Trophic state and seasonal dynamics of phytoplankton communities in two sand-pit lakes at different successional stages

The seasonal and inter-annual dynamics of phytoplankton in two Italian sand-pit lakes have been analysed over three years (2004-2006). The studied lakes, Ca' Morta (CM) and Ca' Stanga (CS), have a common origin and are contiguous, but are of different ages and have been managed in different ways. CM was created in 2002 and was still being dredged during the study period, while sand quarrying in CS began in 1998 and ended at the beginning of the study period. These conditions were thought to be of particular interest in assessing the influence of dredging activities on algal assemblages. Physical and chemical water parameters were also analysed to characterise the two lakes and investigate the effects of hydro-chemical features on phytoplankton. CS was stratified from April to October each year during the study period, while CM showed weaker thermal stratification. Conductivity, dissolved nitrate and dissolved reactive silica concentrations were greater in CM than in CS. The mixing depth/euphotic depth ratio was also higher in CM, probably because of water column mixing induced by hydraulic dredging. Overall, 185 phytoplankton taxa were found, of which ca 50% were present in both lakes. No significant differences in species richness were observed between lakes, but the decreasing trend of Simpson index values measured in CS showed a reduction in biodiversity after the excavation phase. Phytoplankton biomass was higher in CS than in CM, the latter being oligo-mesotrophic and the former mesotrophic. Diatoms and dinoflagellates were the dominant taxa in both lakes. The main differences in phytoplankton assemblages were related to the persistence of chryptophytes and chlorophytes in CM. Multivariate analyses support the hypothesis that different thermal patterns induced by dredging activities, along with certain physico-chemical parameters of the water, were the major factors shaping phytoplankton communities

Variability in Environmental Conditions Strongly Impacts Ostracod Assemblages of Lowland Springs in a Heavily Anthropized Area

The Po river plain (Northern Italy) hosts artificial, lowland springs locally known as fontanili, which provide important ecosystem services in an area dominated by intensive agricultural activities. Here we present a study carried out in 50 springs. Each spring was visited once from October 2015 to January 2016. The sampled sites were selected to include springs studied in 2001 and 2004, to evaluate changes in water quality and ostracod assemblages that possibly occurred over a period of 10–15 years, and explore the relationships between ostracod community composition and water physical and chemical variables. Our results showed a decrease in the chemical water quality especially, in springs south of the Po river, evidenced by high nitrate levels. Most of the studied springs showed a relevant decrease in dissolved reactive silica, probably related to recent transformations of either agricultural practices or crop typology. Ostracods were mostly represented by common and tolerant species, and communities were characterized by low alpha diversity and high species turnover. Water temperature and mineralization level were the most influential variables in structuring the ostracod communities. We stress the need to implement conservation and restoration measures for these threatened ecosystems, to regain their role as ecosystem services providers

Denitrification, Nitrogen Uptake, and Organic Matter Quality Undergo Different Seasonality in Sandy and Muddy Sediments of a Turbid Estuary

The interaction between microbial communities and benthic algae as nitrogen (N) regulators in poorly illuminated sediments is scarcely investigated in the literature. The role of sediments as sources or sinks of N was analyzed in spring and summer in sandy and muddy sediments in a turbid freshwater estuary, the Curonian Lagoon, Lithuania. Seasonality in this ecosystem is strongly marked by phytoplankton community succession with diatoms dominating in spring and cyanobacteria dominating in summer. Fluxes of dissolved gas and inorganic N and rates of denitrification of water column nitrate (Dw) and of nitrate produced by nitrification (Dn) and sedimentary features, including the macromolecular quality of organic matter (OM), were measured. Shallow/sandy sites had benthic diatoms, while at deep/muddy sites, settled pelagic microalgae were found. The OM in surface sediments was always higher at muddy than at sandy sites, and biochemical analyses revealed that at muddy sites the OM nutritional value changed seasonally. In spring, sandy sediments were net autotrophic and retained N, while muddy sediments were net heterotrophic and displayed higher rates of denitrification, mostly sustained by Dw. In summer, benthic oxygen demand increased dramatically, whereas denitrification, mostly sustained by Dn, decreased in muddy and remained unchanged in sandy sediments. The ratio between denitrification and oxygen demand was significantly lower in sandy compared with muddy sediments and in summer compared with spring. Muddy sediments displayed seasonally distinct biochemical composition with a larger fraction of lipids coinciding with cyanobacteria blooms and a seasonal switch from inorganic N sink to source. Sandy sediments had similar composition in both seasons and retained inorganic N also in summer. Nitrogen uptake by microphytobenthos at sandy sites always exceeded the amount loss via denitrification, and benthic diatoms appeared to inhibit denitrification, even in the dark and under conditions of elevated N availability. In spring, denitrification attenuated N delivery from the estuary to the coastal area by nearly 35%. In summer, denitrification was comparable (~100%) with the much lower N export from the watershed, but N loss was probably offset by large rates of N-fixation

