Salinity regulation of copepod egg production in a large microtidal estuary

Noé Espinosa,Laura M. Rodríguez-Graña,Danilo Luis Calliari,Mariano Martínez

doi:10.1590/s1679-87592019026706708

Noé Espinosa, Laura M. Rodríguez-Graña

Open Access

https://doi.org/10.1590/s1679-87592019026706708

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Abstract

ABSTRACT Salinity is a key variable for ecological processes in estuaries. Acartia tonsa is a typical estuarine copepod whose responses to salinity have been thoroughly studied in the laboratory. However, results cannot be extrapolated to the field, and formal comparisons between lab and field responses to salinity were not attempted. Here we compare lab-based with field copepod egg production rates (EPR) from the Rio de la Plata estuary (RPE), with focus on A. tonsa. Field work was conducted between 2009 and 2011 in the mixing zone of the RPE. Water temperature, salinity and chlorophyll-a varied over ample ranges (temperature: 10.54 - 24.56oC, salinity: 2.83 - 32.99, chlorophyll-a: 0.62 - 7.27 mg m-3). A. tonsa was the strongly dominant species. EPR ranged between 6.7 and 95.7 eggs female-1 day-1, and correlated to salinity, temperature (weakly), but not to chlorophyll. The relationship between A. tonsa’s EPR (EPRAT) and salinity was consistent with that obtained under laboratory conditions: a humped pattern with a maximum at intermediate salinities. However, differences were also evident, e.g., higher EPRAT was measured in the field. We speculate that discrepancies derive from nutritional differences between field and reference (laboratory) data sets. Besides salinity, food quality and quantity may be first order drivers of A. tonsa’s productivity in the RPE.

Highlights

The dilution of marine waters by freshwater runoff is characteristic of most estuaries
The goal of this paper is two-fold: i.- to contribute new information in order to characterize copepods’ egg production rates (EPR), and that by A. tonsa, the dominant copepod species in the brackish region of the Río de la Plata estuary (RPE); and ii.- to evaluate the relationship between EPRAT and salinity in situ, in order to compare it with laboratory estimates obtained under controlled conditions
This study focuses on the outer region of the RPE off Montevideo (Fig. 1)

Summary

Introduction

The dilution of marine waters by freshwater runoff is characteristic of most estuaries. Low and variable salinity imposes stressful conditions for both marine and freshwater fauna (Attrill, 2002), and low taxonomic diversity in brackish areas of estuaries generally match high biomass and biological production (Day et al, 2013). A. tonsa is well known for its tolerance to wide salinity conditions, and has been used as a model organism for investigation of vital responses to salinity. Results derived from experimental studies under laboratory conditions showed that A. tonsa tolerates extremely ample salinity ranges (Lance, 1964; Cervetto et al, 1999; CastroLongoria, 2003), rapid salinity increments (Hubareva et al, 2008), and osmotic shocks (i.e., instantaneous changes, Calliari et al, 2009a). The metabolic balance of A. tonsa was shown to be nearly stable under salinities between 2 and 32: egg production rate by A. tonsa

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