Summer Anoxia Research Articles

Can osmotrophy in Gonyostomum semen explain why lake browning drives an expansion of the alga in parts of Europe?

Gonyostomum semen experiences a remarkable expansion in parts of Europe, partially driven by rising levels of dissolved organic matter (DOM), also called lake browning. One way to explain the stimulating effect of browning is to assume that G. semen combines photosynthesis with osmotrophy, the ability to utilize DOM as source of nutrients or energy. The present work tested this idea on a nearly unialgal G. semen bloom. Continuous vertical profiling found that each night, G. semen moved into the oxic-anoxic interface or the anoxic hypolimnion, i.e., into places where redox processes can give access to DOM via dissolution of ferric oxide-organic matter complexes. In the hypolimnion, conditions were right for a build-up of DOM, which, however, never happened. Optical analyses of hypolimnetic DOM indicated that the build-up was prevented by the removal of terrestrially derived DOM, which was correlated with G. semen. During the daytime, G. semen migrated into the epilimnion. Here, net oxygen production was near zero, indicating that G. semen combined photosynthesis with heterotrophic nutrition. Optical analyses of epilimnetic DOM hinted at the utilization of terrestrially derived DOM. Taken together, the above findings provide circumstantial evidence for G. semen combining photosynthesis with osmotrophy in nature. The proposed utilization of terrestrially derived DOM suggests that lake browning may favor G. semen by improving the access to DOM that the alga can utilize. Browning may also benefit G. semen by promoting summer anoxia that causes a reductive release of DOM from complexes with ferric oxides, improving the alga’s access to DOM.

Anoxia decreases the magnitude of the carbon, nitrogen, and phosphorus sink in freshwaters.

Oxygen availability is decreasing in many lakes and reservoirs worldwide, raising the urgency for understanding how anoxia (low oxygen) affects coupled biogeochemical cycling, which has major implications for water quality, food webs, and ecosystem functioning. Although the increasing magnitude and prevalence of anoxia has been documented in freshwaters globally, the challenges of disentangling oxygen and temperature responses have hindered assessment of the effects of anoxia on carbon, nitrogen, and phosphorus concentrations, stoichiometry (chemical ratios), and retention in freshwaters. The consequences of anoxia are likely severe and may be irreversible, necessitating ecosystem‐scale experimental investigation of decreasing freshwater oxygen availability. To address this gap, we devised and conducted REDOX (the Reservoir Ecosystem Dynamic Oxygenation eXperiment), an unprecedented, 7‐year experiment in which we manipulated and modeled bottom‐water (hypolimnetic) oxygen availability at the whole‐ecosystem scale in a eutrophic reservoir. Seven years of data reveal that anoxia significantly increased hypolimnetic carbon, nitrogen, and phosphorus concentrations and altered elemental stoichiometry by factors of 2–5× relative to oxic periods. Importantly, prolonged summer anoxia increased nitrogen export from the reservoir by six‐fold and changed the reservoir from a net sink to a net source of phosphorus and organic carbon downstream. While low oxygen in freshwaters is thought of as a response to land use and climate change, results from REDOX demonstrate that low oxygen can also be a driver of major changes to freshwater biogeochemical cycling, which may serve as an intensifying feedback that increases anoxia in downstream waterbodies. Consequently, as climate and land use change continue to increase the prevalence of anoxia in lakes and reservoirs globally, it is likely that anoxia will have major effects on freshwater carbon, nitrogen, and phosphorus budgets as well as water quality and ecosystem functioning.

Lake thermal structure drives interannual variability in summer anoxia dynamics in a eutrophic lake over 37 years

