Riina Klais

Decadal-scale changes of dinoflagellates and diatoms in the anomalous baltic sea spring bloom.

The algal spring bloom in the Baltic Sea represents an anomaly from the winter-spring bloom patterns worldwide in terms of frequent and recurring dominance of dinoflagellates over diatoms. Analysis of approximately 3500 spring bloom samples from the Baltic Sea monitoring programs revealed (i) that within the major basins the proportion of dinoflagellates varied from 0.1 (Kattegat) to >0.8 (central Baltic Proper), and (ii) substantial shifts (e.g. from 0.2 to 0.6 in the Gulf of Finland) in the dinoflagellate proportion over four decades. During a recent decade (1995-2004) the proportion of dinoflagellates increased relative to diatoms mostly in the northernmost basins (Gulf of Bothnia, from 0.1 to 0.4) and in the Gulf of Finland, (0.4 to 0.6) which are typically ice-covered areas. We hypothesize that in coastal areas a specific sequence of seasonal events, involving wintertime mixing and resuspension of benthic cysts, followed by proliferation in stratified thin layers under melting ice, favors successful seeding and accumulation of dense dinoflagellate populations over diatoms. This head-start of dinoflagellates by the onset of the spring bloom is decisive for successful competition with the faster growing diatoms. Massive cyst formation and spreading of cyst beds fuel the expanding and ever larger dinoflagellate blooms in the relatively shallow coastal waters. Shifts in the dominant spring bloom algal groups can have significant effects on major elemental fluxes and functioning of the Baltic Sea ecosystem, but also in the vast shelves and estuaries at high latitudes, where ice-associated cold-water dinoflagellates successfully compete with diatoms.

Rehabilitating the cyanobacteria – niche partitioning, resource use efficiency and phytoplankton community structure during diazotrophic cyanobacterial blooms

Summary Blooms of nitrogen‐fixing cyanobacteria are recurrent phenomena in marine and freshwater habitats, and their supplying role in aquatic biogeochemical cycles is generally considered vital. The objective of this study was to analyse whether an increasing proportion of nitrogen‐fixing cyanobacteria affects (i) the composition of the non‐diazotrophic component of ambient phytoplankton communities and (ii) resource use efficiency (RUE; ratio of Chl a to total nutrients) – an important ecosystem function. We hypothesize that diazotrophs increase community P use and decrease N use efficiencies, as new N is brought into the system, relaxing N, and concomitantly aggravating P limitation. We test this by analysing an extensive data set from the Baltic Sea (> 3700 quantitative phytoplankton samples), known to harbour conspicuous and recurrent blooms of Nodularia spumigena and Aphanizomenon sp. System‐level phosphorus use efficiency (RUEP) was positively related to high proportion of diazotrophic cyanobacteria, suggesting aggravation of phosphorus limitation. However, concomitant decrease of nitrogen use efficiency (RUEN) was not observed. Nodularia spumigena, a dominant diazotroph and a notorious toxin producer, had a significantly stronger relationship with RUEP, compared to the competing non‐toxic Aphanizomenon sp., confirming niche differentiation in P acquisition strategies between the major bloom‐forming cyanobacterial species in the Baltic Sea. Nodularia occurrences were associated with stronger temperature stratification in more offshore environments, indicating higher reliance on in situ P regeneration. By using constrained and unconstrained ordination, permutational multivariate analysis of variance and local similarity analysis, we show that diazotrophic cyanobacteria explained no more than a few percentage of the ambient phytoplankton community variation. The analyses furthermore yielded rather evenly distributed negative and positive effects on individual co‐occurring phytoplankton taxa, with no obvious phylogenetic or functional trait‐based patterns. Synthesis. Our study reveals that despite the widely acknowledged noxious impacts of cyanobacterial blooms, the overall effect on phytoplankton community structure is minor. There are no predominantly positive or negative associations with ambient phytoplankton species. Species‐specific niche differences in cyanobacterial resource acquisition affect important ecosystem functions, such as biomass production per unit limiting resource.

Impact of extreme climate and bioinvasion on temporal coupling of spring herring (Clupea harengus m.) larvae and their prey.

