known as these organisms produce bioactive metabolites (cyanotoxins) that constitute a serious environmental problem with serious implications on human and animal health. Therefore, the need to resolve the problems associated with cyanobacteria has led to the use of multiple techniques to alleviate their uncontrolled development and improve water quality. This paper is a review of methods for cyanobacterial blooms control in inland waters. All these techniques are related to the factors that regulate the primary production. Thus, the reduction of the external phosphorus loading is sought through the control of inputs in the watershed (agriculture, sanitation, industry, etc.), but also by acting directly on the water body. Moreover, within this intervention strategy on the primary factors controlling the growth of autotrophs, techniques to limit the incident light have also been developed. Mechanical control has been addressed by physical removal of the biomass of cyanobacteria, application of ultrasounds, etc. For the chemical controls a variety of compounds with very different effects, ranging from the application of algaecides or herbicides to oxidants and other substances such as Cu, MnO4K2, H2O2, etc. have been used. Simultaneously, a large number of agents for biological control have been tested with varying success (e.g. viruses, bacteria, protozoa, fungi, aquatic plants, fish) and plant materials in aerobic decomposition have been widely used.

TMDL reevaluation: reconciling internal phosphorus load reductions in a eutrophic lake

Total maximum daily loads (TMDLs) are assigned to impaired waterbodies and provide a prescription for recovery. It is implicit that both the existing conditions and targets identified in a TMDL are based on credible and accurate information, ensuring that management actions directed to recovery are appropriate and effective. We evaluated the TMDL for phosphorus in Bear Lake, Michigan, to assess if the prescribed reduction was appropriately based on existing annual internal loads. We developed 5 different annual internal load scenarios based on phosphorus release rates from laboratory-based sediment incubations and diel dissolved oxygen measurements, ranging from very conservative to very liberal estimates of phosphorus release. The most realistic scenarios indicate that the previously established TMDL for internal phosphorus loading is 3–7 times greater than what actually occurs in Bear Lake. Based on our assessment, Bear Lake is likely already meeting the TMDL internal loading target, without the implementation of any management action. Thus, management efforts aimed at reducing the water column phosphorus concentration to reach the TMDL target should instead be directed at controlling the external phosphorus load.

Screening of Ca- and Fe-rich materials for their applicability as phosphate-retaining filters

Ecology of a large number of lakes and estuaries is affected by external phosphorus (P) loading. Many studies have investigated the applicability of different materials as P-filters for treating runoff or wastewaters. We performed flow-through retention tests and water extractions to estimate the P retention and release properties of potential Ca- and Fe-rich filter materials. In addition to the fresh Ca-rich materials, the P retention tests involved their weathered counterparts. During weathering, the materials were immersed into water to leach out soluble species (e.g., Ca2+ and OH−), thus mimicking aged filters. For the most efficient P-retainers (Filtra P®, steel slag, Sachtofer PR® and mine drainage residual), the cumulative P retentions ranged between 12 and 24mg P/g. The weathering reduced the P retentions of Filtra P® and of slag by 70–80%, but did not affect the abilities of other materials to retain P. No clear relationship existed between the P retentions and water-extractable Ca or oxalate-extractable Fe and Al amounts in the materials. During one month of extraction with a large volume of water, the maximum release of P from the P-saturated materials was 35% of the total P. To recognize the P retention mechanism (precipitation of Ca-phosphates or sorption onto metal hydroxides) and thus predict possible P release, we performed equilibration of P-saturated materials in solutions with variable pH. We suggest incorporation of weathering and P release tests in the screening scheme for identifying potential P-retaining filters.

