Modeling the bottom temperature variation patterns on a coastal marine ecosystem of the Southwestern Atlantic Ocean (El Rincón), with special emphasis on thermal changes affecting fish populations

Abstract

The detailed knowledge of temperature variation patterns in marine ecosystems is important to understand marine communities? dynamics. The present study modeled the historical (1980-2016) daily bottom temperatures on a shallow marine ecosystem of the Southwestern Atlantic Ocean (El Rincón), which is a reproductive ground for several commercially important fish. A detailed description of bottom temperature variation patterns was performed mainly focusing on those changes influencing fish populations. The model used in the present study is a modification from previous models applied to lakes and tidal inlets, and predicts daily water temperatures from air temperatures and radiation, considering in turn the change of thermal inertia with depth. Bottom temperature data for model calibration, and also salinity, were recorded on 666 oceanographic stations between July 1993 and December 2012. More than 95% of daily bottom temperature variation was explained by the fitted model, and predictability decreased from 98 to 82% in relation to the increase in thermal inertia with depth. High model deviations (>1.5 °C) were associated with a particular salinity range (33.6-33.9) and evidenced eventual occurrence of cold water advections more evident during the warm season. In response to the changes in thermal inertia with depth, the seasonal occurrence of two opposite horizontal thermal gradients was evident each year. These gradients involved a shoreward warming during the warm season, which promotes fish reproductive aggregations, and a shoreward cooling (warming toward deeper zones) during the cold season, which is related with adult fish dispersion or even migration. Up to more than one month of difference in the onset-end pattern of these thermal gradients was observed depending on the year. Long-term linear variation trends in temperatura changed seasonally, with higher cooling rates during spring months, which was particularly evident during October. Also, irregular 3 to 7 year thermal oscillations appeared to occur, indicating the influence 56 of one or more climate variability phenomena. These results depict the general dynamics of bottom temperature patterns in a shallow temperate marine ecosystem, with particular emphasis on their implications for understanding different aspects of fish populations. Particularly, the model and specific data provided in this study will help to further advance our knowledge of this marine ecosystem, in which fishes play an important economical role.