Drivers of Cyanobacterial Blooms in a Hypertrophic Lagoon

The Curonian Lagoon is Europe's largest lagoon and one of the most seriously impacted by harmful blooms of cyanobacteria. Intensive studies over the past 20 years have allowed us to identify the major drivers determining the composition and spatial extent of hyperblooms in this system. We summarize and discuss the main outcomes of these studies and provide an updated, conceptual scheme of the multiple interactions between climatic and hydrologic factors, and their influence on internal and external processes that promote cyanobacterial blooms. Retrospective analysis of remote sensed images demonstrated the variability of blooms in terms of timing, extension and intensity, suggesting that they occur only under specific circumstances. Monthly analysis of nutrient loads and stoichiometry from the principal tributary (Nemunas River) revealed large interannual differences in the delivery of key elements, but summer months were always characterized by a strong dissolved inorganic N (and Si) limitation, that depresses diatoms and favors the dominance of cyanobacteria. Cyanobacteria blooms occurred during high water temperatures, long water residence time and low-wind conditions. The blooms induce transient (night-time) hypoxia, which stimulates the release of iron-bound P, producing a positive feedback for blooms of N-fixing cyanobacteria. Consumer-mediated nutrient recycling by dreissenid mussels, chironomid larvae, cyprinids and large bird colonies, may also affect P availability, but their role as drivers of cyanobacteria blooms is understudied

The distribution of carbon stable isotopes as an indicator of temporal and spatial dynamic and distribution of autochtonic and allochtonic organic matter in a shallow estuarine lagoon

Physical and chemical parameters were measured in a mostly freshwater estuarine lagoon in the SE Baltic. Present paper demonstrates an attempt to trace the sources and analyse the seasonal and spatial patterns of distribution of   POC, DIC and DOC in the Curonian lagoon mostly by the isotopic content in different forms of carbon. Samples were collected in 2012-2014 in 9 stations in the Curonian lagoon including riverine and marine input/output stations. Riverine inputs and summarizing outflow to the Baltic sea locations (Nemunas river delta and Klaipeda channel stations were sampled monthly, while  POC, DIC and DOC samples in other stations were collected on a seasonal basis.  The observed results allow easily differentiate between estuarine and riverine POM samples, while the differences in DOC δ13C content between sampling stations were found to be not statistically reliable.  The high biological productivity of the Nemunas river along with the minor contribution of the Baltic Sea inflows to the overall hydrodynamics of the lagoon explain similarity of content between riverine and estuarine material in the spring and autumn.  However, the δ13C content of DIC and DOC could serve as indicator of external inputs only in connection with seasonal water residence variations.DOI: 10.15181/csat.v3i1.1132</p

The distribution of carbon stable isotopes as an indicator of temporal and spatial dynamic and distribution of autochtonic and allochtonic organic matter in a shallow estuarine lagoon

Physical and chemical parameters were measured in a mostly freshwater estuarine lagoon in the SE Baltic. Present paper demonstrates an attempt to trace the sources and analyse the seasonal and spatial patterns of distribution of   POC, DIC and DOC in the Curonian lagoon mostly by the isotopic content in different forms of carbon. Samples were collected in 2012-2014 in 9 stations in the Curonian lagoon including riverine and marine input/output stations. Riverine inputs and summarizing outflow to the Baltic sea locations (Nemunas river delta and Klaipeda channel stations were sampled monthly, while  POC, DIC and DOC samples in other stations were collected on a seasonal basis.  The observed results allow easily differentiate between estuarine and riverine POM samples, while the differences in DOC δ13C content between sampling stations were found to be not statistically reliable.  The high biological productivity of the Nemunas river along with the minor contribution of the Baltic Sea inflows to the overall hydrodynamics of the lagoon explain similarity of content between riverine and estuarine material in the spring and autumn.  However, the δ13C content of DIC and DOC could serve as indicator of external inputs only in connection with seasonal water residence variations.DOI: 10.15181/csat.v3i1.113

The distribution of carbon stable isotopes as an indicator of temporal and spatial dynamic and distribution of autochtonic and allochtonic organic matter in a shallow estuarine lagoon

Physical and chemical parameters were measured in a mostly freshwater estuarine lagoon in the SE Baltic. Present paper demonstrates an attempt to trace the sources and analyse the seasonal and spatial patterns of distribution of   POC, DIC and DOC in the Curonian lagoon mostly by the isotopic content in different forms of carbon. Samples were collected in 2012-2014 in 9 stations in the Curonian lagoon including riverine and marine input/output stations. Riverine inputs and summarizing outflow to the Baltic sea locations (Nemunas river delta and Klaipeda channel stations were sampled monthly, while  POC, DIC and DOC samples in other stations were collected on a seasonal basis.  The observed results allow easily differentiate between estuarine and riverine POM samples, while the differences in DOC δ13C content between sampling stations were found to be not statistically reliable.  The high biological productivity of the Nemunas river along with the minor contribution of the Baltic Sea inflows to the overall hydrodynamics of the lagoon explain similarity of content between riverine and estuarine material in the spring and autumn.  However, the δ13C content of DIC and DOC could serve as indicator of external inputs only in connection with seasonal water residence variations.DOI: 10.15181/csat.v3i1.1132</p

