Probabilistic Estimate of a Threshold for Eutrophication

Regime shifts, or massive changes in ecosystems, are often associated with thresholds in drivers such as climate, land-use change, nutrient fluxes, or other factors. A frequently studied example is eutrophication, which is a serious environmental problem of lakes and reservoirs associated with phosphorus (P) enrichment above a threshold. We estimated probability distributions of thresholds for eutrophication of Lake Mendota, Wisconsin, USA using 30 years of annual P budgets. Although thresholds were likely to influence eutrophication of the lake (probability 96.6%), the probability distributions of thresholds spanned a wide range of P loading rates. Management recommendations are consistent with simpler models that recommend P load targets near or below the lowest P loads observed in the past 30 years. If loads increase, there is considerable risk of crossing a threshold to sustained eutrophication with high in-lake P concentrations and poor water quality. On the other hand, if loads decrease there is a chance of crossing a mitigation threshold, causing substantial reductions in P concentrations and improvements in water quality. Consideration of these risks will increase estimates of the net economic benefits of lower P loading. Our analysis illustrates a process for estimating probability distributions for thresholds of ecosystem regime shifts. Even though threshold probability distributions may be wide with thick tails, they provide crucial information about potential consequences of alternative policy choices. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Vertical distribution and release characteristics of phosphorus forms in the sediments from the river inflow area of Dianchi Lake,China

Columnar sediment samples were collected from five representative river inflow areas of Dianchi Lake, China. The vertical distribution of each form of P were tested. Results showed that the concentration of TP in the sediments from areas A, B, C, D and E in the order of D > B > A > C > E, and the average concentration of D, B, A, C and E were 2991, 2064, 1308, 879, and 759 mg?kg^?1, respectively. The concentration of Ex-P, Fe/Al-P, Ca-P and Org-P all decreased with increasing depth. The release of Ex-P was significantly related to TP whereas the Fe/Al-P was not significantly related to TP in the samples from areas polluted by domestic sewage. However, the release of Ex-P and Fe/Al-P were both significantly related to TP in the samples from areas polluted by phosphate mining and phosphate fertilizer application. The results of equilibrium P concentration (EPC0) analysis showed that P in the sediments of areas A, D and E were the source of P in Dianchi Lake, and the P in the sediments of areas B and C were in relative equilibrium with the overlying water.     

Long-term changes in Wadden Sea nutrient cycles: importance of organic matter import from the North Sea

The Wadden Sea is a shallow tidal area along the North Sea coast of The Netherlands, Germany and Denmark. The area is strongly influenced by rivers, the most important of which are the rivers Rhine, Meuse and Elbe. Due to the increased nutrient load into the coastal zone the primary production in the Wadden Sea almost tripled during the past few decades. A conceptual model is presented that links nitrogen input (mainly nitrate) via Rhine and Meuse with the annual nitrogen cycle within the Wadden Sea. Three essential steps in the model are: (1) nitrogen limits the primary production in the coastal zone, (2) a proportional part of the primary produced organic matter is transported into the Wadden Sea and (3) the imported organic matter is remineralized within the Wadden Sea and supports the local productivity by nitrogen turn-over. The conceptual model predicts that during years with a high nutrient load more organic matter is produced in the coastal zone and more organic matter is transported into and remineralized within the Wadden Sea than during years with low nutrient loads. As a proxy for the remineralisation intensity ammonium plus nitrite concentrations in autumn were used. Based on monitoring data from the Dutch Wadden Sea (1977–1997) the above mentioned model was statistically tested. In autumn, however, a significant correlation was found between autumn values of ammonium and nitrite and river input of nitrogen during the previous winter, spring and summer. The analysis supports that in years with a high riverine nitrogen load more organic matter is remineralized within the Wadden Sea than in years with a low nitrogen load. A comparison with older data from 1960 to 1961 suggests that the remineralisation intensity in the Wadden Sea has increased by a factor of two to three. This is not reflected by a two to three-fold increase in riverine nitrogen load from 1960 to present. It is suggested that the increased remineralisation rates in the Dutch Wadden Sea between the 1960s and the 1980s/1990s are largely caused by an increased nitrogen flux through the Channel and the Strait of Dover and by an increased atmospheric nitrogen input.     