3. Water Physicochemistry in IRES

Peer reviewe

Influence of Potamogeton pectinatus and microphytobenthos on benthic metabolism in a freshwater littoral sediment in an agricultural landscape: N assimilation vs N removal

The influence of Potamogeton pectinatus colonization on benthic nitrogen dynamics was quantified in the littoral zone of a lowland pit lake with high nitrate availability (~200 µM). Our hypothesis was that in aquatic environments where nitrogen availability is not limiting, colonization by rooted macrophytes changes the dynamics of the benthic nitrogen cycle, stimulating N assimilation and denitrification, and increasing the system capacity to mitigate external nitrogen loads. To test this hypothesis we quantified and compared light and dark seasonal variations of benthic metabolism, dissolved inorganic nitrogen fluxes, denitrification and N assimilation rates in an area colonized by P. pectinatus and a reference site colonized by microphytobenthos. In both areas the benthic system was net autotrophic and a sink for dissolved inorganic nitrogen (2241-2644 mmol m-2 y-1). Plant colonization increased nitrogen losses via denitrification by 30% compared to the unvegetated area. In contrast to what is generally observed in coastal marine systems, where the presence of rooted macrophytes limits denitrification rates, under the very high nitrate concentrations in the studied lake, both denitrification (1237-1570 mmol m-2 y-1) and N-assimilation (1039-1095 mmol m-2 y-1) played important and comparable roles in the removal of dissolved inorganic nitrogen from the water column

Do oxic–anoxic transitions constrain organic matter mineralization in eutrophic freshwater wetlands?

This study aims at investigating decomposition processes in wetlands that are daily or seasonally exposed to intermittent oxic and anoxic conditions. We hypothesized that in wetland ecosystems where anoxia regularly establishes, decomposition rates are not affected by oxygen shortage, especially when nitrates are available. Monitoring and experiments were performed from December 2003 to January 2005 in one of the widest (81 ha) freshwater wetlands in the Po river floodplain (Natural Reserve Paludi del Busatello, Italy). Intact sediment cores were sampled on a seasonal basis. Sediment–water fluxes of oxygen, dissolved inorganic carbon, methane, and inorganic nitrogen were determined under oxic and anoxic conditions. Oxic–anoxic transitions always resulted in enhanced ammonium, dissolved inorganic carbon, and methane effluxes. At high temperatures, the methane release from sediments was inversely related to both nitrate concentrations and uptake. Likely, nitrate can compensate for the oxygen deficiency while maintaining an oxidative metabolism, either supporting microbial decomposition as an electron acceptor or stimulating the oxidation of the byproducts of the anaerobic metabolism, e.g., methane. This is a key point as the number of temperate wetlands with concurrent nitrate pollution and oxygen shortage is increasing throughout the world

Seasonal nitrogen and phosphorus dynamics during benthic clam and sospende mussel cultivation

Effects of suspended mussel and infaunal clam cultivation on sediment characteristics, and benthic organic and inorganic nitrogen and phosphorus fluxes were compared in a shallow coastal lagoon. The two species had different impacts on sediment features, but both created "hotspots" of nutrient fluxes with annual N and P regeneration rates being 4.9 and 13.5 (mussel) and 4.5 and 14.9 (clams) fold greater than those of unfarmed control sediments. Mussel farming also caused considerable nutrient regeneration within the water column with the mussel ropes contributing similar to 25% of total inorganic N and P production and at times dominating the sediments (e.g. 95% of SRP production in summer and 45% of DIN production in winter). Such nutrient regeneration rates seriously question the proposal that suspension-feeding bivalves act as a eutrophication buffer, especially during summer when nutrient regeneration rates are maximal, but other nutrient sources (freshwater run-off and unfarmed sediments) are at their lowest. (C) 2011 Elsevier Ltd. All rights reserved

Trophic state and seasonal dynamics of phytoplankton communities in two sand-pit lakes at different successional stages