Abstract. The concentration of oxygen is fundamental to lake water quality and ecosystem functioning through its control over habitat availability for organisms, redox reactions, and recycling of organic material. In many eutrophic lakes, oxygen depletion in the bottom layer (hypolimnion) occurs annually during summer stratification. The temporal and spatial extent of summer hypolimnetic anoxia is determined by interactions between the lake and its external drivers (e.g., catchment characteristics, nutrient loads, meteorology) as well as internal feedback mechanisms (e.g., organic matter recycling, phytoplankton blooms). How these drivers interact to control the evolution of lake anoxia over decadal timescales will determine, in part, the future lake water quality. In this study, we used a vertical one-dimensional hydrodynamic–ecological model (GLM-AED2) coupled with a calibrated hydrological catchment model (PIHM-Lake) to simulate the thermal and water quality dynamics of the eutrophic Lake Mendota (USA) over a 37 year period. The calibration and validation of the lake model consisted of a global sensitivity evaluation as well as the application of an optimization algorithm to improve the fit between observed and simulated data. We calculated stability indices (Schmidt stability, Birgean work, stored internal heat), identified spring mixing and summer stratification periods, and quantified the energy required for stratification and mixing. To qualify which external and internal factors were most important in driving the interannual variation in summer anoxia, we applied a random-forest classifier and multiple linear regressions to modeled ecosystem variables (e.g., stratification onset and offset, ice duration, gross primary production). Lake Mendota exhibited prolonged hypolimnetic anoxia each summer, lasting between 50–60 d. The summer heat budget, the timing of thermal stratification, and the gross primary production in the epilimnion prior to summer stratification were the most important predictors of the spatial and temporal extent of summer anoxia periods in Lake Mendota. Interannual variability in anoxia was largely driven by physical factors: earlier onset of thermal stratification in combination with a higher vertical stability strongly affected the duration and spatial extent of summer anoxia. A measured step change upward in summer anoxia in 2010 was unexplained by the GLM-AED2 model. Although the cause remains unknown, possible factors include invasion by the predacious zooplankton Bythotrephes longimanus. As the heat budget depended primarily on external meteorological conditions, the spatial and temporal extent of summer anoxia in Lake Mendota is likely to increase in the near future as a result of projected climate change in the region.

Fifty years of ecological changes: Regime shifts and drivers in a coastal Mediterranean lagoon during oligotrophication

Thau lagoon is a large Mediterranean coastal lagoons and it supports traditional shellfish farming activities. It has been subject to eutrophication leading to major anoxic events associated with massive mortalities of shellfish stocks. Since the 1970s, improvements have been made to wastewater treatment systems, which have gradually led to oligotrophication of the lagoon. The aim of our study was to determine how the decrease in nutrient inputs resulted in major ecological changes in Thau lagoon, by analysing five decades of time-series (1970–2018) of observations on pelagic and benthic autotrophic communities. We were able to identify two periods during the oligotrophication process. Period 1 (1970–1992) was considered a eutrophic period, characterised by the shift from seagrass dominance to dominance of red macroalgae. Period 2 (1993–2018), characterised by improved eutrophication status, was further divided into three: a transition phase (1993–2003) during which the water column continued to recover but the benthic community lagged behind in recovery and in partial resilience; a regime shift (2003–2006), after which the water column became oligotrophic and seagrass began to recover (2007–2018). Considering anoxia crises as indicators of ecosystem resilience and resistance, we used a generalised linear model to analyse meteorological and environmental data with the aim of identifying the triggers of summer anoxia over the study period. Among the meteorological variables studied, air temperature had the strongest positive effect, followed by the period and wind intensity (both negative effects) and by rainfall in July (positive effect). The risk of triggering anoxia was lower in period 2, evidence for the increasing resistance of the ecosystem to climatic stress throughout the oligotrophication process. At the ecosystem scale and in the long term perspective, the ecological gains related to oligotrophication are especially important in the context of climate change, with more frequent and severe heat waves predicted.

The mechanism of bottom water DO variation in summer at the northern mouth of Isahaya Bay, Japan

Recently, bivalves have been massively killed by anoxia or hypoxia in summer at the northern part of Isahaya Bay, Japan, which constituted a major problem for fisheries. However, the mechanism behind the occurrence of hypoxic water masses is unclear. It is known that the bottom water dissolved oxygen (DO) in this area is affected by the inflow of seawater into the northern mouth of Isahaya Bay. To understand the mechanism of hypoxia, it is necessary to determine the physical processes that cause changes in the bottom DO concentrations in this area. This study shows that there is a neap-spring tidal variation in bottom DO due to a change in vertical tidal mixing, and it also suggests that the decrease in bottom DO was generated by a baroclinic flow, which is due to the internal tide, and a shear flow, which is induced by the external tide in the bottom boundary layer. In addition, our study suggests that the source of cold and hypoxic water that appears in the bottom layer at low tide is the inner area of the Ariake Sea.

