Reet Laugaste

Zooplankton as indicators in lakes: a scientific-based plea for including zooplankton in the ecological quality assessment of lakes according to the European Water Framework Directive (WFD)

With the implementation of the EU Water Framework Directive (WFD), the member states have to classify the ecological status of surface waters following standardised procedures. It was a matter of some surprise to lake ecologists that zooplankton were not included as a biological quality element (BQE) despite their being considered to be an important and integrated component of the pelagic food web. To the best of our knowledge, the decision of omitting zooplankton is not wise, and it has resulted in the withdrawal of zooplankton from many so-far-solid monitoring programmes. Using examples from particularly Danish, Estonian, and the UK lakes, we show that zooplankton (sampled from the water and the sediment) have a strong indicator value, which cannot be covered by sampling fish and phytoplankton without a very comprehensive and costly effort. When selecting the right metrics, zooplankton are cost-efficient indicators of the trophic state and ecological quality of lakes. Moreover, they are important indicators of the success/failure of measures taken to bring the lakes to at least good ecological status. Therefore, we strongly recommend the EU to include zooplankton as a central BQE in the WFD assessments, and undertake similar regional calibration exercises to obtain relevant and robust metrics also for zooplankton as is being done at present in the cases of fish, phytoplankton, macrophytes and benthic invertebrates.

Water level as the mediator between climate change and phytoplankton composition in a large shallow temperate lake

We studied the effect of water level changes in Lake Võrtsjärv (270 km2, mean depth 2.8 m) on the abundance and composition of phytoplankton based on a 35-year database. Analysis of long time-series (since 1884) revealed a strong influence of the North Atlantic Oscillation on the water level in this lake. The registered maximum range of water level in L. Võrtsjärv is 3.2 m, which corresponds to 1.4 times difference in the lake area, 2.5 times difference in the mean depth and 3.5 times difference in volume. Cyanobacteria build up 2/3 of the average phytoplankton biomass during the ice-free period (May–October). On average 75% of the cyanophyte biomass is formed by four filamentous species. Centric diatoms from the genera Aulacoseira and Cyclotella dominate in the biomass of diatoms. Phytoplankton biomass was significantly lower in years of high water level and the changes were unrelated to nutrient loading. The share of filamentous blue-greens among phytoplankton followed the changes in the water level while there was a succession of dominants. Planktolyngbya limnetica reached its maximum in the low water period in the middle of the 1970s. Limnothrix redekei and L. planktonica started to dominate in the high water period in the 1980s while the total biomass decreased substantially in spite of high nutrient loading. During the low water period in the 1990s, the role of the nitrogen-fixing species Aphanizomenon skujae increased. The succession was caused by changes in light and nutrient availability in the fully mixed environment. As low light species, filamentous Limnothrix spp. were most successful in competition for light and phosphorus in deeper water while A. skujae was favoured by high light intensities and a low N/P ratio in shallow water.

Reaction of large and shallow lakes Peipsi and Võrtsjärv to the changes of nutrient loading

More than 20-year monitoring of Estonian rivers reveals that the loading of nitrogen to large shallow lakes Peipsi (3,555 km2, mean depth 7.1 m) and Võrtsjärv (270 km2, mean depth 2.8 m) decreased substantially in the 1990s. Phosphorus loading decreased to a much smaller extent than nitrogen loading. In L. Võrtsjärv both N and P concentrations followed the decreasing trends of loading, which show the high sensitivity of large shallow lakes to catchment processes. Our study showed a positive relationship between P content in sediments and the relative depth of the lake. Assumingly the resilience of a lake in responding to the reduction of nutrient loading decreases together with the decrease of its relative depth. In L. Peipsi the concentration of P has not decreased since the 1990s. Our data show indirectly that P loading from Russia to L. Peipsi may have increased. The N/P ratio has decreased in both lakes. Cyanobacterial blooms have been common in both lakes already at the beginning of the 20th century. The blooms disappeared during heavy nitrogen loading in the 1980s but started again in L. Peipsi in recent years together with the drop of the N/P ratio. In L. Võrtsjärv the N/P ratio is higher and the ecosystem is more stable although the share of N2-fixing cyanobacteria increased from the 1990s. Reappearing cyanobacterial blooms in L. Peipsi have caused fish-kills in recent years. In L. Peipsi summer/autumn fish-kills during water-blooms are a straightforward consequence of reduced nitrogen level at remaining high phosphorus level while in L. Võrtsjärv the climatic factors affecting water level are more critical––at low water level winter fish-kills may occur. In L. Võrtsjärv nutrient loading has decreased and water quality has improved, present ecological status seems to be mostly controlled by climatic factors through changes of water level. The most important measure to improve water quality in L. Peipsi would be the reduction of phosphorus loading from both Estonian and Russian subcatchments.