Long-term changes in the phytobenthos of the southern Åland Islands,northern Baltic Sea

Marine macrophytes and -algae have undergone major changes in abundance and species composition over the last decades, primarily due to eutrophication. However, comparable studies conducted in the mid 20th century are rare, but potentially valuable for enabling insight into changes in the benthic communities from the early onset of the eutrophication of the Baltic Sea. In the present study, the submerged phytobenthic community in the exposed southern archipelago of the Åland Islands was examined in 2018 and compared with surveys conducted in 1956 and 1993, respectively. The aim was to analyze long-term changes in the phytobenthic community in relation to the general large-scale anthropogenic drivers since the 1950s. Between 1956 and 1993, a decrease in the total number of species/taxa, an increase of filamentous algae coverage and a decrease in the depth range of Fucus vesiculosus was observed. These changes in the phytobenthic community continued between 1993 and 2018, suggesting no changes in the previously described negative trends. Between 1956 and 2018, a general shift in the distribution of phytobenthic functional groups, (grouped according to morphology and type of algae; green, brown and red) occurred, with increased coverage of filamentous brown and green algae, and decline in red algae coverage. The depth range of F. vesiculosus also decreased by >50% between 1956 and 2018. The results support findings that the eutrophication of the northern Baltic Sea is still at a high level, which slows down or prevents the recovery of offshore phytobenthic communities, despite the progress seen in other areas. Thus, the likely main drivers behind the changes are the direct and indirect effects of eutrophication in combination with warmer water, i.e. an effect of climate change.     

Influence of phosphorus loads and of some limnological processes on the purity of lake water

Various aspects of the relation between nutrient load-mainly phosphorus-and phytoplankton biomass, the influence of drainage from surrounding forest and land under agriculture, removal of phosphorus from lakes by sedimentation, internal loading and the role of biota, especially fish, are considered and commented upon.     

Organic Nitrogen Retention in the Atchafalaya River Swamp

Freshwater diversions from the lower Mississippi River into the region’s wetlands have been considered an alternative means for reducing nitrogen loading. The Atchafalaya River Swamp, the largest freshwater swamp in North America, carries the entire discharge of the Red River and 30% of the discharge of the Mississippi River, but it is largely unknown how much nitrogen actually can be retained from the overflowing waters of the Mississippi–Atchafalaya River system. Nitrogen discharge from the upper Mississippi River Basin has been implicated as the major cause for the hypoxia in the Northern Gulf of Mexico, which threatens not only the aquatic ecosystem health, but also Louisiana’s fishery industry, among other problems. This study was conducted to determine the change in organic nitrogen mass as water flows through the Atchafalaya River Swamp and into the Gulf of Mexico. By utilizing the river’s long-term discharge and water quality data (1978–2002), monthly and annual organic nitrogen fluxes were quantified, and their relationships with the basin’s hydrologic conditions were investigated. A total Kjeldahl nitrogen (TKN) mass input–output balance between the upstream (Simmesport) and downstream (Morgan City and Wax Lake Outlet) locations was established to examine the organic nitrogen removal potential for this large swamp. The results showed that on average, TKN input into the Atchafalaya was 200 323 tons year⁻¹ and TKN output leaving the basin was 145 917 tons year⁻¹, resulting in a 27% removal rate of organic nitrogen. Monthly TKN input and output in the basin were highest from March to June (input vs. output: 25 000 vs. 18 000 tons month⁻¹) and lowest from August to November (8000 vs. 6000 tons month⁻¹). There was a large variation in both annual and inter-annual organic nitrogen removals. The variability was positively correlated with the amount of inflow water at Simmesport, suggesting that regulating the river’s inflow at the Old River flood control structures may help reduce nitrogen loading of the Mississippi River to the Gulf of Mexico. Furthermore, the in-stream loss of organic nitrogen indicates that previous studies may have overestimated nitrogen discharge from the Mississippi–Atchafalaya River system.     