Distinctive Patterns in the Taxonomical Resolution of Bacterioplankton in the Sediment and Pore Waters of Contrasted Freshwater Lakes.

Bacteria assemblages in lake sediments play a key role in various biogeochemical processes, yet their association with interstitial pore waters has been scarcely investigated. In this study, we utilized Illumina next-generation amplicon sequencing of the 16S rRNA gene to characterize the seasonal bacterial communities in the sediments and pore waters of three contrasted temperate freshwater lakes, namely Pavin, Aydat, and Grangent (French Massif Central). Despite occupying seemingly similar habitats, bacterial communities differed substantially between sediments and pore waters at all seasons with low sharing of operational taxonomic units (OTUs, 6.7 to 20.3%) between them. Sediment-associated bacteria were more rich and diverse than pore water bacteria, indicating a high heterogeneity in the sediment microhabitat. The changes in both sediment and pore water bacterial communities were lake and season specific. The bacterial community showed distinct differences between the lakes, with larger presence of strict anaerobes such as Syntrophus, Syntrophorhabdus, and Sulfuricurvum in the pore water and sediments of Pavin responsible for carbon and sulfur cycling. In both Aydat and Grangent, the hgcI_clade dominated throughout the study period in the pore waters. The higher representation of lesser-known transient members of lake communities such as Methylotenera in the pore waters of Aydat, and Clostridium and Sulfuricurvum in the pore and sediments of Grangent, respectively, were observed during the period of temporary anoxia in summer caused by lake stratification. Our study revealed that in the investigated lakes, the prevailing environmental factors across time and space structured and influenced the adaptation of bacterial communities to specific ecological niches.

Geochemistry of a large impoundment – Part II: Fe and Mn cycling and metal transport

Depletion of oxygen in lakes and reservoirs due to summer stratification has significant effects on many aspects of water chemistry, including lake productivity, elemental cycling and water quality. We have studied the redox conditions of Lake Texoma, a large impoundment lake on the border of Texas and Oklahoma, USA, formed from the confluence of the Red and Washita rivers, in order to understand the impact of summer anoxia on metal distribution. In summer, dissolved oxygen decreases with depth (from c. 7.0 to 0.1 mg/L), whereas dissolved (<0.45 µm) Fe (and Fe 2+ ), Mn and HS − concentrations show complementary increases in the hypolimnion. Summer anoxia is, to a large degree, responsible for vertical variations in Fe and Mn concentrations, and Fe speciation is controlled by Fe-oxyhydroxide reduction and subsequent pyrite precipitation in sulphide-rich bottom waters. Summer anoxia is also responsible for vertical variations in the concentration of other metals including Ba, Pb and Ni in the deepest portion (main lake) of the lake. Lake Texoma and its two river arms show relatively minor variation in δ 34 S CDT (from +11.5 to +13.4‰), mirroring variation in Permian/Cretaceous marine gypsum and anhydrite deposits (δ 34 S CDT +10 to +15‰) in the headwater regions of the catchment. Increasing δ 34 S CDT with depth (>16 m) in the main lake is consistent with fractionation associated with sulphate reduction in anoxic bottom waters. δ 13 C PDB values of dissolved inorganic carbon (DIC) become more negative (from –2.5 to –8.2 ‰) with depth in summer, due to bacterial oxidation of organic matter linked to sulphate reduction. Summer anoxia may induce temporal degradation of water quality in the central three zones of the lake with elevated Fe and Mn concentrations owing to breakdown of oxyhydroxides and release of adsorbed heavy metals, such as Pb and Ni. However, complete turnover of the water column in the autumn lowers dissolved Fe, Mn and heavy metal concentrations by oxidation and formation of oxyhydroxides . The characterization of anoxia in Lake Texoma provides the background for further water quality research and management for the rapidly increasing population of North Texas and improves our understanding of redox cycling and metal mobility in reservoirs.