The composition and density of epiphyton on some macrophyte species in the partly meromictic Lake Verevi

The epiphyton on 22 macrophyte species was studied in the hypertrophic stratified Lake Verevi mainly in the midsummer of 2000 and 2001. Some material from 1998 and 1999 was used as well. Chlorophyll a (Chl a) level was high: 330–360 μg g dw−1 on emergent plants, and an average of 117–200 μg on floating-leaved plants and 820–920 μg g dw−1 on submerged plants. Biomass was 15–23, 5–10 and 35–53 mg g dw−1, respectively. The richest in epiphyton were submerged plants with densely growing and fine branchlets such as Ranunculus, Ceratophyllum, Myriophyllum, Utricularia, Potamogeton pectinatus L. and P. friesii Rupr. The share of Chl a in biomass was higher in 2001 (2.3%) than in 2000 (1.7%), which can be associated with lower irradiance in summer 2001. Filiform chlorophytes were dominating on most plants; 60% of biomass on submerged, 69% on emergent and 80% on floating-leaved plants; in some cases, the share of filamentous species was 95%. Diatoms formed 29, 12 and 7%, cyanobacteria 8, 16 and 10% of the same ecotopes, respectively. As a rule, the epiphyton was quite sparse on large Potamogeton leaves. Cyanobacteria were more abundant on large Potamogeton and Nuphar leaves, Elodea, on stems of P. natans L., Nuphar and on some emergent plants with a smooth and soft stem surface, as Butomus and Typha. Diatoms played the most important role on some Potamogeton species and in single samples of Ceratophyllum and Ranunculus. The morphology of plant species appears to be the main factor of epiphyton richness in L. Verevi.

Long-term changes and seasonal development of phytoplankton in a strongly stratified, hypertrophic lake

Changes in the phytoplankton community of the hypertrophic, sharply stratified Lake Verevi have been studied over eight decades. Due to irregular discharge of urban wastewater, the trophic state of the lake has changed from moderately eutrophic to hypertrophic. We found that the trophic state in summer increased in the 1980s and remained at a hypertrophic level since then. Planktothrix agardhii was recorded first in the 1950s and became the dominant species in the 1980s, forming biomass maxima under the ice and in the metalimnion during the vegetation period. In summer 1989, P. agardhii contributed almost 100% of the phytoplankton biomass. Generally, the highest biomass values occurred in the metalimnion. In spring, when P. agardhii was less numerous, diatoms and cryptophytes prevailed. In springs 2000 and 2001 different diatoms dominated – Synedra acus var. angustissima (18.6 g m−3) and Cyclostephanos dubius (9.2 g m−3), respectively. In recent years, the spring overturn has been absent. In the conditions of strong thermal stratification sharp vertical gradients of light and nutrients caused a large number of vertically narrow niches in the water column. During a typical summer stage, the epilimnion, dominated by small flagellated chrysophytes, is nearly mesotrophic, and water transparency may reach 4 m. The lower part of the water column is hypertrophic with different species of cryptophytes and euglenophytes. A characteristic feature is the higher diversity of Chlorococcales. Often, species could form their peaks of biomass in very narrow layers, e.g. in August 2001 Ceratium hirundinella (18.6 g m−3) was found at a depth of 5 m (the lower part of the metalimnion with hypoxic conditions), Cryptomonas spp. (56 g m−3) at 6 m (with traces of oxygen and a relatively high content of dissolved organic matter) and euglenophytes (0.6 g m−3) at 7 m and deeper (without oxygen and a high content of dissolved organic matter).

