Estuaries are a significant source of nutrients to the marine environment. The magnitude of this source is a function of the nutrient load reaching the estuary and its removal (attenuation) within the estuary. Most estuarine research is conducted in large estuaries, which do not reflect the processes in small estuaries in urban and semi-arid regions, where flood water is a substantial portion of the annual discharge and the estuarine baseflow is often low and dominated by wastewater. To improve the understanding of nutrient attenuation and load into the Mediterranean, we conducted high-resolution nutrient sampling in the eutrophic Alexander micro-estuary as a test case. We sampled once per month during baseflows (2014-2019) and hourly during floods (2016-2018). The concentrations of inorganic nutrients (phosphorous and nitrogen) were extremely high during baseflows. Dissolved ammonium and particulate phosphorous were the only nutrients retained in the estuary (by 55% and 30%, respectively).
Floods were rare, occurring ~4% of the time, but contributed 62% of the annual water discharge of the Alexander micro-estuary (14.7 ± 3.8 ×10⁶ m³/y). The concentration of all dissolved nutrients decreased during floods but remained higher than expected (DIN 584 ± 50 µmol/L, phosphate 21 ± 2 µmol/L), accounting for 42% and 55% of the overall annual DIN (123.5 ± 44.9 ton/y) and P (6.7 ± 1.9 ton/y) loads to the sea, respectively. The N:P ratios were 16 during baseflow and 34 during flood events.
Previously, regional nutrient loads were calculated by multiplying baseflow-measured concentrations by the total water volume of baseflow and floods combined. Our calculations, based on high-resolution spatio-temporal sampling, revealed lower annual loads of phosphorous and nitrogen to the sea, at 56% and 89% of previous estimates, respectively — a considerable difference for an oligotrophic system such as the eastern Mediterranean.
Citation: Suari, Y., Topaz, T., Bassa, O., Gilboa, M., Sedaka, H., Sade, T., Chefetz, B., & Yahel, G. (2024). Nutrient concentration, loads and retention in a semiarid micro-estuary: The relative contribution of baseflow and flood events. Science of The Total Environment, 931, 172805. https://doi.org/10.1016/j.scitotenv.2024.172805