The invasive amphipod Gammarus tigrinus Sexton, 1939 displaces native gammarid amphipods from sheltered macrophyte habitats of the Gulf of Riga

The North-American amphipod Gammarus tigrinus Sexton, 1939 is a successful invader in European waters due to its high reproductive potential and tolerance to severe environmental conditions and various pollutants. In this study, we followed the invasion and establishment of this exotic species in a species-poor ecosystem of the northern Baltic Sea. Two years after the establishment of G. tigrinus, over half of the sampling sites were occupied exclusively by G. tigrinus, whereas G. tigrinus coexisted with native gammarids in only one tenth of all sites. There was a clear separation of habitat occupancy between native species and G. tigrinus in terms of abiotic environment and macrophytic habitat. G. tigrinus preferred shallow sheltered areas dominated by vascular plants, while native species mainly occurred in more exposed, deeper habitats with phaeophytes and rhodophytes. In its suboptimal habitats, G. tigrinus exhibited moderate abundances, which allowed for the coexistence of native gammarids and the invasive gammarid. Since its establishment, the abundance of G. tigrinus has showed no signs of decline, with abundances exceeding almost fifteen times those of native gammarids at some locations. The results suggest that, irrespective to the competitive superiority of G. tigrinus over the native gammarids, the invasive G. tigrinus does not monopolize the entire coastal area of the northern Baltic Sea but mostly outcompetes native species in its favoured habitats.

Cleaning up seas using blue growth initiatives : Mussel farming for eutrophication control in the Baltic Sea

Eutrophication is a serious threat to aquatic ecosystems globally with pronounced negative effects in the Baltic and other semi-enclosed estuaries and regional seas, where algal growth associated with excess nutrients causes widespread oxygen free “dead zones” and other threats to sustainability. Decades of policy initiatives to reduce external (land-based and atmospheric) nutrient loads have so far failed to control Baltic Sea eutrophication, which is compounded by significant internal release of legacy phosphorus (P) and biological nitrogen (N) fixation. Farming and harvesting of the native mussel species (Mytilus edulis/trossulus) is a promising internal measure for eutrophication control in the brackish Baltic Sea. Mussels from the more saline outer Baltic had higher N and P content than those from either the inner or central Baltic. Despite their relatively low nutrient content, harvesting farmed mussels from the central Baltic can be a cost-effective complement to land-based measures needed to reach eutrophication status targets and is an important contributor to circularity. Cost effectiveness of nutrient removal is more dependent on farm type than mussel nutrient content, suggesting the need for additional development of farm technology. Furthermore, current regulations are not sufficiently conducive to implementation of internal measures, and may constitute a bottleneck for reaching eutrophication status targets in the Baltic Sea and elsewhere. Highlights • Mussel farming is a viable internal measure to address Baltic Sea eutrophication. • Rates of nutrient removal depend on salinity at the regional scale and food availability at the local scale. • Cost effectiveness of nutrient removal by mussel farming depends also on farm type. • Total farm area needed for achieving HELCOM nutrient reduction targets is realistic

Füüsikalise häiringu ja elupaika kujundavate liikide mõju põhjasette omadustele ja põhjakooslustele Läänemere põhjaosas

Väitekirja elektrooniline versioon ei sisalda publikatsioone.Häiringud on ebakorrapäraselt toimuvad sündmused, mis kutsuvad esile muutusi ökosüsteemi, koosluse või populatsioonide struktuuris ning loovad vaba ruumi uutele tulijatele. Häiringud on oluliseks looduslikke kooslusi kujundavaks teguriks põhjustades koosluste ruumilist ja ajalist muutlikkust. Elupaika kujundavad liigid muudavad keskkonnatingimusi ja toetavad suuremat liigirikkust. Doktoritöö eesmärgiks oli uurida füüsikaliste häiringute (põhjasette eemaldamine ja lisandumine, süvendustööd), häiringu toimumise aja ja elupaika kujundavate võtmeliikide (pikk merihein, söödav rannakarp jt) mõju põhjasette omadustele (orgaanilise materjali sisaldus, granulomeetriline koosseis, hapnikuvoog) ja põhjaelustiku kooslustele Läänemere põhjaosas. Pealmise settekihi eemaldamine vähendas põhjaelustiku liigirikkust ja asustustihedust, kuid ei mõjutanud orgaanilise materjali sisaldust põhjasettes. Häiringu mõju oli tugevam kevadel. Põhjaloomastiku domineerimisstruktuur taastus kiiremini kui liikide asustustihedused. Settekihi akumuleerumine ei põhjustanud olulisi muutusi põhjasette omadustes, küll aga langes meriheina eemaldamise tagajärjel liigirikkus ja põhjaorganismide asustustihedus. Meriheina eemaldamine vähendas peeneteralise fraktsiooni ja orgaanilise materjali hulka settes ja suurendas ajutiselt sette hapnikutarvet. Koloniseeriva põhjaloomastiku koosluse domineerimisstruktuur ei sõltunud koosluse esialgsest liigilisest koosseisust vaid avatusest lainetusele. Söödav rannakarp suurendas niitjate vetikate, herbivooride ja detrivooride hulka kuid vähendas mändvetikate biomassi. Rannakarbi mõju oli suurem lainetusele mõõdukalt avatud piirkonnas võrreldes lainetuse eest varjatud piirkonnaga. Ulatuslike süvendustööde mõju põhjaloomastikule oli nõrk ja koosluste taastumine oli kiire, mis näitab, et kõrge stressitaluvusega Läänemere põhjaloomastiku liigid elavad kergesti üle süvendustöödega seotud häiringud.Disturbance is mainly described as discrete event in time that disrupts ecosystem, community, or population structure, and creates opportunities for new individuals to become established. Disturbance is a key factor regulating the structure and functioning of natural communities and causing spatial and temporal heterogeneity. Most habitats are generated by the presence of a single or a few habitat-modifying species that alter local environmental conditions and support species diversity. The aim of this thesis was to study the effects of physical disturbances (sediment removal and addition, dredging activities) and habitat-modifying species (e.g. Zostera marina, Mytilus trossulus) on sediment properties (organic matter content, granulometric composition, oxygen flux) and benthic communities in the northern Baltic Sea. The removal of the upper sediment layer decreased species diversity and species’ densities but did not influence the organic matter content of sediment. The impact of disturbance was stronger in spring than in summer. The dominance structure of zoobenthos recovered faster than the densities of zoobenthos species. The addition of sediment did not cause changes in sediment organic matter content and oxygen flux. The removal of Z. marina significantly decreased diversity and density of benthic organisms, content of fine particles and organic matter, and temporarily increased the sediment oxygen consumption. The dominance structure of the colonising invertebrates depended on the wave exposure rather than on the initial composition of the invertebrate community. M. trossulus enhanced the growth of filamentous algae, herbivores, and deposit feeders and decreased the biomass of charophytes. Stronger effects of M. trossulus emerged in the moderately exposed than in the sheltered area. Large-scale dredging had weak effects on benthic invertebrates and the recovery of the communities took place within a year