The seasonal and inter-annual dynamics of phytoplankton in two Italian sand-pit lakes have been analysed over three years (2004-2006). The studied lakes, Ca' Morta (CM) and Ca' Stanga (CS), have a common origin and are contiguous, but are of different ages and have been managed in different ways. CM was created in 2002 and was still being dredged during the study period while sand quarrying in CS began in 1998 and ended at the beginning of the study period These conditions were thought to be of particular interest in assessing the influence of dredging activities on algal assemblages. Physical and chemical water parameters were also analysed to characterise the two lakes and investigate the effects of hydro-chemical features on phytoplankton. CS was stratified from April to October each year during the study period, while CM showed weaker thermal stratification. Conductivity, dissolved nitrate and dissolved reactive silica concentrations were greater in CM than in CS. The mixing depth/euphotic depth ratio was also higher in CM, probably because of water column mixing induced by hydraulic dredging. Overall. 185 phytoplankton taxa were found, of which ca 50% were present in both lakes. No significant diffrences in species richness were observed between lakes, but the decreasing trend of Simpson index values measured in CS showed a reduction in biodiversity after the excavation phose. Phytoplankton biomass was higher in CS than in CM, the latter being oligo-mesotrophic and the former mesotrophic. Diatoms and dinoflagellates were the dominant taxa in both lakes. The main differences in phytoplankton assemblages were related to the persistence of chryptophytes and chlorophytes in CM. Multivariate analyses support the hypothesis that different thermal patterns induced by dredging activities, along with certain physico-chemical parameters of the water, were the major factors shaping phytoplankton communities

Benthic primary production and bacterial denitrification in a Mediterranean eutrophic coastal lagoon

Microphytobenthos and macroalgal mats are simultaneously present in eutrophic lagoons and are expected to have different direct and indirect effects on nitrogen related processes through uptake and inhibition or stimulation of microbial activity. To assess the relative contribution of different primary producer communities and heterotrophic processes to benthic nitrogen cycling, we studied nitrogen uptake and bacterial denitrification in the eutrophic Sacca di Goro lagoon (northern Italy). Benthic fluxes of oxygen and dissolved inorganic nitrogen (DIN), and rates of nitrification-coupled (Dn) and water-nitrate (Dw) denitrification were measured every 30-45. days for one year at two shallow sites. Station Giralda is close to the main freshwater inlet, has turbid waters and muddy-organic and bioturbated sediments with microphytobenthos (MPB). Station Gorino is brackish with muddy-sand sediments which are covered by macroalgal mats of the genus Ulva. Here, sediment patches with and without macroalgae (MA) were simultaneously studied.Sediments with MPB were net heterotrophic and regenerated large amounts of ammonium to the water column. At Gorino, sediments with MA were net autotrophic through the year, and DIN fluxes were mainly controlled by macroalgal uptake. On an annual basis, denitrification rates were three fold higher at Giralda (2.27±0.06molNm-2yr-1) than at Gorino (0.83±0.01molNm-2yr-1), due to higher nitrate in the water column and nitrification in surface sediments. At Gorino, denitrification was one order of magnitude lower than DIN uptake by macroalgae (10.39±1.30molNm-2yr-1). Nevertheless, the differences between denitrification rates in sediments with and without MA were unexpectedly negligible, showing that the denitrification capacity was not suppressed by macroalgal competition. Results from this study suggest that in eutrophic lagoons nitrogen cycling seems less affected by MPB compared to more oligotrophic coastal waters and that most of the available DIN flows through benthic macroalgae. However, Ulva is only a temporary N-sink and most of its nitrogen pool can be either rapidly recycled or exported by tidal currents to the open sea

Denitrification in a meromictic lake and its relevance to nitrogen flows within a moderately impacted forested catchment

We analysed the spatial and temporal variability of benthic nitrogen fluxes and denitrification rates in a sub-alpine meromictic lake (Lake Idro, Italy), and compared in-lake nitrogen retention and loss with the net anthropogenic nitrogen inputs to the watershed. We hypothesized a low nitrogen retention and denitrification capacity due to meromixis. This results from nitrate supply from the epilimnion slowing down during stratification and oxygen deficiency inhibiting nitrification and promoting ammonium recycling and its accumulation. We also hypothesized a steep vertical gradient of sedimentary denitrification capacity, decreasing with depth and oxygen deficiency. These are important and understudied issues in inland waters, as climate change and direct anthropic pressures may increase the extent of meromixis. Nearshore sediments had high denitrification rates (87 mg mâ\u88\u922 dayâ\u88\u921) and efficiency (~ 100%), while in the monimolimnion denitrification was negligible. The littoral zone, covering 10% of the lake surface, contributed ~50% of total denitrification, while the monimolimnion, which covered 70% of the sediment surface, contributed to < 13% of total denitrification. The persistent and expanding meromixis of Lake Idro is expected to further decrease its nitrogen removal capacity (31% of the incoming nitrogen load) compared to what has been measured in other temperate lakes. Values up to 60% are generally reported for other such lakes. Results of this study are relevant as the combination of anthropogenic pressures, climate change and meromixis may threaten the nitrogen processing capacity of lakes