Circulation and distribution of suspended mesozooplankton carcasses in a mid-latitude estuary

Abstract

The coast of southern Chile, located between 37° and 41°S, has many estuaries, which, together with their wetlands, still comprise a region with high biological richness and a high ecosystem value. The Valdivia River Estuary (VRE) stands out due to its relative size and commercial importance. This study analyses time series of environmental forcings and idealized model output to identify estuarine regimes and the subtidal circulation, and its influence on the distribution of Suspended Meso-Zooplankton Carcasses (SMZC), which may affect the local labile organic carbon input. Field campaigns were conducted in 2014 and 2015, and time series of metocean forcings span from 2012 to 2018. The characteristic regimes of the estuary that control the seasonal suspended particulate matter distribution were identified as Normal, Extreme, Exceptional Rainy, and Exceptional Dry. When mapped on the Geyer & MacCready circulation-stratification diagram, they coincide with partially-mixed, strongly-stratified, salt-wedge, and well-mixed water column structures, respectively. The idealized numerical model allows determining the subtidal circulation patterns induced by the along-channel density gradient, river discharge, and wind. The equilibrium SMZC distribution within the VRE during Normal and Exceptional Dry regimes was determined from the modeled circulation patterns. Model results indicated that, during the austral autumn (April 2014; Normal regime), the SMZC distribution shows a maximum concentration as low as 128 individuals per m3 located near the head of the estuary. This suggests low residence times of suspended carcasses within the estuary. During the austral summer (January 2015; Exceptional Dry regime), the SMZC distribution exhibits its maximum value near the mouth, exceeding 15,000 ind. m?3. These results suggest a massive carcass-derived carbon export towards the estuary mouth during dry seasons. Remarkably, the cross-section where maximum SMZC was predicted during this regime matches the area where large beds of Tagelus dombeii have been reported. Overall, this study highlights the importance of circulation-stratification relationships on the control of SMZC trapping mechanisms and on the identification of hotspots of benthic biomass in mid-latitude estuaries. Other estuaries with similar relationships and regimes might experience similar shifts in suspended meso-zooplankton distribution patterns in response to seasonal changes in forcings.