Nutrients and phytoplankton in Lake Peipsi during two periods that differed in water level and temperature

Data for the vegetation periods (May–November) of 1985–2003 were used to collate the nutrient content and biomass of the most important phytoplankton groups in Lake Peipsi (Estonia). Two periods differing in external nutrient load and water level were compared by analysis of variance. The years 1985–1988 were characterized by the highest loads of nitrogen and phosphorus, high water level and cool summers. The years 2000–2003 were distinguished by low or medium water levels and warm summers. The first period showed statistically significantly higher values of total nitrogen (Ntot) and a higher Ntot:Ptot mass ratio. The second period showed a higher content of total phosphorus (Ptot), a higher ratio of dissolved inorganic compounds N to P and higher phytoplankton and cyanobacterial biomasses. Comparison between parts of the lake demonstrated that the differences between the two periods were more evident in the shallower and strongly eutrophic parts, Lake Pihkva and Lake Lämmijärv, than in the largest and deepest part, the moderately eutrophic Lake Peipsi s.s. Temperature and water level acted synergistically and evidently influenced phytoplankton via nutrients, promoting internal loading when the water level was low and the temperature high. The effect of water level was stronger in the shallowest part, Lake Pihkva. The difference in Ptot content between the southern and northern parts was twofold; the Ntot:Ptot mass ratio was significantly lower in the southern parts, and phytoplankton biomass (particularly the biomass of cyanobacteria) was significantly higher for Lake Pihkva and Lake Lämmijärv than for Lake Peipsi s.s.

The role of cladocerans reflecting the trophic status of two large and shallow Estonian lakes

The role of pelagic cladoceran communities is discussed on the basis of a comparative study conducted in two Estonian lakes, the moderately eutrophic Lake Peipsi (Ntot 700, Ptot 40 μg l−1 as average of ice-free period of 1997–2003) and in a strongly eutrophic Lake Vortsjarv (Ntot 1600, Ptot 54 μg l−1). The cladoceran community was found to reflect the differences in the trophic state of these lakes. In L. Peipsi, characteristic species of oligo-mesotrophic and eutrophic waters co-dominated (making up 20% or more of total Zooplankton abundance or biomass), whereas in L. Vortsjarv only species of eutrophic waters occurred. In L. Peipsi, the dominant cladocerans were Bosmina berolinensis and Daphnia galeata, while Chydorus sphaericus was the most abundant cladoceran in L. Vortsjarv. The cladocerans of L. Peipsi (mean individual wet weight 25 μg) were significantly (threefold) larger than those of L. Vortsjarv (8 μg). The mean wet biomass of cladocerans was higher and total cladoceran abundance was lower in L. Peipsi compared to L. Vortsjarv (biomass varied from 0.133 to 1.570 g m−3; mean value 0.800 g m−3 in L. Peipsi and from 0.201 to 0.706 g m−3, mean 0.400 g m−3 in L. Vortsjarv; the corresponding data for abundances were: 8,000-43,000 ind m−3, mean 30,000 ind m−3 for L. Peipsi, 50,000–100,000, mean 52,000 ind m−3 for L. Vortsjarv). Based upon differences in body size, cladocerans were more effective transporters of energy in L. Peipsi than in L. Vortsjarv. Cladocerans proved to be informative indicators of the trophic status and of the efficiency of the food web in studied lakes.

