Drivers of Cyanobacterial Blooms in a Hypertrophic Lagoon

Abstract

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. Consumermediated 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.

Synergistic Use of Synthetic Aperture Radar and Optical Imagery to Monitor Surface Accumulation of Cyanobacteria in the Curonian Lagoon

“…As mentioned in Section 1, given the lack of a continuous and dedicated field campaign and the unavailability of synchronous acquisition of optical and radar images, a rigorous validation for the proposed method cannot be performed. Indeed, the dependencies of cyanobacteria on solar radiation and on air temperature and their ability to consequently perform vertical migration via buoyancy regulation makes their concentration highly variable within the time span of a daily cycle [16,21,41]. It is however viable to assess whether and to what extent the condition WR < 0.1 can be adopted as "SCUM alert" in terms of qualitative interpretation.…”

“…The parametrization of 6SV code in case of the Curonian Lagoon is performed with Aerosol Optical Thickness (AOT) values gathered from AERONET station Gustav Dalen Tower located in the Baltic Sea and with water vapour values retrieved from daily MODIS products, via NASA Giovanni interface [36]. Chl-a concentration is produced after the application of a semi-empirical band-ratio model that uses the reflectance in the red and near-infrared (NIR) spectral regions [21,37]. The scum presence is calculated according to the approach by Bresciani et al [20], with the ratio between reflectances in the near infrared (NIR) range.…”

“…Indeed, while S1-L2 products are generally available only for open ocean, the closeness of the lagoon to the Baltic Sea makes that an exception. Moreover, cyanobacteria blooms occurring in the Curonian Lagoon have been widely investigated with dedicated field campaigns [21] and the optical water properties of the Curonian Lagoon in the presence of cyanobacteria have been ascertained [22,23].…”

Feces from Piscivorous and Herbivorous Birds Stimulate Differentially Phytoplankton Growth

“…We investigate with a laboratory experiment the effects produced by feces from herbivorous and piscivorous birds on microbial activity and on phytoplankton growth and community composition in the water of the Curonian Lagoon, Lithuania. Much evidence supports the relevance of birds in the biogeochemistry of the lagoon and, potentially, in supporting algal blooms [19]. First of all, this freshwater estuary displays blooms of cyanobacteria (up to 400 µg Chl-a L −1 ) during summer, when external nutrient input is minimum and it is characterized by strong inorganic nitrogen and silica limitation [2,19].…”

“…Much evidence supports the relevance of birds in the biogeochemistry of the lagoon and, potentially, in supporting algal blooms [19]. First of all, this freshwater estuary displays blooms of cyanobacteria (up to 400 µg Chl-a L −1 ) during summer, when external nutrient input is minimum and it is characterized by strong inorganic nitrogen and silica limitation [2,19]. The latter is due to a steep late spring-early summer decrease of discharge and of N and Si loads from the Nemunas River, the dominant freshwater input [2,6,20].…”