Impacts of a buoyant strait outflow on the plankton production characteristics of an adjacent semi-enclosed basin: A case study of the Marmara Sea

Abstract

Abstract As documented by observational studies, the buoyant outflows emanating from straits and rivers with large amounts of nutrients and biogenic materials cause marked changes in the biochemical characteristics of the adjacent receiving water bodies. Here, using a three-dimensional biophysical model of the Marmara Sea-Bosphorus Strait two-layer exchange flow system and configuring it for the winter phytoplankton blooming period with limited top-down control, we show that complex buoyancy-induced basin-scale circulation is driven by the buoyant jet emanating from a strait, which sustains enhanced production even in the absence of any nutrient flux from its upstream source region and lateral point sources around the sea. In the supercritical flow regime downstream of the strait exit, strong upward motion introduced by the hydraulically controlled outflow dynamics injects subsurface nutrients into the upper layer. Those accumulated within the adjacent anticyclonic bulge to the right of the outflow plume then support relatively high phytoplankton production, whereas strong currents limit phytoplankton production along the main jet axis. Furthermore, topographically controlled anticyclonic circulation within the lower layer around the deep northern basin induces upwelling due to the divergence of cross-isobath, uphill flow and causes the nutrient enrichment of the upper layer. This cumulative response, together with the additional contribution of nutrient recycling and the horizontal distribution of nutrients and biota by the mesoscale-dominated circulation system, maintain a highly productive system within the sea, which is consistent with previous observations.