Development of a subsurface chlorophyll maximum at the entrance to the Gulf of Finland, Baltic Sea

Abstract

Horizontal, vertical, and temporal variability of chlorophyll a (Chl a), stratification, currents, and pelagial biology (phytoplankton, size-fractionated primary productivity and Chl a, ciliates, picocyanobacteria, bacterial production) were followed at the entrance to the Gulf of Finland during a 12-day cast study in July-August 1994. The study was carried out during weak wind conditions when hydrodynamic activity was minimal and when an intense blooming of diazotrophic cyanobacteria Aphanizomenon flos-aquae (Linne) Ralfs, Anabaena lemmermannii Richter, and Noduluria spumigena Mertens was in its decaying phase. The property conservation equation was used to differentiate the contributions of physical and biological processes to the local Chl a changes in the layer above the thermocline. Subtraction of the Chl a change due to physical processes, mainly advection, gave a biologically induced mean decrease of 0.18 mg Chl a m-3 d-l. Sampling data confirmed the calculated decrease and revealed the basin-wide character of the phenomenon. The observed plankton dynamics showed that the pelagial system was "top-down" controlled and that about half the decrease of Chl a was due to a size-specific grazing of ciliates on the dominating flagellates Chrysochromulina spp. 2-5 µm and Ochromonas spp. 7-8 µm in the upper layer. The grazers were confined to the layer above the thermocline, while the flagellates inhabited both the thermocline and the layer above. This led to the formation of a subsurface Chl a maximum in the thermocline.Horizontal and vertical inhomogeneities in waterbodies and their role in sustaining favorable growth conditions for opportunistic plankton species are of increasing interest in marine and freshwater science. Subsurface Chl a maxima (SCM) are well-documented phenomena in tropical, subtropical, and temperate seas. Chl a is a summarizing parameter of the pigment from several phytoplankton species and functional groups, and typically, the vertical distributions of single species show more distinct subsurface maxima than the SCM, even in decimeter scale (Bjørnsen and Nielsen 1991; Carpenter et al. 1995). The role of SCMs in the population dynamics of harmful algal species has recently been addressed by Nielsen et al. (1990) and Maestrini and Graneli (1991).Usually, SCMs develop in waters where, or when, the Acknowledgments We thank the officers and crew of RV Aranda for their cooper-

Spatial and temporal variability of a dinoflagellate-cyanobacterium community under a complex hydrodynamical influence:a case study at the entrance to the Gulf of Finland

“…Nutnents were exhausted much deeper in the water mass dunng our study than in late July 1994, when calm weather prevailed in the northern Baltic Sea for several weeks. For example, the mean N o 3 concentration at 40 m was 0.3 + 0.3 pM in 1996 compared with 1.7 + 1.1 pM in 1994 , Kononen et al 1998). This suggests that in 1996 a considerably larger amount of nutrients had been transferred into the plankton ecoSystem due to the extreme hydrometeorological conditions.…”

“…The good and significant correlations of A. flos-aquae abundance with chl a (r2 = 0.79, p < 0.001) and pnmary production (r2 = 0.72, p < 0.01) in the >20 pm size fraction and the weaker correlation of H. triquetra with chl a (r2 = 0.38, p < 0.08) in the <20 pm size fraction Support this. Using a carbon to chl a ratio of 64, determined in our earlier study in the same area (Kononen et al 1998), we calculate that H. triquetra contributed an average of 19 % (range 3 to 55 %) of chl a in the < 20 pm size fraction, and A. £los-aquae 24 % (range 17 to 33 %) of chl a in the > 20 pm size fraction.…”

“…Thus it was not possible by use of the property conservation equation to separate quantitatively the vanability due to biological factors (growth, sinking, grazing) from the variability due to pure advective transport (e.g. Kononen et al 1998). The hydrodynamical background at the AN (downwelling, penphery of anticyclonic eddy, cyclonic side of the jet) suggests that vertical downward motion prevailed throughout the study penod.…”

Long-term monitoring data reveal pH dynamics, trends and variability in the western Gulf of Finland

“…The pH measurements extended from 29 April 1972 to 9 December 2009. At the entrance to the Gulf of Finland, the majority of the pelagic primary production occurs in the surface water between 0 and 10 m depth (Kononen et al 1998), and therefore we selected pH data from 4 or 5 m depth as a proxy for the most active zone (total N = 651).…”

Use of different phosphorus sources by the bloom-forming cyanobacteria Aphanizomenon flos-aquae and Nodularia spumigena

“…Water column stratification affects both the temperature and light climate experienced by the species, but also phosphorus availability and quality at different depths, with DIP concentrations rapidly increasing below the thermocline (Laanemets et al 2004). The biomass maxima of the 2 bloom-forming species are usually vertically separated, A. flos-aquae having a deeper biomass maximum than N. spumigena (Niemistö et al 1989, Kononen et al 1998, Vahtera et al 2005. Thus, in addition to the effects of the physical environment, the variation in prevalent phosphorus source with depth and sub-basin may affect the species composition of the cyanobacteria blooms.…”