Spatio-temporal variability of surface sediment phosphorus fractions and water phosphorus concentration in Lake Peipsi (Estonia/Russia)

Spatial and temporal variations in the contents of total phosphorus and five P-fractions were investigated in the surface sediments of large and shallow Lake Peipsi (area 3555 km2, mean depth 7.1 m) and compared with total phosphorus (TP), dissolved inorganic P (PO4-P) and dissolved oxygen (O2) concentration in the overlying water column. The aim of the study was to determine the relationships between the sediment P and water P concentration dynamics. Samples from the uppermost 10-cm layer of the sediment and water samples from surface and near-bottom layers were collected simultaneously at six monitoring stations twice a year (in March and August) from 2004 to 2009. The results indicated that the concentrations of different P-fractions in the studied sediments varied greatly. Total P in sediment ranged from 470 to 1400 mg kg–1 dry mass (DM), while the loosely bound (labile) P (NH4Cl-P) was the smallest and the most variable fraction (range from 3.6 to 43 mg kg–1 DM). The metal oxide-bound P (NaOH-P) was also highly variable (range from 48 to 660 mg kg–1 DM), whereas the concentration of calcium-bound P (HCl-P) was relatively even (range from 300 to 550 mg kg–1 DM) in all the studied sediments. Redundancy analysis results revealed that different phosphorus fractions in sediment had statistically significant relationships with bottom-water variables like TP (in winter) and O2 (in summer). Our results show that the oxygen conditions have deteriorated in the lake during the last decades, which may have enhanced the release of P from the sediments

Ecology under lake ice

Winter conditions are rapidly changing in temperate ecosystems, particularly for those that experi-ence periods of snow and ice cover. Relatively little is known of winter ecology in these systems,due to a historical research focus on summer ‘growing seasons’. We executed the first global quan-titative synthesis on under-ice lake ecology, including 36 abiotic and biotic variables from 42research groups and 101 lakes, examining seasonal differences and connections as well as how sea-sonal differences vary with geophysical factors. Plankton were more abundant under ice thanexpected; mean winter values were 43.2% of summer values for chlorophyll a, 15.8% of summerphytoplankton biovolume and 25.3% of summer zooplankton density. Dissolved nitrogen concen-trations were typically higher during winter, and these differences were exaggerated in smallerlakes. Lake size also influenced winter-summer patterns for dissolved organic carbon (DOC), withhigher winter DOC in smaller lakes. At coarse levels of taxonomic aggregation, phytoplanktonand zooplankton community composition showed few systematic differences between seasons,although literature suggests that seasonal differences are frequently lake-specific, species-specific,or occur at the level of functional group. Within the subset of lakes that had longer time series,winter influenced the subsequent summer for some nutrient variables and zooplankton biomas

Ecology under lake ice

Winter conditions are rapidly changing in temperate ecosystems, particularly for those that experience periods of snow and ice cover. Relatively little is known of winter ecology in these systems, due to a historical research focus on summer 'growing seasons'. We executed the first global quantitative synthesis on under-ice lake ecology, including 36 abiotic and biotic variables from 42 research groups and 101 lakes, examining seasonal differences and connections as well as how seasonal differences vary with geophysical factors. Plankton were more abundant under ice than expected; mean winter values were 43.2% of summer values for chlorophyll a, 15.8% of summer phytoplankton biovolume and 25.3% of summer zooplankton density. Dissolved nitrogen concentrations were typically higher during winter, and these differences were exaggerated in smaller lakes. Lake size also influenced winter-summer patterns for dissolved organic carbon (DOC), with higher winter DOC in smaller lakes. At coarse levels of taxonomic aggregation, phytoplankton and zooplankton community composition showed few systematic differences between seasons, although literature suggests that seasonal differences are frequently lake-specific, species-specific, or occur at the level of functional group. Within the subset of lakes that had longer time series, winter influenced the subsequent summer for some nutrient variables and zooplankton biomass