Pontogammarus robustoides

Paidere, Jana, Brakovska, Aija, Vezhnavets, Vasil, Škute, Artūrs & Savicka, Marina, 2019, Effects Of The Environmental Variables On The Alien Amphipod Pontogammarus Robustoides In The Daugava River And Its Reservoirs, Acta Biologica Universitatis Daugavpiliensis 19 (2), pp. 169-180 : 175-176

Pontogammarus robustoides population structure by body length seasonally changed from larger individuals in May to smaller in June and July, the size increased in August and September in the Pļaviņas Reservoir ( Fig. 4 ). Presence of small or juvenile individuals in June, July, and August and in September (ovigerous females were always observed, personal comments) indicated that in the Pļaviņas Reservoir of the Daugava River P. robustoides is able to reproduce throughout a season. Research of the Neva Estuary on P. robustoides has also demonstrated that the development of three or two generations in year depended on warm or cold weather ( Berezina et al. 2017). Similar results about the traits of reproductivity of P. robustoides in the Kuibyshev and Saratov reservoirs demonstrate polivoltine life cycle with three generations in a year ( Kurina 2016). Further investigations of P. robustoides are needed to reveal the role of life-history traits promoting the species invasion in the Daugava River especially compared to indigenous amphipod.

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values, but the Pļaviņas Reservoir is with higher chlorophyll α and pH values. While the adults or large individuals are weakly related to lower oxred potential in the Pļaviņas Reservoir, seasonally it is May and July. Such division is supported by the other investigation and could be explained by the species feeding behaviour during ontogenetic or size development, by environment conditions, by type of habitats and by season. Detailed studies of the feeding behaviour within different environments (the eutrophic/hypereutrophic Neva Estuary, Curonian Lagoon and the mesotrophic Lake Plateliai) of P. robustoides based on stable isotope and gut content analysis showed that P. robustoides change from herbivores/detritivores at juvenile stages within both eutrophic and mesotrophic environment to detritivores/omnivores or carnivores within more eutrophic environment at adult stage that depends on quality and quantity of feeding resources. Furthermore, P. robustoides is characterized by high stoichiometric plasticity, and this may explain why alien peracaridan species from the high eutrophic freshwaters indicate the substantially higher invader potential comparison with freshwaters of low trophy ( Berezina et al. 2017, Arbačiauskas et al. 2013, Bacela-Spychalska, Van der Velde 2013, Berezina 2007). Considering that the Daugava River and its reservoirs are characterised as eutrophic but the limiting nutrient there is phosphorus and following the above-mentioned P. robustoides could be characterised as more detritivores/ herbivores and omnivores in the Daugava River and its reservoirs. According to our results, the juveniles of P. robustoides are more herbivore (plant detritus, periphyton ( Berezina et al. 2009)) especially in the sites with lower eutrophy, and large individuals are more detritivore or omnivore in the sites of the Daugava River and its reservoirs with higher eutrophy. Detritus or particulate organic materials contain both living and dead organic parts. The lower oxred potential could indicate the organic-rich sediments and the presence of other macro-invertebrates ( Poznańska et al. 2010). The studies of trophic position and predatory abilities of invasive P. robustoides using stable isotope and gut content analysis in Poland water-bodies showed that adult P. robustoides took a high trophic position. Detritus and especially animal materials (animal tissue, oligochaetes and other) are the most important food dietary ( Bacela-Spychalska, Van der Velde, 2013). Similar results about food spectra in the Neva Estuary and the Curonian Lagoon showed that P. robustoides adult individuals are omnivorous, consuming aquatic insect larvae, oligochaetes, isopods, and other crustaceans as well as filamentous algae ( Berezina et al. 2017).

One of the important abiotic factors in running waters and reservoirs is hydrological regime, especially water level changes ( Lampert, Sommer, 2007). Very shallow depth and shorenear zones by sands, gravels, stones, filamentous algae and macrophytes stands are typical freshwater habitats for P. robustoides , but at the same time these habitats are the most exposed to water level changes. It was observed that P. robustoides occurred at the submerged sandy bottom sites in the flooded areas in the Włocławek Reservoir in Poland ( Poznańska et al. 2010). Our results showed that the medium size individuals of P. robustoides prefer habitats with the lowest water and seasonally it coincides with August. It is could be due to the low level of warm stagnant water, food availability and decrease of competition. It is supported by experimental studies about abiotic factors affecting microhabitat selection by alien P. robustoides . For example, P. robustoides generally prefer warm water and avoided flow ≥ 15 cm s-1. According to studies about variable depth and substrata preferences of P. robustoides , the species selected shallower locations, limiting inter-specific competition, allowing optimal utilization of feed and decreasing predatory pressure (Kobak et al. 2017, Kobak et al. 2017a, Jermacz et al. 2015). On the other hand, in our subsequent studies in 2017 in the Pļaviņas Reservoir due to heavy rainfall and comparatively higher water levels (July, August, and September) than in 2016, P. robustoides was obtained in May and June in one site out of two, but the rest of the season it was not detected (unpublished data, personal comments).

Figures

Fig. 4. Seasonal changes of individuals groups by body length in the Pļaviņas Reservoir.