Using paleolimnology to find restoration solutions: the case of Lake Muzzano, Switzerland

Lake Muzzano (45°59′50″N 8°55′41″E, 337 m a.s.l.) is a hyper-eutrophied lake located in the Tessin region of Switzerland. Almost every year, algal blooms (Microcystis) cover the lake with a thickness of 1-2 cm. These blooms associated with periods of anoxia in summer have led to fish kills in 1967 and 1994. In the hope of avoiding these blooms, a bypass bringing water away from the lake has been established in 1999. This solution was not adequate as blooms kept reoccurring. Sediment removal was then proposed by the Tessin Canton as a possible remediation technique and The L.A.K.E.S Institute had a mandate in 2010 to study the lake (present and past state) to determine the reasons creating anoxia and algal blooms. The present state of the lake shows that anoxia is still occurring when the algal bloom covers the lake’s surface. Subfossil diatom and chironomid analyses show that the baseline conditions were those found before 1922 AD when the lake was oligotrophic and supported a diversified community of chironomids suggesting good oxygenation. After 1922 AD, circulation to the lake was cut out and nutrients accumulated in the lake leading to anoxia and the establishment of Microcystis. Heavy metal analysis in the sediment shows that the concentration is above the national recommendation and thus sediment should not be removed or should be stored with hazardous material. Based on the present status of the lake and paleolimnological results, two solutions are proposed: to further decrease the nutrients coming in the lake (possibly using filtrating plants) followed by flushing to increase lake water circulation. Physical capping of the sediment to avoid exchange of heavy metals and phosphorus release at the water/sediment interface could also be envisaged once the two prime solutions are in place.

Erratum to: Relative Importance of Nutrient Load and Wind on Regulating Interannual Summer Hypoxia in the Chesapeake Bay

To analyze the correlations of summer anoxia/hypoxia in the Chesapeake Bay with watershed input and wind conditions, statistics were applied to nearly three decades of monitoring data. The results of Pearson correlation coefficients, multivariate regression analysis, and cluster analysis indicate that anoxia/hypoxia has a strong positive correlation with nutrient load and a moderate negative correlation with wind speed. Physical relationships among the relevant constituents were analyzed. Nutrient loads and the subsequent decay of organic matter are the primary factors that control the oxygen demand causing summer anoxia and hypoxia, while episodic wind can partially erode stratification and reduce anoxia/hypoxia. Although the extent of anoxia/hypoxia reduction differs with wind direction, higher wind speeds result in more destratification and anoxia/hypoxia reduction than lower wind speeds and are more important than the effect of wind direction. The influences of freshwater discharge, stratification, and temperature were also analyzed. Computer modeling results were used to obtain dissolved oxygen conditions at finer temporal and spatial scales to supplement the scattered and discrete observations from monitoring stations and for better understanding of anoxia/hypoxia development under episodic wind events.

Relative Importance of Nutrient Load and Wind on Regulating Interannual Summer Hypoxia in the Chesapeake Bay

To analyze the correlations of summer anoxia/hypoxia in the Chesapeake Bay with watershed input and wind conditions, statistics were applied to nearly three decades of monitoring data. The results of Pearson correlation coefficients, multivariate regression analysis, and cluster analysis indicate that anoxia/hypoxia has a strong positive correlation with nutrient load and a moderate negative correlation with wind speed. Physical relationships among the relevant constituents were analyzed. Nutrient loads and the subsequent decay of organic matter are the primary factors that control the oxygen demand causing summer anoxia and hypoxia, while episodic wind can partially erode stratification and reduce anoxia/hypoxia. Although the extent of anoxia/hypoxia reduction differs with wind direction, higher wind speeds result in more destratification and anoxia/hypoxia reduction than lower wind speeds and are more important than the effect of wind direction. The influences of freshwater discharge, stratification, and temperature were also analyzed. Computer modeling results were used to obtain dissolved oxygen conditions at finer temporal and spatial scales to supplement the scattered and discrete observations from monitoring stations and for better understanding of anoxia/hypoxia development under episodic wind events.

