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RESEARCH PRODUCT

Spatial distribution of phytoplankton in the Gulf of Riga during spring and summer stages

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Distribution patterns of chlorophyll a, phytoplankton species and biomass were studied in the Gulf of Riga, one of the most eutrophicated areas of the Baltic Sea. Quasi-synoptic measurements were carried out during four seasonal stages (spring bloom 1995, early-summer stage 1994, cyanobacterial bloom 1994, and late summer stage 1993). For each stage, common factor analysis was used to simplify the highly correlated patterns of nutrients, salinity, temperature and the depth of mixed layer. Obtained latent variables were used to explain spatial distribution of phytoplankton. Generally, the distribution of phytoplankton variables followed closely the patterns of nutrient rich fresh water. During the spring bloom in 1995, the spatial structures of phytoplankton biomass (4–27 mg l−1) and chlorophyll a (13–50 μg l−1) were well explained (r=0.68 and 0.69, respectively, p<0.01) by common factor loaded by inorganic N, total N and P, salinity and the depth of mixed layer. The early-summer stage in 1994 was characterized by homogenous distribution of algal biomass and low concentrations of inorganic N and P. At this stage, the decreasing amount of total N and P from the southern Gulf of Riga towards the northern part of the Gulf was accompanied with the decrease of chlorophyll a concentrations and the increase of relative importance of picoplankton (algae <2 μm). Intense bloom of cyanobacteria (Aphanizomenon flos-aquae (L.) Ralfs) took place in 1994 after warming up of the surface layer. The abundance of A. flos-aquae was related to the common structure of N, P and fresh water and the bloom was most dense (up to 6.6 mg l−1) in the southern Gulf. At the same time, the contribution of picoplankton from total chlorophyll a was high (up to 60%) in the northern Gulf affected less by the nutrient load. The late summer phase in 1993 was characterized by high heterotrophic activities and the phytoplankton community was dominated by cryptomonads (on the average 50% of total biomass). At that stage the distribution of phytoplankton was obviously governed by biological interactions and the multivariate methods were not especially successful to explain the spatial distribution of the main components of phytoplankton community. Excluding the late summer phase, the statistical methods used in this study revealed well the relationships between phytoplankton variables and nutrient rich fresh water. It is evident that nutrient load from the River Daugava is a reason for higher phytoplankton biomass in the southern Gulf of Riga, especially during bloom occasions.