Combining limnological and palaeolimnological approaches in assessing degradation of Lake Pskov

Limnological monitoring data and palaeorecords from large shallow Lake Pskov were used to study the long-term dynamic pattern of the lake ecosystem and to identify the start of its degradation. Lake Pskov is the southern part of Lake Peipsi s.l., the largest transboundary lake in Europe. The limnological monitoring data collected in the years 1956–2005 show that the water quality of L. Pskov has deteriorated and caused adverse changes in the whole ecosystem (e.g. excessive growth of algae, increased cyanobacterial blooms, silting of the lake bottom, fish kills). Doubled total phosphorus (Ptot), dissolved inorganic P (PO4-P) and chlorophyll a (Chl-a) contents, increased total alkalinity (HCO3 −) and pH, as well as decline in water transparency and oxygenation conditions, indicate a clear increase in the trophic level of the lake. However, the limnological studies do not show when the degradation started. To understand long-term dynamics of the lake ecosystem, a 52 cm sediment core taken from one monitoring station of L. Pskov was studied and dated by the 210Pb method. Palaeodata show that substantial changes in the L. Pskov ecosystem started already in the 1930s when mesotrophic conditions in the lake turned increasingly eutrophic. Since that time, the content of P, nitrogen (N), carbon (C) and relative abundance (RA%) of planktonic diatoms in the sediment have increased significantly (P 0.9 or r <-0.9, P < 0.001) were observed between the contents of P, N, C and sulphur (S) in the sediment and the mean water level (WL) and temperature (WT) registered one to five years earlier in the lake. The C content of the sediment showed a negative relationship with WL and a positive correlation with WT. A significant positive relationship was recorded also between WT, and N and P content in the sediment. It is hypothesized that a higher WT and lower WL result in an increased organic mater accumulation in the sediment in the coming years.

Effect of winter conditions on spring nutrient concentrations and plankton in a large shallow Lake Peipsi (Estonia/Russia)

During the study period 1997–2007, the duration of the ice cover of Lake Peipsi, the largest freshwater lake in Estonia, was variable. The ice-break took place in mid-April instead of generally in May. Compared with data from 1960s onwards, a trend of shortening of the duration of ice-cover period is evident. In under-ice phytoplankton, diatoms, cryptophytes and flagellated chlorophytes predominated from March to June. In zooplankton, while thermophobic rotifers dominated in densities, copepods were dominant in biomass. The monthly sum of positive water temperatures correlated with the zooplankton in the following month and duration of the ice cover correlated with the biomass of all different zooplankton groups in May and June. The sum of water temperatures in April, May and June was positively correlated with the biomass of cladocerans and copepods, and negatively with the biomass of winter rotifers. Mild winters affected nitrogen and silica concentration positively and phosphorus concentration negatively.

Lake Peipsi: Changes in nutrient elements and plankton communities in the last decade

In the 1990s, as a consequence of a decline in agricultural production in the watershed and a decrease in the amount of waste water discharged into rivers, the nutrient load carried from the catchment area into large, shallow, eutrophic Lake Peipsi (area 3,555 km 2 ) decreased. The aim of the present study was to analyze the in-lake response of key physical and chemical variables and the biota to large-scale changes in the nutrient load. Yearly changes in water transparency, nutrient elements of surface water, chlorophyll a content, as well as in phyto- and zooplankton biomass were studied during the growth season of 1992-2000. A clear decline in total nitrogen, ammonium ion, and orthophosphate ions was revealed in the northern part of the lake over the studied years. Beginning from 1996-1997, a decreasing tendency was revealed for the nitrate ion and total phosphorus. A significant decline of the total nitrogen:total phosphorus ratio was also observed from 1992-2000. However, the biomass of phytoplankton (particularly cyanobacteria) and chlorophyll a concentration did not follow the dynamics of nutrients, but displayed an increasing trend. Concentrations of nutrients in the lake during the last decade were not so low as to limit phytoplankton growth directly. In Lake Peipsi, strong and long-lasting algal blooms were observed in recent years, despite a definite decline in the nutrient content of surface water. Weather conditions appear to be very important factors in causing algal blooms in Lake Peipsi.