Mapping benthic biodiversity using georeferenced environmental data and predictive modeling

Biodiversity is critical for maintaining and stabilizing ecosystem processes. There is a need for high-resolution biodiversity maps that cover large sea areas in order to address ecological questions related to biodiversity-ecosystem functioning relationships and to provide data for marine environmental protection and management decisions. However, traditional sampling-point-wise field work is not suitable for covering extensive areas in high detail. Spatial predictive modeling using biodiversity data from sampling points and georeferenced environmental data layers covering the whole study area is a potential way to create biodiversity maps for large spatial extents. Random forest (RF), generalized additive models (GAM), and boosted regression trees (BRT) were used in this study to produce benthic (macroinvertebrates, macrophytes) biodiversity maps in the northern Baltic Sea. Environmental raster layers (wave exposure, salinity, temperature, etc.) were used as independent variables in the models to predict the spatial distribution of species richness. A validation dataset containing data that was not included in model calibration was used to compare the prediction accuracy of the models. Each model was also evaluated visually to check for possible modeling artifacts that are not revealed by mathematical validation. All three models proved to have high predictive ability. RF and BRT predictions had higher correlations with validation data and lower mean absolute error than those of GAM. Both mathematically and visually, the predictions by RF and BRT were very similar. Depth and seabed sediments were the most influential abiotic variables in predicting the spatial patterns of biodiversity

Ecological niche differentiation between native and non-native shrimps in the northern Baltic Sea

Invasions of non-native species are modifying global biodiversity but the ecological mechanisms underlying invasion processes are still not well understood. A degree of niche separation of non-native and sympatric native species can possibly explain the success of novel species in their new environment. In this study, we quantified experimentally and in situ the environmental niche space of caridean shrimps (native Crangon crangon and Palaemon adspersus, non-native Palaemon elegans) inhabiting the northern Baltic Sea. Field studies showed that the non-native P. elegans had wider geographical range compared to native species although the level of habitat specialization was similar in both Palaemon species. There were clear differences in shrimp habitat occupancy with P. elegans inhabiting lower salinity areas and more eutrophicated habitats compared to the native species. Consequently, the non-native shrimp has occupied large areas of the northern Baltic Sea that were previously devoid of the native shrimps. Experiments demonstrated that the non-native shrimp had higher affinity to vegetated substrates compared to native species. The study suggests that the abilities of the non-native shrimp to thrive in more stressful habitats (lower salinity, higher eutrophication), that are sub-optimal for native shrimps, plausibly explain the invasion success of P. elegans

Environmental niche separation between native and non-native benthic invertebrate species: Case study of the northern Baltic Sea

Knowledge and understanding of geographic distributions of species is crucial for many aspects in ecology, conservation, policy making and management. In order to reach such an understanding, it is important to know abiotic variables that impact and drive distributions of native and non-native species. We used an existing long-term macrobenthos database for species presence-absence information and biomass estimates at different environmental gradients in the northern Baltic Sea. Region specific abiotic variables (e.g. salinity, depth) were derived from previously constructed bathymetric and hydrodynamic models. Multidimensional ordination techniques were then applied to investigate potential niche space separation between all native and non-native invertebrates in the northern Baltic Sea. Such an approach allowed to obtain data rich and robust estimates of the current native and non-native species distributions and outline important abiotic parameters influencing the observed pattern. The results showed clear niche space separation between native and non-native species. Non-native species were situated in an environmental space characterized by reduced salinity, high temperatures, high proportion of soft seabed and decreased depth and wave exposure whereas native species displayed an opposite pattern. Different placement of native and non-native species along the studied environmental niche space is likely to be explained by the differences in their evolutionary history, human mediated activities and geological youth of the Baltic Sea. The results of this study can provide early warnings and effectively outline coastal areas in the northern Baltic Sea that are prone to further range expansion of non-native species as climate change is expected to significantly reduce salinity and increase temperature in wide coastal areas, both supporting the disappearance of native and appearance of non-native species