The Continuous Plankton Recorder survey has monitored plankton

No abstract

We examined long‐term changes in the macroalgal vegetation at Stora Bornö Island in the inner Gullmar Fjord on the Swedish Skagerrak coast. This was made possible by access to a 1941 diving investigation. The same sites were reinvestigated in 1998. Community composition and depth distributions of species were compared and changes were analyzed with focus on functional groups (size, thallus shape, and life‐history traits). We discovered a significant decrease in the depth extension of macroalgal species and a dramatic decline of species richness in the lower littoral (below 16 m of depth) compared with 57 years earlier. Ordination analysis revealed that there was a significant difference in the community composition between the two study periods. In general, small (<10 cm), thin, filamentous, and aseasonal ephemerals increased in relative abundance, whereas larger (>10 cm), coarsely branched, and perennial algae decreased. Calibrations of individual species to local sediment cover, using canonical correspondence analysis, indicated that part of the change in species composition was related to sediment load. Furthermore, large‐scale climate differences (NAO Winter Index) between the study periods indicated a higher impact of Baltic Sea and Kattegat water in the nutrient dynamics of the fjord in the 1998 study. We concluded that the observed long‐term changes in the macroalgal community at Stora Bornö Island were consistent with an increased nutrient availability.

The large variation in size and shape in diatoms is shown by morphometric measurements of 515 benthic and pelagic diatom species from the Baltic Sea area. The largest mean cell dimension (mostly the apical axis) varied between 4.2 and 653 μm, cell surface area between 55 and 344,000 μm2, and cell volume between 21 and 14.2 × 106μm3. The shape‐related index, length to width ratio, was between 1.0 and 63.3 and the shape‐ and size‐related index, surface area to volume ratio, was between 0.02 and 3.13. Diatom community analysis by multivariate statistics is usually based on counts of a fixed number of diatom valves with species scores irrespective of cell size. This procedure underestimates the large species for two reasons. First, the importance of a species with higher cell volume is usually larger in a community. Second, larger species usually have lower abundances and their occurrence in the diatom counts is stochastic. This article shows that co‐occurring small and large diatom species can respond very differently to environmental constraints. Large epiphytic diatoms responded most to macroalgal host species and small epiphytic diatoms most to environmental conditions at the sampling site. Large epilithic diatoms responded strongly to salinity, whereas small epilithic diatoms did so less clearly. The conclusion is that different scale‐dependent responses are possible within one data set. The results from the test data also show that important ecological information from diatom data can be missed when the large species are neglected or underestimated.

We show how photosynthesis and UV sensitivity of algae are related to thallus morphology and depth distributions. This was studied for typical depth zonations of red and brown macroalgae in the Skagerrak (ca. 25 psu) and the Baltic Sea (6.5 psu). The algae were collected from the water surface down to 20.5 m of depth, whereby each species was sampled at its maximum abundance depth. Altogether, we measured photosynthetic and respiratory rates of 19 red and 13 brown algal species as O 2 evolution at different light intensities. Photosynthesis versus irradiance curves (PI curves) showed that light-saturated net photosynthetic rates (P max ), respiratory rates in darkness (R d ) and the initial slope (α) were strongly related to algal morphology with higher values for thinner species. The compensation irradiance (I c ) and saturating irradiance (I k ) were strongly related to water depth with lower values at greater depth. A novel approach to analyse PI data with principal component analysis (PCA) is presented. The method makes it possible to assign a quantitative morphological gradient to algal species based on photosynthetic properties. Such a gradient can be used in ecological studies as an alternative to more subjective discrete subdivisions into functional-form groups. Another type of PCA analysis, with the relative shapes of the PI curves as input data, summarises α and convexity but discards all interference of morphology. This results in a gradient of genuine physiological responses, which in our study was strongly correlated to maximum abundance depth. The UV sensitivity of the same 32 algal species was determined as the change in net O 2 evolution after exposure to UV light and the recovery after this treatment. Deeper-growing algae were more sensitive to UV and species with thinner thalli recovered better after UV treatment in the Skagerrak. No such trends were observed for the algae in the northern Baltic Sea, which suggests that no real deepwater species occur here. This is further supported by the lack of a clear pattern in I c and I k values with depth for the algae in the Baltic Sea. Our results advocate that the reduced species diversity of the Baltic Sea is also coupled to a loss of functional groups in the sense of general photosynthetic performance and not only in the sense of pure morphology (loss of canopy-forming species).