Phosphorus Cycling in a Freshwater Estuary Impacted by Cyanobacterial Blooms

The availability of reactive phosphorus (P) may promote cyanobacterial blooms, a worldwide increasing phenomenon. Cyanobacteria may also regulate benthic P cycling through labile organic input to sediments, favouring reduced conditions and P release, ultimately acting as self-sustainment mechanism for the phytoplankton blooms. To analyse P–cyanobacteria feedbacks and compare external versus internal loads, we investigated P cycling in the Curonian Lagoon, a freshwater estuary with recurrent summer blooms. At two sites representing the dominant sediment types, we characterised P pools and mobility, via combined pore water analysis, calculation of diffusive exchanges and flux measurements via sediment core incubations. Annual P budgets were also calculated, to analyse the whole lagoon role as net sink or source. Muddy sediments, representing nearly 50 % of the lagoon surface, displayed higher P content if compared with sandy sediments, and most of this pool was reactive. The muddy site had consequently higher pore water dissolved inorganic phosphorus (DIP) concentrations maintaining high diffusive gradients. However, measured fluxes suggested that both sediment types were mostly P sinks except for a large DIP regeneration (nearly 30 μmol m−2 h−1) recorded at the muddy site during an intense cyanobacteria bloom. Such internal regeneration had the same order of magnitude as the annual external P load and may offset the net annual DIP sink role of the estuary. It may also prolong the duration of the bloom. Our results suggest that positive feedbacks can regulate N-fixing cyanobacteria blooms and internal P recycling, through either diffusive fluxes or sediment settling and resuspension

Benthic metabolism in fluvial sediments with larvae of Lampetra sp.

Lampreys spend their larval stage within fine sand fluvial sediments, where they burrow and act as filter feeders. Lamprey larvae (ammocoetes) can significantly affect benthic-pelagic coupling and nutrient cycling in rivers, due to high densities. However, their bioturbation, feeding and excretion activities are still poorly explored. These aspects were investigated by means of laboratory incubations of intact sediments added with ammocoetes and of animals alone. Oxygen respiration, nutrient fluxes and excretion rates were determined. Individual ammocoete incubations suggested that biomass-specific oxygen consumption and ammonium, reactive phosphorus and silica excretion were size-dependent, and greater in small compared to large individuals. The comparison of ammocoetes metabolic rates with rates measured in intact sediments revealed that ammocoetes activity decreases significantly when they are burrowed in sediments. Furthermore, results suggest that a major fraction of ammonium excreted by ammocoetes was assimilated by benthic microbes or microalgae to overcome in situ N-limitation. Alternatively, part of the excreted ammonium was oxidized and denitrified within sediments, as nitrate uptake rather increased along with ammocoetes density. Ammocoetes excreted reactive phosphorus and silica but such production was not apparent in bioturbated sediments, likely due to microbial or microalgal uptake or to immobilization in sediments

Microphytobenthos and chironomid larvae attenuate nutrient recycling in shallow-water sediments

In shallow-water sediments, the combined action of microphytobenthos and bioturbating fauna may differentially affect benthic nutrient fluxes and exert a bottom-up control of pelagic primary production. In many cases, the effects of microphytobenthos and macrofauna on nutrient cycling were studied separately, ignoring potential synergistic effects. We measured the combined effects of microphytobenthos and chironomid larvae on sediment-water fluxes of gas (O2, TCO2 and N2) and nutrients (NH4+, NO3-, NO2-, PO43- and SiO2) in shallow-water sediments of a hypertrophic freshwater lagoon. Fluxes were measured in the light and in the dark in reconstructed sediments with low (L = 600 ind/m2), high (H = 1,800 ind/m2) and no (C) addition of chironomid larvae, after 3 weeks of pre-incubation under light/dark regime to allow for microalgal growth. Besides flux measurements, pore water nutrient (NH4+, PO43- and SiO2) and dissolved metal concentrations (Fe2+ and Mn2+) were analysed and diffusive fluxes were calculated. Chironomid larvae increased sediment heterotrophy, by augmenting benthic O2 demand and TCO2 and N2 dark production. However, on a daily basis, treatments C and L were net O2 producing and N2 sinks while treatment H was net O2 consuming and N2 producing. All treatments were net C sink regardless of chironomid density. Microphytobenthos always affected benthic nutrient exchange, as significantly higher uptake or lower efflux was measured in the light compared with dark incubations. Theoretical inorganic N, P and Si demand by benthic microalgae largely exceeded both dark effluxes of NH4+, PO43- and SiO2 and their net uptake in the light, suggesting the relevance of N-fixation, water column NO3- and solid-phase associated P and Si as nutrient sources to benthic algae. Chironomid larvae had a minor effect on inorganic N and P fluxes while they significantly stimulated inorganic Si regeneration. Their bioturbation activity significantly altered pore water chemistry, with a major reduction in nutrient (highest for NH4+ and lowest for SiO2) and metal concentration. Underlying mechanisms are combinations of burrow ventilation and bioirrigation with stimulation of element-specific processes as coupled nitrification-denitrification, co-precipitation and inhibition of anaerobic paths such as Fe3+ or Mn4+ reduction or re-oxidation of their end products. The combined activity of benthic algae and chironomid larvae may significantly attenuate internal nutrient recycling in shallow eutrophic ecosystems, and contribute to the control of pelagic primary production

Essence of the patterns of cover and richness of intertidal hard bottom communities: a pan-European study

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