Jukka Seppälä

Recent Changes in Trophic State of the Baltic Sea along SW Coast of Finland

Abstract Drawing reliable conclusions on changes in the trophic state of various subareas of the Baltic Sea is problematic, in large part because the monitoring of productivity parameters exhibiting high natural variability is based on sparse sampling. This emphasizes the importance of long-term data sets. Here we present a 30-year chlorophyll α data set from the western Gulf of Finland. The trophic state of the study area showed an increasing trend in the 1970s and 1980s manifested mainly as strengthened vernal blooms. This trend did not continue in the 1990s, and the seasonal phytoplankton biomass maxima since then has begun to show some bias toward the late summer. The changes in seasonal maxima of phytoplankton blooms probably reflect i) decreased availability of N suppressing the magnitude of the vernal bloom, which increases the P reserves for the summer, and ii) enhanced internal P loading, which further increases the summer P reserves.

Specific Affinity for Phosphate Uptake and Specific Alkaline Phosphatase Activity as Diagnostic Tools for Detecting Phosphorus-limited Phytoplankton and Bacteria

We analyzed responses of soluble reactive phosphorus (SRP), bioavailable phosphate (PO4), particulate phosphorus, turnover time of orthophosphate (Tt), and alkaline phosphatase activity (APA) to varying degrees of nutrient stress. The nutrient stress was evoked by different treatments in concentration and combination of inorganic nitrogen (N) and phosphorus (P), and labile organic carbon (glucose) to mesocosms in experiments carried out in eutrophic southern (Odense Fjord, Denmark) and northern (Tvärminne Archipelago, Finland) coastal zones of the Baltic Sea. Despite seasonal and geographical differences, similar responses were observed in both experiments. Low SRP (<100 nmol l−1), shortTt (<10 h), and increased levels of APA were observed in both N+P balanced and P deficient treatments, while the opposite trend was observed in P replete treatments. The shortestTt and the highest APA were found when glucose was combined with N treatment. Bioavailable PO4 was estimated usingTt and P uptake rates as derived from stoichiometric conversion of carbon based primary and bacterial production. With shorterTt, the PO4 pool declined to <1 nmol-P l−1, whereas the SRP background pool (difference between SRP and PO4) remained relatively constant (c. 50 nmol l−1). APA was inversely related to PO4 but not to SRP. Responses of specific APA and specific affinity for PO4 uptake, which are APA and PO4 uptake rates (inverse ofTt), respectively, normalized to the summed P biomass of phytoplankton and bacteria, responded consistently to the pattern and magnitude of nutrient limitation evoked in our experiments. Our results, together with a literature survey, suggest that both parameters can be useful in examining PO4 availability for the natural phytoplankton and bacteria community in P starved aquatic systems.

Spectral absorption and fluorescence characteristics of phytoplankton in different size fractions across a salinity gradient in the Baltic Sea

Spectral absorption and fluorescence properties of living phytoplankton were measured across a salinity gradient in the northern Baltic Sea in August 2000. The discrete water samples (nu200a=u200a22) were size‐fractionated (<2u2009µm and <20u2009µm) to study the origins of different optical signals. The concentration of chlorophyll‐a (Chla) ranged from 1.6u2009mgu2009m−3 to 6.0u2009mgu2009m−3, from which picophytoplankton (<2u2009µm) contributed 18–54%. Phytoplankton community structure had a clear gradient; from the northern part, dominated by chlorophytes, to the southern part, dominated by cyanophytes (mainly Aphanizomenon sp.). The regional variability in the Chla‐specific absorption coefficient was partly explained by variations in phytoplankton community size‐structure, or species composition. Phycoerythrin absorption was notable, especially for picophytoplankton. Accordingly, phycoerythrin fluorescence arose mainly from picophytoplankton (average 74%, range 35–100%). Thus, pico‐sized cyanophytes – predominated by phycoerythrin‐rich cells – were the main source of the phycoerythrin signal. Large filamentous cyanobacteria, in turn, were responsible for the phycoerythrocyanin and phycocyanin signals. The observed spectral differences between size fractions contribute to the development of operational optical detection systems for cyanobacterial blooms and provide the background for other absorption and fluorescence‐based algal detection systems and for remote sensing algorithm development.

Recent advances in ferrybox monitoring on board Finnmaid ferry

Finnish Institute of Marine Research (FIMR) as a founding member of Alg@line consortium has been a forerunner in the field of monitoring research using commercial ferries. In 1992 FIMR started continuous measurements on board the ferry Finnjet, crossing the Baltic Sea Proper, using unattended recording and sampling system. During the spring of 2007 the ferrybox monitoring system was reinstalled in a new ferry Finnmaid providing real time observed data transmission with satellite connection. Chlorophyll-a (Chla) still remains the principal monitoring parameter. However, the distribution of cyanobacteria cannot be evaluated using Chla in vivo fluorescence, as most of their Chla is located in the poorly-fluorescing photosystem I. Instead, phycocyanin (PC) fluorescence is used in the detection of cyanobacterial blooms in 2005-07. PC fluorescence shows a linear relation to the biomass of the bloom forming filamentous cyanobacteria. During blooms of filamentous cyanobacteria the variability in Chla concentrations is better explained by PC fluorescence than by Chla fluorescence. Additionally, Chla records have been applied in validation of MODIS satellite monitoring for the water quality.