Academic journal article Estonian Journal of Ecology

Cool Winters versus Mild Winters: Effects on Spring Plankton in Lake Peipsi/Kulm Talv versus Soe Talv: Nende Moju Peipsi Jarve Planktonile Ja Vee Biogeenisisaldusele

Academic journal article Estonian Journal of Ecology

Cool Winters versus Mild Winters: Effects on Spring Plankton in Lake Peipsi/Kulm Talv versus Soe Talv: Nende Moju Peipsi Jarve Planktonile Ja Vee Biogeenisisaldusele

Article excerpt

INTRODUCTION

Mean global air temperature has increased steadily in recent decades (Weyhenmeyer et al., 2008). Global average surface temperature of the Earth rose approximately by 0.6[degrees]C in the 20th century. Regional climatic models for Sweden simulate increasing air temperatures over the following decades, especially temperatures in winter (Rummukainen et al., 2001). The warming trend in recent decades has led to an earlier ice break-up in lakes and to increased water temperature in many northern temperate lakes, particularly in spring (Gerten & Adrian, 2000; Weyhenmeyer, 2001; Noges, 2004; Adrian et al., 2006). Magnuson et al. (2000) reported a 6.3 days earlier ice break-up in lakes over the northern hemisphere in the past 100 years in response to the 1.2[degrees]C air temperature increase. The ice cover on lakes is not only a robust indicator of climate change and variation but it represents also a limnological response to climate change, which influences the ecology of the aquatic system: several studies showed that the ice cover has an impact on underwater light conditions (Lepparanta et al., 2003) and nutrient recycling (Livingstone, 1993), as well as on the production and biodiversity of phytoplankton (Phillips & Fawley, 2002; Weyhenmeyer et al., 2008), and on the dynamics of zooplankton (Talling, 2003; Richardson, 2008; Blank et al., 2009). Other studies (Noges, 2004; Wagner & Benndorf, 2007; Blank et al., 2009) demonstrated that biological systems can exhibit responses to subtle climatic signals and could be sensitive indicators of climatic indices. Temperature-driven changes in interactions between populations are crucial to estimation of the impact of global warming on aquatic food webs (Wagner & Benndorf, 2007). Climate warming is known to affect most winter and spring conditions (Sorvari et al., 2002; Weyhenmeyer et al., 2005). It is possible that winter conditions determine the 'inoculum' of species in spring and affect thus the succession pattern in spring and, possibly, also over the entire year (Adrian et al., 1999). The spring development of phyto- and zooplankton depends differently on physical conditions in early spring: phytoplankton depends on light (and on stratification in deep waters), while zooplankton depends on the availability of food and temperature (Sommer et al., 2007).

In the case of Lake Peipsi it is worth stressing that shallow freshwater ecosystems are particularly susceptible to externally imposed change, and are therefore likely to be the ecosystems facing considerable perturbation under scenarios of climate warming (McKee et al., 2002). According to Kohler & Hoeg (2000), shallow lakes are more strongly influenced by stochastic meteorological events than deep lakes. In Lake Peipsi, the warm winters of 2007 and, particularly, of 2008 offer a possibility of comparing some spring parameters of water chemistry and plankton with those of the two previous moderately cold winters of 2005 and 2006. We presume that significant differences exist between these periods in the water nutrient content and plankton dynamics immediately after ice break-up and possibly also in the following months.

STUDY SITE

Lake Peipsi sensu lato (s.l.) is a large (3555 [km.sup.2]) and shallow (mean depth 7.1 m) mainly unstratified lowland water body. Located on the Estonian-Russian border, Lake Peipsi is the largest international lake in Europe. Its water volume is 25 [km.sup.3] at the long-term mean water level (30 m above sea level) and the mean residence time of water is about two years. The entire catchment area of 47 800 [km.sup.2] includes Estonian (34%), Russian (58%), and Latvian (8%) territories. The catchment area is largely located in an agricultural region. The largest inflows are the Velikaya River in Russia and the Emajogi River in Estonia. The Narva River, the only outflow of Lake Peipsi, falls into the Gulf of Finland. In the 1960s the lake was covered by ice for up to 6 months, but in recent decades the period of ice-cover has shortened to 3-4 months. …

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