Regime shift in the ecosystem of the eastern Gulf of Finland caused by the invasion of the polychaete Marenzelleria arctia

The changes in the concentrations of the mineral nitrogen (nitrates and nitrites) and phosphorus and in the state of the planktic communities in the eastern Gulf of Finland following a largescale invasion of the polychaete Marenzelleria arctia were analyzed. The bioirrigation and bioturbation of the bottom deposits by polychaetes resulted in a dramatic increase in the nitrogen/phosphorus ratio in the waters of the gulf, thus leading to cascade changes in the plankton. As a result of the decrease in the abundance of colonial nitrogen� fixing cyanobacteria, which cause "blooming" in surface waters, the total biomass of the phytoplankton and chlorophyll a concentration decreased. Because of the disappearance of large colonies of cyanobacteria and the surge of small algae, the food supply for the zooplankton improved and its biomass increased. According to our calculations, the amount of phosphorus deposited in the bottom sediments as a result of the polychaete activity markedly exceeded the external phosphorus load on the Gulf of Finland from the Russian coast. We conclude that the invasion of Marenzelleria arctia resulted in the cardinal reconstruction of the entire ecosys� tem of the eastern Gulf of Finland.

Modeling Lake Erie's hypoxia response to nutrient loads and physical variability

A 1-dimensional, linked hydrodynamic and eutrophication model was developed and calibrated with 19years of observations (1987–2005) for the summer stratification period in the central basin of Lake Erie, corroborated by comparison with observed process rates and areal hypoxic extents, and confirmed with observations from the 1960s and 1970s. The model effectively captures observations of both vertical and temporal trends in dissolved oxygen, as well as temporal trends in chlorophyll-a, phosphorus, zooplankton biomass, and several key processes. The model was used to develop a relationship between external phosphorus load and hypolimnion oxygen conditions, and then to establish load–response envelopes that account for inter-annual variability in physical conditions driven by variation in meteorological drivers. The curves provide a valuable tool for reassessing phosphorus loading targets with respect to reducing hypoxia in Lake Erie.

A New Phosphorus Paradigm for the Baltic Proper

The external phosphorus (P) loading has been halved, but the P content in the water column and the area of anoxic bottoms in Baltic proper has increased during the last 30 years. This can be explained by a temporary internal source of dissolved inorganic phosphorus (DIP) that is turned on when the water above the bottom sediment becomes anoxic. A load-response model, explaining the evolution from 1980 to 2005, suggests that the average specific DIP flux from anoxic bottoms in the Baltic proper is about 2.3 g P m−2 year−1. This is commensurable with fluxes estimated in situ from anoxic bottoms in the open Baltic proper and from hydrographic data in the deep part of Bornholm Basin. Oxygenation of anoxic bottoms, natural or manmade, may quickly turn off the internal P source from anoxic bottoms. This new P-paradigm should have far-reaching implications for abatement of eutrophication in the Baltic proper.Electronic supplementary materialThe online version of this article (doi:10.1007/s13280-013-0441-3) contains supplementary material, which is available to authorized users.

Understanding seasonal phosphorus dynamics to guide effective management of shallow, hypereutrophic Grand Lake St. Marys, Ohio

Grand Lake St. Marys, hereafter Grand Lake, is a large shallow reservoir in western Ohio that has become hypereutrophic in recent decades owing to external phosphorus (P) loading. To restore water quality in Grand Lake, managers must decide whether to reduce external P loading from tributaries or continue to reduce internal P loading from sediments using aluminum sulfate additions. To help managers identify the most effective strategies for water quality improvements in Grand Lake, we quantified the reservoir's seasonal P dynamics using tributary and in-reservoir P measurements during 2009–2011. We also tested the short-term effects of reduced external P loading on Grand Lake's water quality using a pond experiment performed at the adjacent St. Marys State Fish Hatchery. The highest P concentrations in Grand Lake (1660, 391, and 311 μg/L in 2009, 2010, and 2011, respectively) occurred in summer when external P loads from tributaries were minimal, suggesting internal P release from sediments was the major source of P to fuel cyanobacterial blooms. Results from our pond experiment showed that decoupling the reservoir from external P loading during summer months does not improve short-term water quality. We conclude that external P loading during winter and spring, coupled with internal P loading from sediments during summer, drive the extremely high P concentrations in summer that support dense cyanobacterial blooms. Long-term improvement in Grand Lake water quality will first require major reductions to the external P loads from tributaries, followed by manipulations to reduce internal P loading from sediments.

