Invertebrate predation and trophic cascades in a pelagic food web : The multiple roles of Chaoborus flavicans (Meigen) in a clay-turbid lake

In lake ecosystems, both fish and invertebrate predators have dramatic effects on their prey communities. Fish predation selects large cladocerans while invertebrate predators prefer prey of smaller size. Since invertebrate predators are the preferred food items for fish, their occurrence at high densities is often connected with the absence or low number of fish. It is generally believed that invertebrate predators can play a significant role only if the density of planktivorous fish is low. However, in eutrophic clay-turbid Lake Hiidenvesi (southern Finland), a dense population of predatory Chaoborus flavicans larvae coexists with an abundant fish population. The population covers the stratifying area of the lake and attains a maximum population density of 23000 ind. m-2. This thesis aims to clarify the effects of Chaoborus flavicans on the zooplankton community and the environmental factors facilitating the coexistence of fish and invertebrate predators. In the stratifying area of Lake Hiidenvesi, the seasonal succession of cladocerans was exceptional. The spring biomass peak of cladocerans was missing and the highest biomass occurred in midsummer. In early summer, the consumption rate by chaoborids clearly exceeded the production rate of cladocerans and each year the biomass peak of cladocerans coincided with the minimum chaoborid density. In contrast, consumption by fish was very low and each study year cladocerans attained maximum biomass simultaneously with the highest consumption by smelt (Osmerus eperlanus). The results indicated that Chaoborus flavicans was the main predator of cladocerans in the stratifying area of Lake Hiidenvesi. The clay turbidity strongly contributed to the coexistence of chaoborids and smelt at high densities. Turbidity exceeding 30 NTU combined with light intensity below 0.1 μE m-2 s-1provides an efficient daytime refuge for chaoborids, but turbidity alone is not an adequate refuge unless combined with low light intensity. In the non-stratifying shallow basins of Lake Hiidenvesi, light intensity exceeds this level during summer days at the bottom of the lake, preventing Chaoborus forming a dense population in the shallow parts of the lake. Chaoborus can be successful particularly in deep, clay-turbid lakes where they can remain high in the water column close to their epilimnetic prey. Suspended clay alters the trophic interactions by weakening the link between fish and Chaoborus, which in turn strengthens the effect of Chaoborus predation on crustacean zooplankton. Since food web management largely relies on manipulations of fish stocks and the cascading effects of such actions, the validity of the method in deep clay-turbid lakes may be questioned.

Benthic-pelagic coupling in the northern Baltic Sea: importance of bioturbation and benthic predation