Whole-lake nitrate addition for control of methylmercury in mercury-contaminated Onondaga Lake, NY

Methylmercury (MeHg) strongly bioaccumulates in aquatic food webs resulting in exposure to humans and wildlife through consumption of fish. Production of MeHg is promoted by anaerobic conditions and the supply of inorganic Hg (Hg2+), sulfate (SO42−), and labile organic carbon. The anaerobic sediments of stratified lakes are particularly active zones for methylation of Hg2+ and can be an important source of MeHg to the water column during summer anoxia and fall turnover. Nitrate (NO3–) addition has recently been proposed as a novel approach for the control of MeHg accumulation in the hypolimnia of Hg-contaminated lakes. In 2011, a whole-lake NO3– addition pilot test was conducted in Hg-contaminated Onondaga Lake, NY with the objective of limiting release of MeHg from the pelagic sediments to the hypolimnion through maintenance of NO3––N concentrations >1mgN/L. A liquid calcium-nitrate solution was added to the hypolimnion as a neutrally buoyant plume approximately three times per week during the summer stratification interval. Maximum hypolimnetic concentrations of MeHg and soluble reactive phosphorus (SRP) decreased 94% and 95% from 2009 levels, suggesting increased sorption to Fe and Mn oxyhydroxides in surficial sediments as the regulating mechanism. Increased MeHg concentrations in the upper waters during fall turnover, which had been a generally recurring pattern, did not occur in 2011, resulting in decreased exposure of aquatic organisms to MeHg. Over the 1992–2011 interval, the hypolimnetic NO3– supply explained 85% and 95% of the interannual variations in hypolimnetic accumulations of SRP and MeHg, respectively.

Rates and regulation of nitrogen cycling in seasonally hypoxic sediments during winter (Boknis Eck, SW Baltic Sea): Sensitivity to environmental variables

This study investigates the biogeochemical processes that control the benthic fluxes of dissolved nitrogen (N) species in Boknis Eck – a 28 m deep site in the Eckernförde Bay (southwestern Baltic Sea). Bottom water oxygen concentrations (O 2−BW) fluctuate greatly over the year at Boknis Eck, being well-oxygenated in winter and experiencing severe bottom water hypoxia and even anoxia in late summer. The present communication addresses the winter situation (February 2010). Fluxes of ammonium (NH 4 +), nitrate (NO 3 −) and nitrite (NO 2 −) were simulated using a benthic model that accounted for transport and biogeochemical reactions and constrained with ex situ flux measurements and sediment geochemical analysis. The sediments were a net sink for NO 3 − (−0.35 mmol m −2 d −1 of NO 3 −), of which 75% was ascribed to dissimilatory reduction of nitrate to ammonium (DNRA) by sulfide oxidizing bacteria, and 25% to NO 3 − reduction to NO 2 − by denitrifying microorganisms. NH 4 + fluxes were high (1.74 mmol m −2 d −1 of NH 4 +), mainly due to the degradation of organic nitrogen, and directed out of the sediment. NO 2 − fluxes were negligible. The sediments in Boknis Eck are, therefore, a net source of dissolved inorganic nitrogen (DIN = NO 3 − + NO 2 − + NH 4 +) during winter. This is in large part due to bioirrigation, which accounts for 76% of the benthic efflux of NH 4 +, thus reducing the capacity for nitrification of NH 4 +. The combined rate of fixed N loss by denitrification and anammox was estimated at 0.08 mmol m −2 d −1 of N 2, which is at the lower end of previously reported values. A systematic sensitivity analysis revealed that denitrification and anammox respond strongly and positively to the concentration of NO 3 − in the bottom water (NO 3 − BW). Higher O 2−BW decreases DNRA and denitrification but stimulates both anammox and the contribution of anammox to total N 2 production (%R amx). A complete mechanistic explanation of these findings is provided. Our analysis indicates that nitrification is the geochemical driving force behind the observed correlation between %R amx and water depth in the seminal study of Dalsgaard et al. (2005). Despite remaining uncertainties, the results provide a general mechanistic framework for interpreting the existing knowledge of N-turnover processes and fluxes in continental margin sediments, as well as predicting the types of environment where these reactions are expected to occur prominently.

