Hypoxia in deep waters of moderately eutrophic marine lakes, Island of Mljet, eastern Adriatic Sea

Abstract

In this study, we explored the impact of eutrophication and stratification on hypoxia in deep waters of moderately warm Croatian marine lakes. Although the Mljet Lakes (MLs) are predominantly oligotrophic, mesotrophic conditions are present at depths below 20 m in the Small Lake (SL) and below 30 m in the Big Lake (BL), along with higher apparent oxygen utilization (AOU). Hypoxia at depths ≥ 25 m in SL and and ≥ 40 m in BL was observed between October 2009 and January 2010, and in SL in summer (July and September 2010). Significant differences (p < 0.05) in several physical, biological and chemical parameters were detected between the lakes, while AOU, derived oxygen utilization rate (OUR) and organic carbon remineralization rate (OCRR) were not significantly different (p >0.05) between the lakes. An intense and persistent pycnocline throughout the year, comparatively high water temperature, extended water renewal time and summer phytoplankton bloom were identified as physical and biological parameters which might have significantly contributed to increased frequency of hypoxic events in a shallow SL. Significantly (p < 0.05) higher ammonium concentration in SL, especially in its deep water, seems to be a long-term chemical feature related to the poor ventilation and higher sediment oxygen demand. At the current level of eutrophication and the present climate change trends, the MLs and similar systems may experience more persistent and intense stratification, which could further prevent mixing between upper and deep waters, likely leading to increasing duration of hypoxia and its negative impacts on the biodiversity of benthic communities.