Article

Changes in water temperature and chemistry preceding a massive kill of bottom-dwelling fish: An analysis of high-frequency buoy data of shallow Lake Vortsjarv (Estonia)

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Abstract

Although massive fish kills are wide-spread and can be economically devastating, there is little information on exact causal mechanisms of fish kills in nature. In large shallow Lake Vortsjarv, sporadic fish kills have been registered mainly in cold winters, yet in 2013, an unexpected fish kill occurred beginning mid-June. At the time of the fish kill, an investigation was conducted to determine species composition, number, and sizes of dead fish along the lake shore. To determine possible causes of the fish kill, we analysed the dynamics of key physical and chemical parameters of lake water, including diurnal fluctuations of water temperature (WT), pH, dissolved oxygen (DO), ammonium ion concentrations (NH4-N), and the development of water stratification, during the growing season of 2013 using high-frequency water quality monitoring buoy and monthly manual monitoring data. Environmental data between 2010 and 2012 were used as a reference because no fish kill occurred. The results suggest that the fish kill was induced by a combination of successive and co-occurring extreme water parameters such as high WT (up to 24.5 °C), pH (up to 9.2), and NH4-N (up to 0.13 mg L⁻¹), short-term stratification, and low DO concentration in the bottom water (0.49 mg L-1, saturation 5.4%) induced by quick warming of this shallow lake after a long ice-covered period and leading to a likely ammonia poisoning and hypoxia. The main target species was the bottom-dwelling ruffe (Gymnocephalus cernuus), indicating that the summer kill started at the bottom of the lake. The event highlights the significance of short-term disturbances on fish populations, which can be detected only using high-frequency monitoring data.