Geochemical impact of aquifer storage and recovery operation on fate and transport of sediment phosphorus in a large shallow lake

In response to the impact of climate change, the US Army Corps of Engineers proposed a large-scale implementation plan for an aquifer storage and recovery (ASR) project in the Kissimmee River Basin, Florida, in 2009. It is envisaged that the routine operation of the ASR will deliver recovered water from ASR wells into Lake Okeechobee with inherently different water quality parameters. However, the addition of ASR well water into such a large, shallow lake has raised concerns about sediment phosphorus stability, which could lead to increased eutrophication in Lake Okeechobee. This paper presents a geochemical assessment to explore possible impacts of the addition of ASR well water on lake sediment in terms of phosphorus adsorption, desorption, and diffusion processes via laboratory-scale batch and column tests. Based on five different mixing ratios of ASR well water and lake water, estimated isotherms, and piston velocity calculations, a mechanistic modeling analysis provided a better understanding of the fate of sediment phosphorus and its transport processes. A final multicriteria decision analysis suggests that the mixing ratio of 1:10 between ASR well water and lake water is deemed more applicable than others based on the given composition of ASR well water, which might buffer more external phosphorus loading in the long run.

Benthic Phosphorus Dynamics in the Gulf of Finland, Baltic Sea

Benthic fluxes of soluble reactive phosphorus (SRP) and dissolved inorganic carbon (DIC) were measured in situ using autonomous landers in the Gulf of Finland in the Baltic Sea, on four expeditions between 2002 and 2005. These measurements together with model estimates of bottom water oxygen conditions were used to compute the magnitude of the yearly integrated benthic SRP flux (also called internal phosphorus load). The yearly integrated benthic SRP flux was found to be almost 10 times larger than the external (river and land sources) phosphorus load. The average SRP flux was 1.25 ± 0.56 mmol m−2 d−1 on anoxic bottoms, and −0.01 ± 0.08 mmol m−2 d−1 on oxic bottoms. The bottom water oxygen conditions determined whether the SRP flux was in a high or low regime, and degradation of organic matter (as estimated from benthic DIC fluxes) correlated positively with SRP fluxes on anoxic bottoms. From this correlation, we estimated a potential increase in phosphorus flux of 0.69 ± 0.26 mmol m−2 d−1 from presently oxic bottoms, if they would turn anoxic. An almost full annual data set of in situ bottom water oxygen measurements showed high variability of oxygen concentration. Because of this, an estimate of the time which the sediments were exposed to oxygenated overlying bottom water was computed using a coupled thermohydrodynamic ocean–sea and ecosystem model. Total phosphorus burial rates were calculated from vertical profiles of total phosphorus in sediment and sediment accumulation rates. Recycling and burial efficiencies for phosphorus of 97 and 3%, respectively, were estimated for anoxic accumulation bottoms from a benthic mass balance, which was based on the measured effluxes and burial rates.

Modelled long-term development of hypoxic area and nutrient pools in the Baltic Proper

The development of nutrient pools and sediment areas covered by hypoxic water in the Baltic Proper was modelled for the 1900–2009 period. Reconstructed physical forcing and estimated past nutrient loads were used when direct observations were unavailable. The modelled pool of dissolved inorganic phosphorus (DIP) more than doubled in size over the last century — partly a direct effect of an increased external phosphorus load, and partly an indirect effect of less effective phosphorus retention in sediments associated with deteriorating oxygen conditions. As opposed to the DIP pool, the modelled pool of dissolved inorganic nitrogen (DIN) decreased over the last ~50years due to more effective denitrification associated with spreading hypoxia. The results of a hindcast scenario with low external nutrient loads indicate that long-term climate change had no significant effects on hypoxic conditions or nutrient pools during the study period. The deteriorating oxygen conditions since the 1950s were mainly related to increased external nutrient loads in combination with internal nutrient feedback processes. Furthermore, simultaneously reducing both nitrogen and phosphorus loads would have a larger positive effect on the oxygen conditions than would phosphorus reduction alone. Reducing only nitrogen loads would eventually be compensated for by nitrogen-fixing cyanobacteria.