Pelagic food webs, with particular emphasis on microbial loops, were investigated in European alpine lakes. During two ice-free seasons (1996-1997) investigations of biomass structure in pelagic food webs were performed at 7 lakes in low-alkalinity mountain regions (Lake Districts). In 2000 (in some cases, 2001), late-summer (autumn) samples from 3041akes from Lake Districts were analysed for pelagic bacterial biomass (BAC), chlorophyll a (CHL) and zooplankton (ZOO) abundance. In two districts (Northern Finland-NF and Tyrolian Alps-TY), phytoplankton (PHY) species structure was also determined, together with ciliate (CIL) species structure in NE BAC abundances (0.02 - 2.7 x 106 mi-I) generally corresponded to values found in other oligotrophic lakes, and to winter values from lowland meso- to eutrophic lakes. Values from two regions, the Julian Alps (lA) and Western Greenland (SS), however, were exceptionally high (0.10 - 2Sx106 ml'). Here, bacterial cells often were large, elongated and even filamentous. Mean cell volumes (per sample) reached values up to 0.91 f-lm3 and cell lengths up to 4.1 um, thus surpassing the values known from lowland eutrophic lakes. Consequently, BAC biomasses frequently reached 40-S0 ug C L-I, and in lakes with filamentous bacteria up to 100 - lSO ug C L-'. In Greenland lakes values of more than 200 ug C L-I were observed. Autotrophic picoplankton (APP) were scarce «SOO mi-I) or absent in 6 of the 7 low-alkalinity lakes studied. APP abundances regularly surpassing 103 mI-I (biomass IS-ISO f-lg C L-I) were only observed in Lochnagar (Scotland), which is characterised by a high sodium and chloride concentration. APP biomass at this site surpassed that of BAC during late summer and autumn. Biomass of heterotrophic flagellates regularly reached S-20% (exceptionally SO%) of BAC biomass, whereas CIL biomass usually reached only 1-8% ofBAC biomasses, except for NF lakes (8 - SO% ofBAC biomass). The most abundant CIL species belonged to five groups: large mixotrophic oligotrichs, algivorous prostomatids, large hymenostomatids, gymnostomatids and minute omnivorous oligotrichs. Besides heterotrophic protists, mixotrophic (containing Chi) bacterivores were always present. PHY biomass varied considerably among lakes and within individuai lakes, and sometimes reached values found in meso- to eutrophiclakes. The smallest variation (0.3 - 153 ug C L-') was observed in lakes within a pH range of 5.3 - 5.9. In lakes with either lower or higher pH values (ranges 4.5 - 5.2 and 5.9 - 9.4), PHY biomass varied from 1.8 up to 1000flg C L-'. Among PHY species, motile and potentialIy phagotrophic bacterivorous taxa dominated from the Cryptophyceae,Chrysophyceae ancl Dinophyceaefamilies. Representatives of Chlorophyceae and Bacillariophyeeae usualIy occurred in lower abundance. In Northern Finland Conjugatophyceae were frequent. Redundaney Analysis was applied to alI lake districts except SS to test the response of BACto chlorophylI a (CHL) and to the abundance of aggregatedZOO groups. 12.8% of variation in BAC abundanees is explained by Daphnidae (mostly), Cyclopidae and Diaptomidae. Partial Redundancy Analysis proved that, though the BAC response to ZOO plus CHL is statistically significant, the majority of the explained variation is accounted for by proximal variables attributed to the physico-chemical environment of the lakes, such as nitrate, ehloride and littoral organic substrate. In the set of 7 intensely studied low-alkalinity lakes, the share of BAC in the total pelagic microbial biomass was significantly inversely correlated with trophy.
Structure of pelagic microbial assemblages in European mountain lakes during icefree season
Bertoni R;Callieri C;
2009
Abstract
Pelagic food webs, with particular emphasis on microbial loops, were investigated in European alpine lakes. During two ice-free seasons (1996-1997) investigations of biomass structure in pelagic food webs were performed at 7 lakes in low-alkalinity mountain regions (Lake Districts). In 2000 (in some cases, 2001), late-summer (autumn) samples from 3041akes from Lake Districts were analysed for pelagic bacterial biomass (BAC), chlorophyll a (CHL) and zooplankton (ZOO) abundance. In two districts (Northern Finland-NF and Tyrolian Alps-TY), phytoplankton (PHY) species structure was also determined, together with ciliate (CIL) species structure in NE BAC abundances (0.02 - 2.7 x 106 mi-I) generally corresponded to values found in other oligotrophic lakes, and to winter values from lowland meso- to eutrophic lakes. Values from two regions, the Julian Alps (lA) and Western Greenland (SS), however, were exceptionally high (0.10 - 2Sx106 ml'). Here, bacterial cells often were large, elongated and even filamentous. Mean cell volumes (per sample) reached values up to 0.91 f-lm3 and cell lengths up to 4.1 um, thus surpassing the values known from lowland eutrophic lakes. Consequently, BAC biomasses frequently reached 40-S0 ug C L-I, and in lakes with filamentous bacteria up to 100 - lSO ug C L-'. In Greenland lakes values of more than 200 ug C L-I were observed. Autotrophic picoplankton (APP) were scarce «SOO mi-I) or absent in 6 of the 7 low-alkalinity lakes studied. APP abundances regularly surpassing 103 mI-I (biomass IS-ISO f-lg C L-I) were only observed in Lochnagar (Scotland), which is characterised by a high sodium and chloride concentration. APP biomass at this site surpassed that of BAC during late summer and autumn. Biomass of heterotrophic flagellates regularly reached S-20% (exceptionally SO%) of BAC biomass, whereas CIL biomass usually reached only 1-8% ofBAC biomasses, except for NF lakes (8 - SO% ofBAC biomass). The most abundant CIL species belonged to five groups: large mixotrophic oligotrichs, algivorous prostomatids, large hymenostomatids, gymnostomatids and minute omnivorous oligotrichs. Besides heterotrophic protists, mixotrophic (containing Chi) bacterivores were always present. PHY biomass varied considerably among lakes and within individuai lakes, and sometimes reached values found in meso- to eutrophiclakes. The smallest variation (0.3 - 153 ug C L-') was observed in lakes within a pH range of 5.3 - 5.9. In lakes with either lower or higher pH values (ranges 4.5 - 5.2 and 5.9 - 9.4), PHY biomass varied from 1.8 up to 1000flg C L-'. Among PHY species, motile and potentialIy phagotrophic bacterivorous taxa dominated from the Cryptophyceae,Chrysophyceae ancl Dinophyceaefamilies. Representatives of Chlorophyceae and Bacillariophyeeae usualIy occurred in lower abundance. In Northern Finland Conjugatophyceae were frequent. Redundaney Analysis was applied to alI lake districts except SS to test the response of BACto chlorophylI a (CHL) and to the abundance of aggregatedZOO groups. 12.8% of variation in BAC abundanees is explained by Daphnidae (mostly), Cyclopidae and Diaptomidae. Partial Redundancy Analysis proved that, though the BAC response to ZOO plus CHL is statistically significant, the majority of the explained variation is accounted for by proximal variables attributed to the physico-chemical environment of the lakes, such as nitrate, ehloride and littoral organic substrate. In the set of 7 intensely studied low-alkalinity lakes, the share of BAC in the total pelagic microbial biomass was significantly inversely correlated with trophy.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