Simulations of climate effects on water temperature, dissolved oxygen, and ice and snow covers in lakes of the contiguous U.S. under past and future climate scenarios

Daily water temperature, dissolved oxygen (DO) profiles, and ice and snow covers (where applicable) were simulated for 27 types of small lakes (up to 10 km2 surface area) at 209 locations in the contiguous United States under past climate (observed from 1961 to 1979) and for projected doubled atmospheric carbon dioxide (2×CO2) climate conditions. A verified, process‐oriented, dynamic, and one‐dimensional year‐round lake water quality model (MINLAKE96) was applied to simulate water temperature and DO profiles continuously in daily time steps over a 19‐yr simulation period. This regional lake model has no geographic constraints on the model's physical and chemical processes, but the climate forcing is a function of geographic location. Model calibration parameters and initial conditions are correlated to lake geometry, trophic state, and location. The 2×CO2 climate scenario is projected to increase lake surface temperatures by up to 5.2°C when the climate scenario projects an increase of mean annual air temperature up to 6.7°C. The 2×CO2 climate scenario is projected to increase the duration of seasonal summer stratification by up to 67 d, to shorten ice cover by up to 90 d, and to reduce the maximum ice thickness by up to 0.44 m. Under a 2×CO2 climate scenario, lake anoxia during the period of ice cover is projected to shorten so that fish winterkill can be avoided, but the periods of hypolimnetic summer anoxia are projected to lengthen. These projected changes will have many significant effects on ecological conditions and aquatic habitats in lakes in the contiguous United States.

Controls on biochemical oxygen demand in the upper Klamath River, Oregon

A series of 30-day biochemical oxygen demand (BOD) experiments were conducted on water column samples from a reach of the upper Klamath River that experiences hypoxia and anoxia in summer. Samples were incubated with added nitrification inhibitor to measure carbonaceous BOD (CBOD), untreated to measure total BOD, which included demand from nitrogenous BOD (NBOD), and coarse-filtered to examine the effect of removing large particulate matter. All BOD data were fit well with a two-group model, so named because it considered contributions from both labile and refractory pools of carbon: BOD t = a 1(1 − e − a 0 t ) + a 2 t. Site-average labile first-order decay rates a 0 ranged from 0.15 to 0.22/day for CBOD and 0.11 to 0.29/day for BOD. Site-average values of refractory zero-order decay rates a 2 ranged from 0.13 to 0.25 mg/L/day for CBOD and 0.01 to 0.45 mg/L/day for BOD; the zero-order CBOD decay rate increased from early- to mid-summer. Values of ultimate CBOD for the labile component a 1 ranged from 5.5 to 28.8 mg/L for CBOD, and 7.6 to 30.8 mg/L for BOD. Two upstream sites had higher CBOD compared to those downstream. Maximum measured total BOD 5 and BOD 30 during the study were 26.5 and 55.4 mg/L; minimums were 4.2 and 13.6 mg/L. For most samples, the oxygen demand from the three components considered here were: labile CBOD > NBOD > refractory CBOD, though the relative importance of refractory CBOD to oxygen demand increased over time. Coarse-filtering reduced CBOD for samples with high particulate carbon and high biovolumes of Aphanizomenon flos-aquae. There was a strong positive correlation between BOD, CBOD, and the labile component of CBOD to particulate C and N, with weaker positive correlation to field pH, field dissolved oxygen, and total N. The refractory component of CBOD was not correlated to particulate matter, instead showing weak but statistically significant correlation to dissolved organic carbon, UV absorbance at 254 nm, and total N. Particulate organic matter, especially the alga A. flos-aquae, is an important component of oxygen demand in this reach of the Klamath River, though refractory dissolved organic matter would continue to exert an oxygen demand over longer time periods and as water travels downstream.

