Kaire Torn

Seasonal variation in invertebrate grazing on Chara connivens and C. tomentosa in Kõiguste Bay, NE Baltic Sea

Charophytes are a highly endangered group of algae. In the Baltic Sea, the number of species, distribution area and biomass of charophytes have significantly decreased in recent decades. Although eutrophication triggers their initial decline, the mechanism of the final extinction of charophyte populations is not fully understood. An in situ experiment was performed to study the role of the mesoherbivores Idotea baltica, Gammarus oceanicus and Palaemon adspersus in the decline of charophytes in the north-eastern Baltic Sea. Invertebrate grazing showed a clear seasonality: grazing pressure was low in April, moderate in July, and high in October. Grazing on charophytes by P. adspersus was negligible, whereas I. baltica and G. oceanicus significantly reduced the biomass of charophytes in the field. Low photosynthetic activity (high decomposition rate) of the charophytes favoured grazing. The invertebrates studied preferred Chara tomentosa to C. connivens. Low consumption of C. connivens may reflect its non-native origin. The experiment suggests that, under moderately eutrophic conditions, grazers are not likely to control charophyte populations. However, grazers have the potential to eliminate charophytes in severely eutrophic systems under the stress of filamentous algae.

Seasonality Pattern of Biomass Accumulation in a Drifting Furcellaria Lumbricalis Community in the Waters of the West Estonian Archipelago, Baltic Sea

A free-floating, loose form of Furcellaria lumbricalis (Huds.) Lamour is rare in the Baltic Sea area. Kassari Bay, situated in the West Estonian Archipelago Sea area contains the largest known community of this kind. Here the free-floating mixed Furcellaria lumbricalis-Coccotylus truncatus (Paela) M. J. Wynne et J. N. Heine community inhabits sandy bottom, covering up to 120 km2. Commercial exploitation of the community started in 1966 and has led to regular monitoring surveys for the quantification of the commercial resource. The aim of the present study was to determine the potential growth rates of the two community-forming species as well as to test different environmental factors affecting their growth. Results showed that the highest growth rates were measured in shallower depths (4 m) for both species. The seasonal growth pattern was also very similar for both species, showing the highest growth rates during the beginning of summer. Incubation of both species in another sea area with apparently similar basic environmental conditions (the northern part of the Gulf of Riga, Kõiguste Bay) resulted in significantly lower growth rates during the whole incubation period.

Direct effects of increased CO2 concentrations in seawater on the net primary production of charophytes in a shallow, coastal, brackish-water ecosystem

Charophytes are found in fresh and brackish waters across the globe and play key roles in coastal ecosystems. However, their response to increasing CO2 is not well understood. The aim of the study was to detect the effects of elevated CO2 on the physiology of charophyte species growing in the brackish Baltic Sea by measuring net primary production. Mesocosm experiments were conducted in the Kõiguste Bay (N Gulf of Riga) during the field season of 2012. Separate mesocosms were maintained at different pCO2 levels: ~2000, ~1000 and ~200 μatm. The experiments were carried out with three species of charophytes: Chara aspera, C. tomentosa and C. horrida. The short-term photosynthetic responses of charophytes to different treatments were measured by the oxygen method. The results show that elevated CO2 levels in brackish water may enhance the photosynthetic activity of charophyte species and suggest that increasing CO2 in the Baltic Sea could have implications for interspecific competition and community structure in a future high CO2 world.

Variability of Benthic Communities in Relation to Hydrodynamic Conditions in the North-Eastern Baltic Sea

ABSTRACT Herkül, K.; Torn, K.; Suursaar, Ü.; Alari, V., and Peterson, A., 2016. Variability of benthic communities in relation to hydrodynamic conditions in the north-eastern Baltic Sea. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 867–871. Coconut Creek (Florida), ISSN 0749-0208. Wave forcing is an important hydrodynamic variable that influences spatial patterns and temporal dynamics of marine benthos. In this study, we used three principally different wave models – simplified wave model (SWM), spectral wave model SWAN (simulating waves nearshore), and locally calibrated point model (LCPM) – to assess the effects of wave forcing on the distribution and temporal dynamics of macrobenthos in the north-eastern Baltic Sea. Other important environmental variables like depth, salinity, turbidity etc., were also included into analyses of benthos distribution in order to quantify the relative importance of wave forcing. Following depth, SWM was the second most important environmental predictor of spatial distribution of benthos. The importance of SWAN was the lowest among environmental variables due to its low spatial resolution (1 nautical mile) compared to SWM (25 m). Considering the temporal dynamics of benthos, wave height significantly correlated with several benthic variables, but the direction and magnitude of effects were site-specific.

The environmental niche separation between charophytes and angiosperms in the northern Baltic Sea

Abstract Charophytes and angiosperms are the most important primary producers and habitat modifiers in shallow soft-bottom fresh and brackish water bodies. The sheltered bays of the low-salinity northern Baltic Sea provide unique habitats, where euryhaline macrophytes of both freshwater and marine origin can thrive. The regional cohabitation of several species, with generally similar biological traits, raises questions about the mechanisms underlying the coexistence. Generally, sympatric populations rely on niche separation to reduce competition pressure that, in turn, facilitates long-term coexistence. A large benthos biomass data-set, covering the whole extent of the Estonian sea area, the northern Baltic Sea, together with a set of 13 environmental variables (bathymetrical, hydrodynamical, chemical, biological) was used to test the potential niche separation between regionally coexisting charophytes and angiosperms. A constrained correspondence analysis indicated both the grouping of some species and a distinct niche separation of others. The niche centers of angiosperms Zostera marina, Zannichellia palustris, and Potamogeton perfoliatus diverged most strongly from all the other species, while those of Chara connivens and Myriophyllum spicatum nearly coincided. Depth, salinity, and duration of ice were the most influential environmental gradients that discerned the environmental niches of the species. Comparison of the breadths of the environmental niche spaces, occupied by the studied species, showed highly species-specific results. Within the studied species, Z. palustris and Z. marina had the lowest niche specialization, and C. connivens had the highest specialization. Altogether, among the angiosperms the variability of the environmental niche breadth was higher compared to the charophytes.