| | | 19.6 Spatial variability of phytoplankton |
During the spring bloom in 1995, the spatial structures of the high phytoplankton biomass and chlorophyll a were well explained by common factors such as inorganic N, total N and P, salinity and the depth of the mixed layer [418]. Contrary to our expectations, the SW part of the Gulf was more influenced by the freshwater flow from the river Daugava than the SE part. Furthermore, the bloom was more pronounced in the SW part of the Gulf. Also in the central region of the southern gulf are influenced by the freshwater load. The river plume can move towards the centre and the eastern part of the Gulf instead of the anticipated north-easterly direction suggested by the general circulation pattern. This is obviously caused by the predominating wind strength and direction and suggests that wind plays a significant role for the spreading of surface and river plume water in the Gulf of Riga.
During the middle of the spring bloom diatoms prevail over the entire Gulf, but algal biomass and chlorophyll a can quickly decrease during week-long periods, although considerable amounts of inorganic N and P are still available [418][456]. The dinoflagellate Peridiniella catenata can be an important constituent of the phytoplankton spring community. The reduction of the silicate pool over the last ten years in the Gulf of Riga [504], which is probably the result of decreased runoff (Laznik et al. 1999), could be one of the factors stimulating the development of dinoflagellates in late spring, as dissolved silicate becomes a limiting nutrient for diatoms. The proportion of diatoms/non-silicate demanding phytoplankton species varies in accordance to the annual and climatological variations in runoff. Strong stratification of the water column appeared to enhance sedimentation of diatoms while the motile dinoflagellates controlled their position in the water column in spring [355][356]. In the most stratified region in the southern Gulf a clear dominance of diatoms, however, remained.
The early summer stage is characterized by a homogenous distribution of algal biomass and low concentrations of inorganic N and P. Picoplankton was dominating the phytoplankton community in the outer Gulf [418]. The small relative fraction of picoplankton in the regions affected by nutrient load in the southern part of the Gulf is in accordance with the conclusions of Kuparinen and Kuosa (1993). The dominating species of green algae, Monoraphidium contortum was probably favoured by eutrophic conditions [269]; the opposite seemed to be true for dinoflagellates.
An intense bloom of the cyanobacterium Aphanizomenon flos-aquae took place in the middle of the summer of 1994 after an increase in temperature of the surface layer. Similar to the situation in spring, the abundance of A. flos-aquae was related to the common structure of N, P and freshwater, and the bloom was most dense in the southern Gulf [418]. It has been postulated that the recent increase of cyanobacteria blooms is caused by the decreasing DIN/DIP ratio [30][31]. During the cyanobacteria bloom in 1994, the DIN/DIP ratio was rather low (0.7-6.4 w:w). As during the early summer stage, the contribution of picoplankton was lower in the most eutrophicated, southernmost areas [456]. Cyanobacteria are obviously supported by the high nutrient status of the river runoff. Less eutrophic conditions in the central and northern parts of the Gulf favoured the growth of cryptomonads and diatoms.
The early autumn phase in 1993 (after an intense bloom of cyanobacteria which covered the entire Gulf; [259]) was characterized by high heterotrophic activity and the phytoplankton community was dominated by cryptomonads (on the average 50 % of total biomass). Diatoms were relatively more abundant in the most nutrient-rich southern Gulf. Cryptomonads, as main constituents of the phytoplankton community, were most abundant on the western side of the Gulf or generally in areas with less total N or P.
Basin-wide the distribution of phytoplankton variables correlated clearly, except during the early autumn stage, with environmental factors such as salinity, stratification and nutrients. Thus, the distribution of phytoplankton variables followed closely the patterns of nutrient-rich freshwater [418]. The nutrient load from the rivers obviously generates a south-north gradient of phytoplankton biomass while the east-west gradient is more variable, probably depending on the predominating winds. The differences in phytoplankton biomass or chlorophyll a along the north-south axis were 3 to 6 fold. It seems to be clear that the nutrient load from rivers in the southern Gulf of Riga is the reason for higher phytoplankton biomass in the southern Gulf, especially during blooms.
| | | 19.6 Spatial variability of phytoplankton |