Deliverable 5.2

Abstract: Lake Beyşehir modelling: final report

This study included a model-based analysis on the effects of near-future climate and land-use changes (represented by years 2031-2060) on the hydrology and trophic state of Lake Beyşehir, the largest freshwater lake in Turkey. A sampling program was implemented and completed in the period April 2010 – April 2012, in which tributary nutrient loads and in-lake nutrient and plankton data were collected at monthly frequencies. The data was collected to understand the dynamics of the Lake Beyşehir ecosystem, and for validating hydrological and ecological models.

The collected data revealed that the lake currently is of low trophic status (oligo- to mesotrophic), and a sensitivity analysis of a hydrological model (SWAT) for the Lake Beyşehir catchment further suggested that snow fall and snow melt dynamics are key factors influencing the seasonality of tributary inflows and nutrient loads to the lake. Scenario output for precipitation and air temperatures from three regional climate models (ECHAM5-KNMI, HadRM3P-HadCM3Q0 and SMHIRCA-BMC), based on the IPCC A1B emission scenario, were used as input to the hydrological model. Projections by the hydrological model on future nutrient loads combined with air temperatures from the climate models were subsequently provided to three individual lake ecosystem models (DYRESM-CAEDYM, PROTECH and PCLake) to project the effect of future scenarios on phytoplankton biomass and dynamics in Lake Beyşehir. For the future scenarios, all three climate models project a considerable decrease in precipitation for the near-future period (up to 20% reduction on an annual basis relative to the baseline period of 1981-2010), and according to the hydrological model the total water supply to Lake Beyşehir may be reduced up to 30%. Consequently, nitrogen and phosphorus loads to the lake are projected to decrease by as much as 36 and 53%, respectively, for the near-future scenarios. Despite the dramatic decrease in nutrient loads, the phytoplankton biomass of Lake Beyşehir will, according to projections by the three lake ecosystem models, only change slightly (reductions up to 10%), and the yield of phytoplankton biomass to nutrient load is thus projected to increase with the warming of the near-future scenarios. When combining the future climate projections with a series of potential land-use changes (e.g., intensification of agriculture by increasing fertilizer use by 20%) only minor additional change on phytoplankton biomass of Lake Beyşehir is projected, relative to the effects of climate change alone. Given the dramatic hydrological changes projected in the near-future scenario, it is recommended that additional research on the hydrology focus on longer-term scenarios (e.g. 100 years from now and beyond) and thus aim to elucidate at which point in the future that Lake Beyşehir may no longer exist, as in recent years has already been the faith of several Turkish lakes.

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