Nutrient Budgets, Dynamics and Storm Effects in a Eutrophic, Stratified Baltic Lake

Abstract

Between 1989 and 1998 the small eutrophic stratified Lake Belau was investigated intensively and multidisciplinarily. This article is a short, comprehensive summary and re-evaluation of the hydrochemistry of the lake, with focus on nitrogen and phosphorus. In several aspects the lake can be regarded as a typical example of the glacial north German lakes. The 1960's and 1970's are characterised by heavy nutrient inputs and fast eutrophication. During the last two decades the external nutrient load, especially the phosphorus load into Lake Belau was significantly reduced. But phosphorus-rich sediments and large areas with summerly anoxic sediment surface conditions cause intensive release of phosphorus from older deeper sediment layers. Annual budgets reveal that despite an average sediment accumulation of 3 mm a -1 the lake has lost its function as net phosphorus sink and it is very likely that internal eutrophication by the sediments will keep the lake in its eutrophic state during the next decades. Despite that, monthly budgets of five vertical layers show that the main phosphorus supplier for the phosphorus depleted epilimnion during summer is the creek Alte Schwentine. The annual nitrogen budget indicates groundwater and interflow water as well as atmospheric input as additional important nitrogen sources. 36 % (98 µmol m -2 h -1 N) of all nitrogen input is lost to atmosphere mainly due to denitrification. The example of a heavy storm shows that about 10 % of the annual nitrogen loss to the atmosphere can take place during a single day and in form of ammonia. The storm further made obvious that these unpredictable events can have strong impact on nutrient cycling and ecology in Lake Belau and the lake can become an unexpected nutrient source for downstream systems. Nährstoffbilanz, -dynamik und Sturmeinwirkung in einem eutrophen, geschichteten norddeutschen SeeDer Belauer See war zwischen 1989 und 1998 Gegenstand umfangreicher, interdisziplinä-rer Untersuchungen im Rahmen des Projektes "Ökosystemforschung im Bereich der Bornhöveder Seenkette". Wesentliche Ergebnisse zum Nährstoffhaushalt dieses typischen, glazial geprägten Sees in Ostholstein werden in diesem Artikel kurz zusammengefasst. Nach rascher Eutrophierung in 1960er und 70er Jahren wurden die Nährstoffeinträge seit 1980 deutlich reduziert. Durch anoxische Verhältnisse im sommerlichen Hypolimnion findet eine starke interne Eutrophierung durch Freisetzung aus den alten Sedimenten statt, die den See noch Jahrzehnte in einem eutrophen Zustand halten kann. Darüber hinaus wird die Nährstoffdynamik im See diskutiert sowie die Bedeutung von Einzelereignissen, wie z. B. die eines außergewöhnlichen Sturms im Sommer des Jahres 1989.

Phosphorus retention as a function of external loading, hydraulic turnover time, area and relative depth in 54 lakes and reservoirs

“…Z R is a good characteristic of the stability and thermal or chemical stratification of water masses, and could also be applied as a predictor of oxygen conditions and redox status in lakes (Håkanson, 2004). Mass balance calculations show that much of the phosphorus load to a lake is retained largely through sedimentation, and the role of sedimentation in phosphorus retention is usually greater than that of assimilation by biota (de Anda et al, 2001;Schernewski, 2003;Kelderman et al, 2005). Håkanson (2005) stressed that gross sedimentation is a function of relative depth as well as total lake phosphorus concentration, so lake morphometry is a very important factor in the gross sedimentation value.…”

The impact of agricultural land use on stream chemistry and inputs to an inland reservoir: case of the Sauce Grande River, Argentina

“…Sediments in the river bed can play an important role in buffering concentrations of SRP under low-flow conditions (House & Denison 2002 , ) and when the stream water enters the reservoir, the P in the sediments begins to re-equilibrate with the SRP already dissolved in the reservoir (Correll 1998 , ). At this critical time, once the sediment-associated P from diffuse sources has settled to the bottom, internal processes within the reservoir may significantly influence the concentration and availability of P and hence algal growth (Correll, 1998;Schernewski, 2003).…”

Phosphorus transfer from agricultural areas and its impact on the eutrophication of lakes—two long-term integrated studies from Norway

“…This model has been adapted to Norwegian conditions by Berge (1987), who found that a non-logarithmic approach for the theoretical residence time gave a better estimate for shallow Norwegian lakes than the Vollenweider-model. In addition to external contributions of TP, internal processes within a lake may significantly influence the concentration and availability of P and hence algal growth (Kauppi et al, 1993;Graneli, 1999;Schernewski, 2003). Furthermore, not only the dissolved P contributes to eutrophication, since when the stream water enters a lake, the particulate P (PP) in the SS of the stream water begins to re-equilibrate with the standing water's dissolved P (Correll, 1998).…”