Long-Term (2002-2021) Trend in Nutrient-Related Pollution at Small Stratified Inland Estuaries, the Kishon SE Mediterranean Case

Nutrient pollution may negatively affect the water quality and ecological status of rivers and estuaries worldwide, specifically in stratified and small inland estuaries. We present a long-term, two-decade data set of dissolved inorganic nutrient concentrations, chlorophyll-a (chl-a), dissolved oxygen (DO), and potentially toxic algal cell concentrations at the Kishon River estuary (Israel) as a case study for assessing nutrient ecological thresholds in such type of estuaries, prevalent along the Mediterranean coast of Israel. In-situ measurements and water samples were collected at 3 permanent stations at the lower part of the estuary every March and October/November in 40 campaigns over the years 2002 to 2021. In spite of an improvement in nutrient loads and concentrations as recorded over the last 2 decades, the nutrient and chl-a levels at the Kishon estuary surface water represent mostly a 'bad' or 'moderate' ecological state, considering the recommended thresholds discussed in this study. It is suggested to develop a combined suite of nutrient and biological variables for assessing Good Environmental Status (GES), considering the relatively high residence time of such small, low-flow estuarine water bodies.

Automated summary

Nutrient pollution has a negative impact on the water quality and ecological status of rivers and estuaries worldwide, particularly in stratified and small inland estuaries. This study presents a two-decade data set from the Kishon River estuary in Israel, showing the nutrient ecological thresholds in such estuaries along the Mediterranean coast. Despite improvements in nutrient loads and concentrations over the past 20 years, the Kishon estuary still has a 'bad' or 'moderate' ecological state based on recommended thresholds.