Coupling between Hydrodynamics and Chlorophyll a and Bacteria in a Temperate Estuary: A Box Model Approach

Élia Fernandes,Adriano Bordalo,Catarina Teixeira

doi:10.3390/w11030588

Élia Fernandes, Adriano Bordalo + Show 1 more

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https://doi.org/10.3390/w11030588

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Abstract

The spatial patterns of chlorophyll a and bacteria were assessed in a temperate Atlantic tidal estuary during seasonal surveys, as well as in consecutive summer spring and neap tides. A box model approach was used to better understand spatial and temporal dynamics of these key estuarine descriptors. The Lima estuary (NW Portugal) was divided into boxes controlled by salinity and freshwater discharge and balance equations were derived for each variable, enabling the calculation of horizontal and vertical fluxes of plankton and, therefore, production or consumption rates. Chlorophyll a tended to burst within the oligohaline zone, whereas higher counts of bacteria were found in the mesohaline stretch. Whenever the water column was stratified, similar tide-independent trends were found for chlorophyll a and bacterial fluxes, with net growth in the upper less saline boxes, and consumption beneath the halocline. In the non-stratified upper estuary, other controls emerged for chlorophyll a and bacteria, such as nitrogen and carbon inputs, respectively. The presented results show that, while tidal hydrodynamics influenced plankton variability, production/consumption rates resulted from the interaction of additional factors, namely estuarine geomorphological characteristics and nutrient inputs. In complex estuarine systems, the rather simple box model approach remains a useful tool in the task of understanding the coupling between hydrodynamics and the behavior of plankton, emerging as a contribution toward the management of estuarine systems.

Highlights

Estuaries all over the world are subject to increased pressure due the alterations in the land uses and increase of the population [1]
The maximum concentrations of chlorophyll a were found in the upper oligohaline area regardless of the tide for all seasons, except during the winter, when the peak moved to the downstream polyhaline area
Freshwater appeared to be the main source of bacteria during neap tides, with the exception of fall, whereas coastal waters have assumed this role during the summer spring tide and for the fall neap tide

Summary

Introduction

Estuaries all over the world are subject to increased pressure due the alterations in the land uses and increase of the population [1]. These alterations result in an accretion of inorganic and organic compounds, microbial loads, and changes in the discharge patterns of freshwater in the estuarine and marine coastal environments, resulting in complex ecosystem behaviors [2]. Estuaries are influenced by both marine and riverine characteristics, and their features depend of tidal pulses, freshwater flow, hydrodynamic, and autochthonous biological processes [3]. Mesotidal temperate estuaries are characterized to have semi-diurnal tidal cycles in the range of 2–4 m. The complexity of estuarine circulation system may be described through a combination of several components, such as tidal forcing, depth and width, freshwater flow, as well as meteorological

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