Ecological assessment of lakes of NW Greece with emphasis on the associations between aquatic macrophytes, zooplankton and water quality

Lakes are extremely invaluable ecosystems due to their significance as acrucial component of the water circle and a source of fresh water. The last fewdecades there is an increasing interest of the scientific community for the functionsand mechanisms associated with the lake ecosystems. Moreover, the WaterFramework Directive requires the implementation of sustainable fresh watermanagement, including the lake ecosystems, in order to achieve a minimum goodecological quality by 2015. Consequently, there is a lot of research focused onimportant biotic groups used as biological indicators by the WFD. However, it is wellknown that the biotic interactions between the various groups of aquatic organismscan often provide key background information regarding the ecological quality of theecosystem.The main objective of the present thesis is the assessment of the ecologicalstatus of seven lakes in West and NW Greece based on physicochemicalcharacteristics, aquatic macrophyte composition and the major zooplanktontaxonomic groups composition. The results of the current thesis are presented in fourbasic parts. The first part is focused on the seasonal and spatial variations among theseven studied lakes regarding the nutrient concentrations and the abiotic parameters.The second part describes the composition of aquatic macrophytes and thecomposition of Cladocera and Rotifera in the studied lakes. In the third part theecological status of the studied lakes was assessed using Trophic State Indices andindices based on aquatic macrophytes and zooplankton. Finally, the fourth partinvestigates the effects of the aquatic vegetation on the spatial distribution andabundances of the major zooplankton taxonomic groups.According to the results of the current thesis, several statistically significantdifferences were found among the studied lakes regarding the abiotic parameters.Most notable differences include those of chlorophyll-a concentrations between thedeeper and the shallower lakes reflecting the variations in nutrient loading,geomorphology and local climate. During the present thesis, high concentrations oftotal phosphorus were recorded indicating the influence of eutrophic conditions.11As far as the aquatic macrophytes are concerned, few submerged species werefound in high abundances which are indicative of the eutrophic conditions.Ceratophyllum demersum, Trapa natans, Potamogeton pectinatus, Vallisneriaspiralis were characterized as dominant indicator species for the lakes Lysimachia,Megali Prespa, Petron and Vegoritis respectively.Regarding the zooplankton composition a total of 14 Cladocera and 39Rotifera were identified in lakes Mikri Prespa, Kastoria, Vegoritis and Petron. Mostcommon Cladocerans were the species Bosmina longirostris, Chydorus sphaericusand Diaphanosoma brachyurum. The small sized Cladoceran Bosmina longirostriswas found in greater density while Keratella cochlearis was the most abundantamong the Rotifera. The high abundance of small Cladocerans combined with thelow contribution of large Daphnia indicates the high predation pressure on thezooplankton. Lakes Mikri Prespa and Vegoritis were characterized by a largercontribution of Cladocera in relation with lakes Kastoria and Petron, including littoralspecies such as Chydorus. Other Rotifera species such as Keratella quadrata,Lecane and Polyarthra were also recorded in significant abundances in lakes MikriPrespa and Vegorits.The ecological assessment of the studied lakes based on the application ofTrophic State Indices classified lakes Mikri Prespa, Kastoria and Petron as eutrophicto hypereutrophic class, lake Lysimachia as mesotrophic to eutrophic class, lakeMegali Prespa as mesotrophic and lake Trichonis as oligotrophic to mesotrophicclass. The results from Macrophyte Index evaluation highlight the high nutrientenrichment and the eutrophic influence on the aquatic macrophyte composition.According to the results of Wetland Zooplankton Index, an index based onzooplankton developed for the assessment of wetlands in Great Lakes of N. America,lakes Mikri Prespa and Vegoritis are characterized by higher ecological status thanlakes Kastoria and Petron. Wetland Zooplankton Index can provide an important toolfor a holistic approach of the ecological assessment of lake ecosystems in Greece.According to the results of the present thesis, dense aquatic macrophyteassemblages contribute to a higher total zooplankton species richness. Specifically, inmacrophyte assemblages occupying the whole water column were found higher12abundances of littoral Cladocerans and Rotifers in relation with the abundances foundamong thinner macrophyte stands. Moreover, Redundancy Analysis revealedPotamogeton pectinatus, Potamogeton lucens, Trapa natans and Myriophyllumspicatum as most statistically significant aquatic macrophytes that influence thespatial distribution of zooplankton. Further analysis conducted separately for theCladocera and Rotifera taxa suggest that Trapa natans and Potamogeton natans havea larger influence on Rotifera spatial variation while the submerged macrophytesMyriophyllum spicatum and Potamogeton perfoliatus will influence mostly thecrustacean spatial distribution. These differences can be attributed to a preference oflittoral Cladocerans for more complex macrophyte structures that provide habitat forforaging. Few abiotic parameters were found to have a significant effect onzooplankton spatial variability which highlights the importance of top-down andbottom-up factors for determination of biotic community interactions.In conclusion, the results of this doctoral thesis underline the influence ofeutrophication on the biological communities of aquatic macrophytes andzooplankton. However, there are indications that other human induced activities, suchas introduction of exotic fish species, may have a significant effect on thezooplankton composition. Additional research, focused on the composition of littoralfish communities combined with studies on the horizontal distribution of zooplanktonin the littoral zone could provide solid information on the role of littoral aquaticvegetation as refuge. Moreover, the results of this thesis indicate that the aquaticvegetation has a significant effect on the zooplankton abundance and there is apossible relationship between complex macrophyte structures and Cladocera density.Further investigation of the spatial variation of zooplankton composition inside themacrophyte assemblages may also elucidate the role of aquatic vegetation asmicrohabitats for the zooplankton communities.Although the interactions between the biotic communities in a lake ecosystemcan be quite complex, the comprehension of the background mechanisms is necessaryin order to implement successive management and restoration strategies. Especially,in Mediterranean region, due to the special climate, environmental and13socioeconomic conditions, the research on lake ecosystems is particularly importantand highly significant for an effective sustainable water managemen