Addressing the trophic status in urban ponds: An evaluation of current trophic state indexes

Ponds are generally understudied. Quality problems in urban freshwaters can arise from eutrophication, and trophic status classification gives information related to their ecological situation. Ponds may strongly differ from larger lakes for which most current trophic status classification methods have been developed. This paper seeks to compare and contrast the suitability of six trophic status indexes (TSI) and OECD trophic classifications system currently used to evaluate the eutrophication level of urban ponds in the Subtropical region. The trophic status was evaluated based on total phosphorus (TP), soluble reactive phosphorus (SRP), total nitrogen (TN) and phytoplankton chlorophyll a (Chl a) in 12 ponds placed in different cities from the Pampean region (Argentina), in the warm and cold seasons. Our results demonstrate that there was no relationship between pond size (0.08–2.45 ha) and trophic status. TSIs estimated with Chl a showed significant differences between seasons and the range oligotrophic-hypertrophic of trophism was encountered. Conversely, TSIs estimated with nutrient concentrations (TP, SRP, TN) did not. The pond's classification in trophic levels fell in the eutrophic-hypertrophic extreme, and only considering TN/TP broadened the trophic range. No relationship was encountered between Chl a and P; however, Chl a and TN/TP were positively correlated during the warm season. Lower trophic levels regarding TSI (Chl a) were generally associated with a dense floating-macrophyte cover. We suggest that the TSI considered should correspond to the latitudinal region in which the ponds are located. Our results indicate that it appears restrictive to consider only P contents in ponds to assess eutrophication. Under the proposed framework, the key issues for the study of pond trophic classification will be to include not only P but N concentrations and TN/TP. Also, phytoplankton Chl a concentration jointly with floating macrophytes biomass/coverage should be considered as diagnosis parameters.     

Is Lake Prespa Jeopardizing the Ecosystem of Ancient Lake Ohrid?

Lake Prespa and Lake Ohrid, located in south-eastern Europe, are two lakes of extraordinary ecological value. Although the upstream Lake Prespa has no surface outflow, its waters reach the 160 m lower Lake Ohrid through underground hydraulic connections. Substantial conservation efforts concentrate on oligotrophic downstream Lake Ohrid, which is famous for its large number of endemic and relict species. In this paper, we present a system analytical approach to assess the role of the mesotrophic upstream Lake Prespa in the ongoing eutrophication of Lake Ohrid. Almost the entire outflow from Lake Prespa is found to flow into Lake Ohrid through karst channels. However, 65% of the transported phosphorus is retained within the aquifer. Thanks to this natural filter, Lake Prespa does not pose an immediate threat to Lake Ohrid. However, a potential future four-fold increase of the current phosphorus load from Lake Prespa would lead to a 20% increase (+0.9 mg P m⁻³) in the current phosphorus content of Lake Ohrid, which could jeopardize its fragile ecosystem. While being a potential future danger to Lake Ohrid, Lake Prespa itself is substantially endangered by water losses to irrigation, which have been shown to amplify its eutrophication.     

Use of diatom indices to categorise impacts on and recovery of a floodplain system in South Africa

The trophic status of the Ramsar-accredited Nyl River floodplain, which is stressed by sewage treatment effluents, and its ability to restore normal conditions, were assessed in 2014–2015 using diatoms as biological indicators. The Trophic Diatom Index, Specific Pollution Sensitivity Index and the Generic Diatom Index were used to characterise water quality under high and low flow conditions in 2014. An additional survey was conducted in May 2015, following an accidental sulphuric acid spill above the Sewage Treatment Works (STW). Significant linear correlations were identified between physico-chemical parameters and the indices, as well as between the three indices themselves, validating their use to infer water quality. Water quality deteriorated from the source of the Klein Nyl River, especially at the STW, but improved within the Nylsvley Nature Reserve wetland sampling site (NYL).