Differences in stress tolerance and brood size between a non-indigenous and an indigenous gammarid in the northern Baltic Sea

Differences in stress tolerance and reproductive traits may drive the competitive hierarchy between nonindigenous and indigenous species and turn the former ones into successful invaders. In the northern Baltic Sea, the non-indigenous Gammarus tigrinus is a recent invader of littoral ecosystems and now occupies comparable ecological niches as the indigenous G. zaddachi. In laboratory experiments on specimens collected between June and August 2009 around Tva¨rminne in southern Finland (59°500N/23°150E), the tolerances towards heat stress and hypoxia were determined for the two species using lethal time, LT50, as response variable. The brood size of the two species was also studied and some observations were made on maturation of juveniles. Gammarus tigrinus was more resistant to hypoxia and survived at higher temperatures than G. zaddachi. Brood size was also greater in G. tigrinus than in G. zaddachi and G. tigrinus matured at a smaller size and earlier than G. zaddachi. Hence, there are clear competitive advantages for the non-indigenous G. tigrinus compared to the indigenous G. zaddachi, and these may be further strengthened through ongoing environmental changes related to increased eutrophication and a warming climate in the Baltic Sea region

Anpassungsfähigkeit indigener und nichtindigener Arten an Umweltstressoren

Successful invasive species are often hypothesized to have a wider ecological tolerance and consequently a reduced sensitivity towards a variety of stressors as compared to local species. Thus, part of the competitive hierarchy between invaders and native species may depend on the stress regime. Another factor of successful invasions is the reproductive potential of a species. The alien amphipod Gammarus tigrinus is a recent successful invader in the northern Baltic Sea and occupies comparable ecological niches and has similar reproductive periods in summer as the native Gammarus zaddachi in littoral ecosystems. In laboratory experiments, we determined the tolerance towards heat stress and hypoxia for the two competing species by exposing them to temperatures between 30°C and 35°C (heat stress) and only 1% dissolved oxygen at 20°C (hypoxia). For all stress tolerance experiments the response was measured as LT50, i.e. the time until mortality reached 50%. In separate experiments, we measured the fecundity of both species by counting their newly hatched juveniles. Gammarus tigrinus were more resistant to hypoxia, survived at higher temperatures and produced more juveniles than corresponding Gammarus zaddachi indicating wider tolerance ranges as well as a higher fecundity of the invasive species and maybe also competitive advantages above the native species

TemBi 2014 mesocosm study: Summer storm impact on Zooplankton abundance in Lake Stechlin

We simulated an experimental summer storm in large-volume (~1200 m3, ~16m depth) enclosures in Lake Stechlin by mixing deeper water masses from the meta- and hypolimnion into the mixed layer (epilimnion). The mixing included the disturbance of a deep chlorophyll maximum (DCM) which was present at the same time of the experiment in Lake Stechlin and situated in the metalimnion of each enclosure during filling. Copepod and Cladocera biomass was monitored for 42 days after the experimental disturbance event (Utermöhl counting at 60x magnification and biomass calculation from length-dry mass relationships). Sampling was performed using a 90 µm mesh size Apstein-cone

Non-native marine invertebrates are more tolerant towards environmental stress than taxonomically related native species: Results from a globally replicated study

To predict the risk associated with future introductions, ecologists seek to identify traits that determine the invasiveness of species. Among numerous designated characteristics, tolerance towards environmental stress is one of the most favored. However, there is little empirical support for the assumption that non-native species generally cope better with temporarily unfavorable conditions than native species. To test this concept, we ran five pairwise comparisons between native and non-native marine invertebrates at temperate, subtropical, and tropical sites. We included (natives named first) six bivalves: Brachidontes exustus and Perna viridis, P. perna and Isognomon bicolor, Saccostrea glomerata and Crassostrea gigas, two ascidians: Diplosoma listerianum and Didemnum vexillum as well as two crustaceans: Gammarus zaddachi and G. tigrinus. We simulated acute fluctuations in salinity, oxygen concentration, and temperature, while we measured respiration and survival rates. Under stressful conditions, non-native species consistently showed less pronounced deviations from their normal respiratory performance than their native counterparts. We suggest that this indicates that they have a wider tolerance range. Furthermore, they also revealed higher survival rates under stress. Thus, stress tolerance seems to be a property of successful invaders and could therefore be a useful criterion for screening profiles and risk assessment protocols. Highlights ► Non-native species showed higher survival rates in the face of stress than native. ► Respiratory performance under stress was closer to normal in non-native species. ► Strong evidence for stress tolerance as a general trait of non-native species. ► Robust results due to a global-scale, modular experimental approach

TemBi 2014 mesocosm study: Summer storm impact on water chemistry and physics in Lake Stechlin

We simulated an experimental summer storm in large-volume (~1200 m3, ~16m depth) enclosures in Lake Stechlin by mixing deeper water masses from the meta- and hypolimnion into the mixed layer (epilimnion). The mixing included the disturbance of a deep chlorophyll maximum (DCM) which was present at the same time of the experiment in Lake Stechlin and situated in the metalimnion of each enclosure during filling. Water physical variables and water chemistry was monitored for 42 days after the experimental disturbance event. Mixing disrupted the thermal stratification, increasing concentrations of dissolved nutrients and CO2 and changing light conditions in the epilimnion. Mixing, thus, stimulated phytoplankton growth, resulting in higher particulate matter concentrations of carbon, nitrogen and phosphorous