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... Fish kills have been linked to multiple factors such as rapid changes in water temperature (temperature stress), algal blooms (cyanobacteria, dinoflagellates), ammonia (NH 3 ) and hydrogen sulphide (H 2 S) toxicity, infectious diseases (bacteria, parasites, fungus and virosis), acidification, oxygen depletion (hypoxia or anoxia), gas bubble trauma, fish overpopulation, dam operation and stress due to spawning activities (Barica, 1975;Hoyer et al., 2009;La and Cooke, 2011;Mericas and Malone, 1984;Moss et al., 2011;Muñoz et al., 1994;Nicholls et al., 1980;Schofield, 1976;Thronson and Quigg, 2008;Zhu et al., 2008). Although fish kills can occur due to direct human actions such as discharge of toxic substances, acid mine drainage and herbicides and pesticides contamination of aquatic environments (Giger, 2009;Mason, 2002;Muñoz et al., 1994;Thronson and Quigg, 2008), studies frequently identify several drivers as likely causes for the mortality of fish, including the combination of natural and human-induced stressors (Kangur et al., 2016). Moreover, the mechanisms underlying many fish kills remain unknown (La and Cooke, 2011), probably because the identification of the factors and paths leading to a fish kill event require monitoring of several parameters with a high frequency, in order to understand the ecosystem dynamics during the mortality event (Kangur et al., 2016). ...
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The toxicity of ammonia to fishes has been attributed to the un-ionized ammonia chemical species present in aqueous solution. Because the percent of total ammonia present as un-ionized ammonia (NH3) is so dependent upon pH and temperature, an exact understanding of the aqueous ammonia equilibrium is important for toxicity studies. A critical evaluation of the literature data on the ammonia–water equilibrium system has been carried out. Results of calculations of values of pKa at different temperatures and of percent of NH3 in aqueous ammonia solutions of zero salinity as a function of pH and temperature are presented.
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Respiratory CO 2 release from inland waters is a major process in the global carbon cycle, retaining more than half of the carbon flux from terrestrial sources that otherwise would reach the sea. The strongly lake type-specific balance between primary production and respiration determines whether a lake acts regionally as a net sink or source of CO 2 . This study presents two-year (2009, 2010) results of high-frequency metabolism measurements in the large and shallow polymictic eutrophic Lake Võrtsjärv (area 270 km 2 ; mean depth 2.8 m). We estimated the net ecosystem production (NEP), com-munity respiration (R) and gross primary production (GPP) from continuous measurements of oxygen, irradiance, wind and water temperature. A sinusoidal model fitted to the calculated metabolic rates showed the prevalence of net autotrophy (mean GPP:R [ 1) from early spring until August/September, whereas during the rest of the year heterotrophy (mean GPP:R \\ 1) prevailed, characterizing the lake as CO 2 neutral on an annual basis. Community respira-tion lagged behind GPP by approximately 2 weeks, which could be explained by the bulk of the phyto-plankton biomass accounted for by filamentous cya-nobacteria that are considered mostly inedible to zooplankton, and the seasonally increasing role of sediment resuspension. In the warmer year 2010, the seasonal peaks of GPP, R and NEP were synchro-nously shifted nearly 1 month earlier compared with 2009. The strong stimulating effect of temperature on both GPP and R and its negative effect on NEP revealed by the multiple regression analysis suggests increasing metabolic rates and increasing heterotro-phy in this lake type in a warmer climate.
Article
Research information on Eurasian ruffe from previously published peer-review literature, reports, and previously unavailable or unpublished research from Slovakia and central Europe is synthesized. The synthesis focuses on geographical distribution, habitat requirements, reproductive biology, early development, diet, morphology, age and growth, and karyotype analysis. In Slovakia, the Eurasian ruffe prefers lentic to lotic environment. It is benthic but does not demonstrate any substrate preferences. However, it requires clean and well-oxygenated water. Males attain sexual maturity at standard length 32 to 80 mm, females at 57 to 90 mm. Eurasian ruffe is a polycyclic species with asynchronous ripening of eggs and protracted spawning. It spawns between mid-April and mid-June at water temperatures 7.1 to 20.2°C. Absolute individual fecundity is very variable, ranging from 1,000 to 150,000 eggs. The diameter of eggs varies from 0.97 mm to 1.07 mm. Embryos attain 3.35 mm to 3.81 mm at hatching. Ruffe feed mainly on larvae of chironomids, being active throughout the year, including winter. Most individuals attain a maximum age of 6 years, exceptionally 7 or 8 years, and a maximum size of 15 cm, exceptionally 20 cm in total length. In the Danube River, the abundance of the ruffe ranges from 49 to 4,254 ind/ha in side-arms, and from 378 to 14,934 ind/ha in oxbow lakes. Ruffe is a prey species for large piscivores, such as pike and pikeperch. Fish eggs have not been found in stomachs of ruffe from the Danube. Therefore, the only impact of the species on local fish assemblages can be competition for food.
Chapter
More than 20-year monitoring of Estonian rivers reveals that the loading of nitro-gen to large shallow lakes Peipsi (3,555 km:2, 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 catch-ment 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 remain-ing 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.
Article
Winter fish kills can be intense under ice in shallow lakes, and have cascading effects on the food web and ultimately on lake water clarity. In maritime Western Europe, winters are usually mild, but occasional colder periods may also have strong effects on lake fish communities. Global warming may have disproportionate effects by delaying freezing and shortening the period of ice coverage. We studied differences in zooplankton (cladocerans, copepods, and rotifers): phytoplankton biomass, zooplankton community structure, and individual body size among 37 Danish lakes of various depths, chemical characteristics, and trophy, by comparing four winters of different severity (mean winter temperatures ranging from -1.19°C in 1996 to +2.9°C in 1995). We found that crustacean mean body sizes were significantly larger in the summer following a severely cold winter. The zooplankton communities in the summer after a cold winter had a significantly larger proportion of larger-bodied species and taxa. Phytoplankton biomass, expressed as chlorophyll-a (chl-a), was lower and zooplankton herbivory (chl-a:TP index), higher, in the summer after the severely cold winter of 1995/1996. All these effects were stronger in shallow lakes than in deep lakes. Changes in zooplankton during summer 1996, compared with other years, were likely caused by fish kills under ice during the preceding severe winter of 1995-1996. Fish kills due to under ice oxygen depletion would be expected to occur earlier and be more complete in the shorter water columns of shallow lakes. With climate change, severe winters are predicted to become less frequent and the winters to be milder and shorter. In general, this is likely to lead to higher winter survival of fish, lower zooplankton grazing of phytoplankton the following summer and more turbid waters, particularly in shallow eutrophic lakes.
Article
Massive mortality of planktivorous fish had a dramatic impact on plankton community dynamics of Lake Mendota, Wisconsin, USA. After fish mortality, the largerDaphnia pulicaria replaced the smallerDaphnia galeata mendotae, resulting in greater grazing pressure on phytoplankton. This was accompanied by a much longer spring clear-water period and lower summer phytoplankton biomass compared to years before the fish mortality. Analysis of historical data (from the mid-1970's) showed that previous fluctuations in planktivorous abundance had similar effects onDaphnia abundance and species composition, and on spring phytoplankton biomass. However, the mid-1970's fish fluctuations had no detectable effect on summer phytoplankton. Concentrations of phosphorus were much higher in the 1970's (spring P 80–135 µg 1−1) than in the 1980's (spring P 19–36 µg 1−1) and it is possible that high P concentrations may reduce trophic cascade effects on summer phytoplankton communities. This suggests that the success of biomanipulation programs may be dependent on lake nutrient status.
Article
Behavioral reactions of fishes are measurable, adaptive features which contribute to their evolutionary fitness and, hence, can serve as biomarkers of stress. For a behavior to serve as an efficacious biomarker, it should be important to an individual's survival, growth and/or reproduction, be relatively stable under control conditions, and have a "concentration-effect' relationship with the stressor. This review summarizes the influence of numerous environmental stimuli on 7 different behaviors in fishes. -from Author
Article
There is an apparent gap between the prominence of present theoretical frameworks involving ecological thresholds and regime shifts, and the paucity of efforts to conduct simple tests and quantitative inferences on the actual appearance of such phenomena in ecological data. A wide range of statistical methods and analytical techniques are now available that render these questions tractable, some of them even dating back half a century. Yet, their application has been sparse and confined within a narrow subset of cases of ecological regime shifts. Our objective is to raise awareness on the range of techniques available, and to their principles and limitations, to promote a more operational approach to the identification of ecological thresholds and regime shifts.
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