Geochemistry of iron in the Salton Sea, California

The Salton Sea is a large, saline, closed-basin lake in southern California. The Sea receives agricultural runoff and, to a lesser extent, municipal wastewater that is high in nutrients, salt, and suspended solids. High sulfate concentrations (4× higher than that of the ocean), coupled with warm temperatures and low-redox potentials present during much of the year, result in extensive sulfate reduction and hydrogen sulfide production. Hydrogen sulfide formation may have a dramatic effect on the iron (Fe) geochemistry in the Sea. We hypothesized that the Fe(II)-sulfide minerals should dominate the iron mineralogy of the sediments, and plans to increase hypolimnetic aeration would increase the amount of Fe(III)-oxides, which are strong adsorbers of phosphate. Sequential chemical extractions were used to differentiate iron mineralogy in the lake sediments and suspended solids from the tributary rivers. Iron in the river-borne suspended solids was mainly associated with structural iron within silicate clays (70%) and ferric oxides (30%). The iron in the bottom sediments of the lake was associated with silicate minerals (71% of the total iron in the sediments), framboidal pyrite (10%), greigite (11%), and amorphous FeS (5%). The ferric oxide fraction was <4% of the total iron in these anaerobic sediments. The morphological characteristics of the framboidal pyrite as determined using SEM suggest that it formed within the water column and experiences some changes in local redox conditions, probably associated with alternating summer anoxia and the well-mixed and generally well-aerated conditions found during the winter. The prevalence of Fe(II)-sulfide minerals in the sediments and the lack of Fe(III)-oxide minerals suggest that the classic model of P-retention by Fe(III)-oxides would not be operating in this lake, at least during anoxic summer conditions. Aeration of the hypolimnion could affect the internal loading of P by changing the relative amounts of Fe(II)-sulfides and Fe(III)-oxides at the sediment/water interface.

Role of the polychaete Neanthes succinea in phosphorus regeneration from sediments in the Salton Sea, California

The Salton Sea currently suffers from several well-documented water quality problems associated with high nutrient loading. However, the importance of phosphorus regeneration from sediments has not been established. Sediment phosphorus regeneration rates may be affected by benthic macroinvertebrate activity (e.g. bioturbation and excretion). The polychaete Neanthes succinea (Frey and Leuckart) is the dominant benthic macroinvertebrate in the Salton Sea. It is widely distributed during periods of mixing (winter and spring), and inhabits only shallow water areas following development of anoxia in summer. The contribution of N. succinea to sediment phosphorus regeneration was investigated using laboratory incubations of cores under lake temperatures and dissolved oxygen concentrations typical of the Salton Sea. Regeneration rates of soluble reactive phosphorus (SRP) were lowest (−0.23–1.03 mg P m−2 day−1) under saturated oxygen conditions, and highest (1.23–4.67 mg P m−2 day−1) under reduced oxygen levels. N. succinea most likely stimulated phosphorus regeneration under reduced oxygen levels via increased burrow ventilation rates. Phosphorus excretion rates by N. succinea were 60–70% more rapid under reduced oxygen levels than under saturated or hypoxic conditions. SRP accounted for 71–80% of the dissolved phosphorus excreted under all conditions. Whole-lake SRP regeneration rates predicted from N. succinea biomass densities are highest in early spring, when the lake is mixing frequently and mid-lake phytoplankton populations are maximal. Thus, any additional phosphorus regenerated from the sediments at that time has potential for contributing to the overall production of the lake.

Ecosystem Responses to Extreme Natural Events: Impacts of Three Sequential Hurricanes in Fall 1999 on Sediment Quality and Condition of Benthic Fauna in the Neuse River Estuary, North Carolina