We used weekly observational data from mid-May to end of July in 1958-2012 in Gulf of Riga to investigate temporal coupling between spring herring larvae and their first prey - copepod nauplii, under the extreme hydroclimatic conditions. We focused on a small shallow estuary that is important nursery ground for larvae of the Gulf of Riga (Baltic Sea) herring population. We quantified the effect of extreme values of the winter air temperatures, time of ice retreat and spring water temperatures on the timing of peak abundance of herring larvae and copepod nauplii. We also assessed whether the invasion of the non-native cladoceran Cercopagis pengoi had notable effect on timing and abundance of copepod nauplii during the peak occurrence of herring larvae. In the years of earliest ice retreat the peak abundance of herring larvae was five weeks earlier than in the years of latest ice retreat, while the timing of nauplii remained unchanged. Abundant presence of the C. pengoi affected neither timing nor maximum abundance of copepod nauplii during the herring larvae first feeding period. Thus, we conclude that processes induced by climate variability are superior to invasion of C. pengoi in determining the timing and coupling of larval herring and copepod nauplii.

Winter–spring climate effects on small-sized copepods in the coastal Baltic Sea

The general positive effect of warmer winters on the abundance of small-sized zooplankton in the following spring and early summer has been reported from different parts of the Baltic Sea, but the mechanism of this link is not clear. Although causal links cannot be deduced with confidence from observational data, sufficiently detailed analyses can nevertheless provide insights to the potential mechanisms. We present an example of such an analysis, scrutinizing the effects of winter and spring hydroclimate on the abundance of small-sized dominant calanoid copepods (Eurytemora affinis and Acartia spp.), using data from 2080 zooplankton samples collected over 55 years (1957–2012) from a shallow coastal habitat (Parnu Bay, Gulf of Riga) in the Baltic Sea. Our results indicated that the milder winters brought about higher abundances, and reduced seasonality of small-sized copepods, whereas ambient sea surface temperature (SST) mostly affected the relative abundance of adult stages. The sliding window correlation tests revealed temporal shifts in the effects of controlling variables: with the continuous increase in SST, the effect of winter temperature on the abundance of Acartia spp. weakened. In contrast, E. affinis was consistently affected by SST, but the effect of winter temperature was more pronounced during the period of on average colder winters.

Selecting for three copepods—feeding of sprat and herring in the Baltic Sea

Original Article Selecting for three copepods—feeding of sprat and herring in the Baltic Sea Henn Ojaveer*, Ain Lankov, Tiit Raid, Arno P~ollumäe, and Riina Klais Estonian Marine Institute, University of Tartu, Lootsi 2a, 80032 Pärnu, Estonia Estonian Marine Institute, University of Tartu, Mäealuse 14, 12618 Tallinn, Estonia *Corresponding author: tel: þ372 5158328; fax: þ372 6718900; e-mail: henn.ojaveer@ut.ee.

Shifting Diatom—Dinoflagellate Dominance During Spring Bloom in the Baltic Sea and its Potential Effects on Biogeochemical Cycling

The Baltic Sea is affected by a range of human induced environmental pressures such as eutrophication. Here we synthesize the ongoing shift from diatom dominance towards more dinoflagellates in parts of the Baltic Sea during the spring bloom and its potential effects on biogeochemical cycling of key elements (e.g. C, N and P). The spring bloom is the period with the highest annual primary production and sinking of organic matter to the sediment. The fate of this organic matter is a key driver for material fluxes, affecting ecosystem functioning and eutrophication feedback loops. The dominant diatoms and dinoflagellates appear to be functionally surrogates as both groups are able to effectively exhaust the wintertime accumulation of inorganic nutrients and produce bloom level biomass that contribute to vertical export of organic matter. However, the groups have very different sedimentation patterns, and the sea floor has variable potential to mineralize the settled biomass in the different sub-basins. While diatoms sink quickly out of the euphotic zone, dinoflagellates sink as inert resting cysts, or lyse in the water column contributing to slowly settling phyto-detritus. The dominance by either phytoplankton group thus directly affects both the summertime nutrient pools of the water column and the input of organic matter to the sediment but to contrasting directions. The proliferation of dinoflagellates with high encystment efficiency could increase sediment retention and burial of organic matter, alleviating the eutrophication problem and improve the environmental status of the Baltic Sea.

Genetic analysis reveals the diversity of larval Gobiidae in a temperate estuary

Using molecular tools to examine Gobiidae, the second most abundant taxon in ichthyoplankton samples in the Gulf of Riga (Baltic Sea), the sand goby Pomatoschistus minutus was the most abundant taxon (82% of all individuals analysed), the common goby Pomatoschistus microps constituted 12% and the black goby Gobius niger 6%. The spatiotemporal distribution of P. microps and G. niger indicated a preference for habitats closer to the river inlet and their abundances increased slightly towards the end of the sampling period in summer. The species composition was interpreted in the context of the prevailing habitat conditions, characterized by extremely low water transparency, low salinity, limited spread of vegetated area and dominance of sandy-muddy substrata.