| Abstract |
This research took place in Kourna Lake (Chania, Crete) from March 2003 to August 2005. During that time specific physical-chemical parameters and biotic elements (phytoplankton, zooplankton and fishfauna) were monitored nearly monthly in 4 sample stations. Kournas is the only natural lake of Crete Island with temperature fluctuating from 12-27oC and water mixing only during winter months. This feature classifies the lake in the warm monomictic category. Water column is well oxidized. Only during middle summer time in specific years (2004 & 2005), diluted oxygen falls to anoxia levels near bottom (2,7ppm). Even H2S was found at the bottom during the 2004 autumn. The water transparency fluctuated from 6,6-17m seeming to be defined mostly of the phytoplankton biomass than the terrestrial loading after rainfalls. This phenomenon determines a deep lake euphotic zone in almost the whole water volume except the deepest area in the centre most of the year. The pH was alkaline (7,4-8,6) with great fluctuations during seasons and after autumn first rains. In the vertical level the parameter differentiates strongly in the autumn and minimizes in the whole column during winter mixing.The entering water from the sub fountains contains significant salt quantities which influence the lake salinity (0,5-1 ppt or even 4 ppt) and electrical conductivity.
The most precious lake nutrient for the phytoplankton growth seems to be phosphates with very low concentrations (0,01-0,935 μΜ/l or 0,3-28,96 μg P /l). Ιn the last 20 years, man activities over the lake area seem to have strong influence through the increase of nitrogen inputs in the littoral zone mainly and the sub fountains water area. The total nitrogen salts fluctuated from 160-890 μg/L (ή 11.57-63.49 μΜ/L) and are contributed mainly from nitrates (120-870 μg/l ή 8.51-61.92 μΜ/L). These nutrients increase mainly during the winter and the spring. The nitrites salts fluctuated from 0,59-12,94 μg/l (ή 0.042-0.924 μΜ/L) and ammonium salts from 3,2-544 μg/L (ή 0.23-38.85 μΜ/L). The last have low contribution in the nitrogen nutrients usually, except in periods with reduction conditions like the 2004 autumn when it topped to 80%. In general nitrate concentrations got higher values during 2003 than 2004. On the contrast in warm 2004 with more reduction conditions we measured higher values of nitrites and ammonium. Silicon in silicate salts seem to be sufficient for time to time for the diatom biosynthesis with no standard periodicity, fluctuating from 271-555,4 μg(SiO2)/l or 4,5-92,4 μΜ/l.Organic loading from the area around the lake seem to play sufficient role depending to the weather conditions, like heavy autumn rains or long draught period and strong south winds.
Rich plankton biodiversity with at least 118 phytoplankton and 20 zooplankton taxa has been registered in Kournas. From the first category 51 belong to Chlorophyceae , 31 to Bacillariophyceae, 10 to Cyanophyceae, 7 to Dinophyceae, 2 to Chrysophyceae, 2 to Cryptophyceae and 1 to Euglenophyceae and Xanthophyceae. The phytoplankton biomass was characterized by the filaments of the greenalgae taxa, Mougeotia and Spirogyra spp. of the Zygnemales Order, the diatomic thecas of the Cyclotella taxa, the capsae of the bluegreen Chroococcus genus and the spiny cells of the dinoflangelate, Ceratium hirundinella.
According to the general patterns of the phytoplankton succeeding process (Reynolds 1984), quite good similarity seem to be between Kourna and other oligotrophic lakes. The succeed of the phytoplankton taxa seem to be characterized by alternation of two volume classes, with the prevalence of r-strategy small taxa like Cyclotella spp. or the prevalence of k-strategy big sized taxa in spring or autumn. The latter being Pennales diatoms and Zygnemales Chlorophyta or the dinoflangelate, C. hirundinella.
The statistical analysis of the richness and biomass phytoplankton data from the Niskin bottle and the plankton net specimens concludes that it grows in space homogeneity but with sufficient differences from year to year. This was clear also with the low biodiversity in 2005 in contrast with 2003 and between 2003 and 2004. Also the vertical distribution of the algae flora has significant differences between surface and bottom specimens (underlining the missing main winter data).
In zooplankton taxa, 11 belong to the Potifera Phylum, 4 to the Cladoceran Family, 3 to the Copepode Family (1 Calanoid and 2 Cyclopoid) and 2 to the flagellate Protozoa, without taking account to the meroplanktonic larvae of insects. 11-14 of them considered as pelagic taxa. Due to that consideration of the zooplankton synthesis Kournas is also similar to other oligotrophic European lakes. Also the increase of trophism in particular seasons and years is simultaneous with the low values of the quota (Calanoid/ Cyclopoid + Cladocera). In the richness level usually prevail the Rotifers (99% max in total richness) and in biomass Copepodes prevail (up to 80%) most of the time. Exception to that the prevalence of the rotifer species Asplanchna priodonta , even in zooplankton biomass. This phenomenon has more longevity by the warm 2004 than in 2003.
Zooplankton doesn’t have homogeneity in lake space. Also zooplankton richness has significant diffence between surface and bottom during the stratification period (in summer mainly). Indeed it doen’t correspond with phytoplankton changes during the year. Calanoid Copepod Order seem to prevail the Cyclopoid order with richness of 0-52 ind/m3 and 0-6 ind/m3 respectively. Farther more Cladocerans have low richness of 0,6-8,8 ind/m3 but have significant frequency as a prey in planktonic fish.
During the research, 6 fish species has been registered. Two of them belonging to the autochthonous taxa (sandsmelt,Atherina boyeri and riverblenny,Blennius (Salaria) fluviatilis). Two other to the diadromous taxa (European eel, Anguilla anguilla and greymullet,Liza ramada). The rest belong to introduced species. First the mosquitofish (Gambusia (affinis) holbrooki) which was transferred 70 years ago and the accidentally recently introduced goldfish (Carassius auratus). In the late years, there was no sign from the rainbow trout, Onkorhynchus mykiss, which has been introduced 25 years ago.
The predation pressure of the sandsmelt fish (even young) is evident in big Cladocerans (f=36,85 +/- 32.73%) and less in Copepods (f= 10,09+/- 9,79%) but also in macroinvertebrates with preference in Planaria (f= 46,8+/-33,82%) and insects larvae (f=39,31+/-20,5%) and adult individuals (f=44,91+/-18,51%).
The same prey frequency has been registered in mosquitofish but with greater preference in smaller life forms, neither zooplanktonic (Calanoid Copepods with frequency f= 36,7+/-37,8, Rotifers with f=23,1+/-29,2% and Cladocerans with f=28,04+/-5,19) nor macroinvertebrates (insects larvae with f=43,61+/-9,93% and adult individuals f=21,78+/- 20,46%).
The newcomer goldfish seem to be in control by the high predators of the lake. Late studies ornithologists have registered increase of the cormorant population in the area.
As a conclusion, the phytoplankton biomass which is left over during the productive years sinks to the bottom. This could happen massively during predator overpopulation in warm years through the trophic cascade, by the minimizing the zooplankton population. This phenomenon seems to backside the rich zoobenthos populations of the lake and maily the numerous gastropods and small cetaceans, like glasshrimp, which play the key role in the energy food chain recycling in the Kournas lake ecosystem.
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Μελέτη οικοσυστήματος λίμνης Κουρνά Χανίων με έμφαση στο πλαγκτόν και την ιχθυοπανίδα