Benthic-pelagic coupling describes processes that operate across and between the seafloor and open-water ecosystems. In soft-sediment communities, bioturbation by sediment-dwelling and epibenthic organisms may strongly shape habitat characteristics and influence processes, e.g. biogeochemical cycling, which supplies bioavailable nutrients to pelagic primary producers. In addition, benthic fauna may mediate benthic-pelagic coupling by affecting the survival and hatching of zooplankton dormant eggs in the sediment. In the shallow waters and seasonally fluctuating environment of the Baltic Sea, emergence from the seafloor essentially contributes to the dynamics of zooplankton pelagic populations. In this thesis, I examine how benthic organisms with different functional traits affect the link between the benthic and pelagic systems in the northern Baltic Sea. By means of experimental laboratory studies, the effects of sediment-dwelling (Monoporeia affinis, Macoma balthica and Marenzelleria spp.) and nectobenthic (Mysis spp.) taxa on the survival and hatching of zooplankton benthic eggs and on benthic nutrient fluxes and sediment structure were investigated. In the predation studies, the nectobenthic mysids Mysis spp. preyed upon benthic eggs of the cladoceran Bosmina longispina maritima (syn. B. coregoni maritima), both in pelagic and benthic environments. Of the sediment-dwelling species, the amphipod M. affinis and the bivalve M. balthica reduced the number of cladoceran eggs in the sediment, whereas the polychaetes Marenzelleria spp. had no effects on cladoceran eggs. Both M. balthica and M. affinis also increased the mortality rates of benthic eggs of copepods and rotifers. It was estimated that zooplankton eggs provide an additional carbon source for food-limited benthic communities. The results indicate that predation pressure on zooplankton benthic eggs may be strong, but varies widely depending on the season and the functional characteristics of the macrofauna. Macoma balthica buried cladoceran eggs and a fluorescent tracer from the sediment surface to a depth of 3 4 cm, indicating efficient sediment mixing. In contrast, the other taxa had fewer effects on particle distributions. In addition to organic matter mineralization, particle mixing is crucial to the success of benthic recruitment of zooplankton, since only eggs close to the sediment surface may hatch. Macoma balthica and M. affinis altered the patterns of zooplankton emergence from the sediment. In general, the highest emergence rates were observed in the absence of macroscopic fauna, and M. balthica exerted a stronger suppressive effect than M. affinis. Moreover, copepods were less severely affected than cladocerans, while only one species (Temora longicornis) clearly benefited from the presence of the macrofauna. These differences probably result from species-specific differences in the resistance of eggs to disturbances. The results show that benthic fauna may considerably alter the patterns of zooplankton emergence from the seafloor, thereby shaping zooplankton pelagic populations. The semi-motile M. balthica and Marenzelleria spp. increased the fluxes of phosphate and ammonium from the sediment to the water, whereas the motile M. affinis and Mysis mixta had a contrasting effect. In the eutrophied Baltic Sea, efficient internal cycling of bioavailable nutrients forms a strong feedback inhibiting the recovery of the ecosystem. Based on the results, a change in species dominance from the two motile taxa, susceptible to oxygen deficiency, to the more tolerant semi-motile taxa provides additional feedback, strengthening internal nutrient cycling and accelerating eutrophication, with deteriorating near-bottom oxygen conditions and changes in the benthic communities. In shallow-water ecosystems, benthic nutrient regeneration plays a key role in determining the overall productivity of the ecosystem. In addition, the results of this study show that the communities in the benthos may essentially contribute to the structure of those in the plankton.

Allelopathic effects of filamentous cyanobacteria on phytoplankton in the Baltic Sea

Eutrophication and enhanced internal nutrient loading of the Baltic Sea are most clearly reflected by increased late-summer cyanobacterial blooms, which often are toxic. In addition to their toxicity to animals, phytoplankton species can be allelopathic, which means that they produce chemicals that inhibit competing phytoplankton species. Such interspecific chemical warfare may lead to the formation of harmful phytoplankton blooms and the spread of exotic species into new habitats. This is the first report on allelopathic effects in brackish-water cyanobacteria. The experimental studies presented in this thesis showed that the filamentous cyanobacteria Anabaena sp., Aphanizomenon flos-aquae and Nodularia spumigena are capable of decreasing the growth of other phytoplankton species, especially cryptophytes, but also diatoms. The detected allelopathic effects are rather transitory, and some co-occurring species show tolerance to them. The allelochemicals are excreted during active growth and they decrease cell numbers, chlorophyll a content and carbon uptake of the target species. Although the more specific modes of action or chemical structures of the allelochemicals remain to be studied, the results clearly indicate that the allelopathic effects are not caused by the hepatotoxin, nodularin. On the other hand, cyanobacteria stimulated the growth of bacteria, other cyanobacteria, chlorophytes and flagellates in a natural phytoplankton community. In a long-term data analysis of phytoplankton abundances and hydrography of the northern Baltic Sea, a clear change was observed in phytoplankton community structure, together with a transition in environmental factors, between the late 1970s and early 2000s. Surface water salinity decreased, whereas water temperature and the concentration of dissolved inorganic nitrogen increased. In the phytoplankton community, the biomass of cyanobacteria, chrysophytes and chlorophytes significantly increased, and the late-summer phytoplankton community became increasingly cyanobacteria-dominated. In contrast, the biomass of cryptophytes decreased. The increased temperature and nutrient concentrations probably explain most of the changes in phytoplankton, but my results suggest that the possible effect of chemically mediated biological interactions should also be considered. Cyanobacterial allelochemicals can cause additional stress to other phytoplankton in the nutrient-depleted late-summer environment and thus contribute to the formation and persistence of long-lasting cyanobacterial mass occurrences. On the other hand, cyanobacterial blooms may either directly or indirectly promote the growth of some phytoplankton species. Therefore, a further increase in cyanobacteria will probably shape the late-summer pelagic phytoplankton community by stimulating some species, but inhibiting others.