A study was conducted in November 1999 to assess sediment quality and condition of benthic fauna in the Neuse River Estuary (NRE), North Carolina, USA, following the passage of three Atlantic hurricanes during the two months prior. Samples for analysis of macroinfauna (>0.5 mm sieve size), chemical contamination of sediments, and other abiotic environmental variables (salinity, dissolved oxygen, pH, depth, sediment granulometry) were collected at 20 sites from the mouth of the Neuse River at Pamlico Sound to approximately 90 km upstream. Results were compared to those obtained from the same area in July 1998 using similar protocols. Depressed salinity, caused by extreme rainfall and associated high freshwater flow, persisted throughout much of the estuary, which had experienced periods of water-column stratification and hypoxia of underlying waters. Fifteen of the 20 sites, representing 299 km2 (76% of the survey area), also showed signs of benthic stress based on a multi-metric benthic index of biotic integrity (B-IBI). Benthic impacts included reductions in the abundance, diversity, and numbers of species and shifts in taxonomic composition, with a notable increase in dominance of the opportunistic polychaete Mediomastus ambiseta as other former dominant species declined. There was no significant increase in the extent of chemical contamination compared to pre-hurricane conditions. Storm-related reductions in dissolved oxygen and salinity were the more likely causes of the observed benthic impacts, though it was not possible, based on these results, to separate storm effects from seasonal changes in the benthos and annual episodes of summer anoxia and hypoxia.

A 3D hydrodynamic fate and transport model for herbicides in Sacca di Goro coastal lagoon (Northern Adriatic)

Sacca di Goro is a shallow coastal microtidal lagoon with a surface area of 26 km 2, and an average depth of about 1.5 m. Fresh water pollutant loads from Po River branches and several drainage canals lead to anthropogenic eutrophication, frequent summer anoxia crises and chemical contamination. Such events not only affect the lagoon ecosystem but also cause serious economic losses, the lagoon being the second largest producer of clams in Italy. The present work aims at using a fate model coupled with COHERENS 3D hydrodynamic model to simulate and to explain the spatial distribution and temporal variations of s-triazines herbicides in the Sacca di Goro lagoon. The simulation results of spatial and temporal dynamic behaviour of atrazine, simazine and terbuthylazine have been compared with experimental data obtained during an annual monitoring programme.

Ecological characteristics of the invasive Asian date mussel,Musculista senhousia, in the Sacca Di Goro (Adriatic Sea, Italy)

A population of the invasive musselMusculista senhousia was monitored bimonthly from May 1999 through April 2000 in the Sacca di Goro, a brackish lagoon of the Po River Delta, Northern Adriatic Sea, in order to give information on the gametogenic cycle, population dynamics, and secondary production of this successful invader. The gonad underwent 4 different stages: spent (December to February), developing (March to May), ripe (June to August), and spawning (September to November). The population was numerically dominated by a single cohort of individuals for most of the year. The mean size of this cohort rapidly increased to 24–25 mm shell length, after which growth slowed and mussels rarely grew larger than 30–32 mm. Summer anoxia may have greatly reduced mussel abundance; annual cohort mortality was 95%. No recruitment was registered on the bed until February 2000, and there was a large pulse of new recruits in April when two cohorts were clearly recognizable. The established bed was the primary site for new recruitment. Secondary production, calculated with two different methods, gave comparable estimates; P:B ratios were 1.5 and 1.7.M. senhousia beds seemed to facilitate the presence of other macrofaunal taxa: abundance of some species (a small gastropod, amphipods, and tube-building polychaetes) were significantly higher within mussel mats at two sites sampled in May 2000 than they were in soft-sediments ∼100 m away.

Management strategies for the control of eutrophication processes in Fogliano lagoon (Italy): a long-term analysis using a mathematical model

Models are developed and used to analyse and test different management strategies aimed at limiting eutrophication processes in Fogliano Lagoon: modification of lagoon hydrodynamics by tidal flow regulation, harvest of algae biomass, reclaim of sediments. Mathematical models, which have been constructed and proposed, simulate, on a multiyear time scale, the main ecological processes responsible for the most important effects of eutrophication: vegetal blooms, summer anoxia. For different management strategies, hydrodynamic fields produced by wind and tide, and three-dimensional concentration fields of significant species in the ecological phenomena, in water and into sediments, are quantified and compared. The species simulated are: in the water column dissolved oxygen, phytoplanktonic biomass, macrophytic biomass, orthophosphate, dissolved organic carbon, particulate organic carbon and hydrogen sulphide; in sediments dissolved oxygen, dissolved organic carbon, particulate organic carbon, orthophosphate, adsorbed phosphorous and hydrogen sulphide. On the basis of the results of the simulations carried out, the best management strategy limiting eutrophication processes in Fogliano lagoon has been pointed out.