Spatial-temporal distribution of salinity and temperature in the Oued Loukkos estuary, Morocco: using vertical salinity gradient for estuary classification

Abstract

Background: Several different classifications to characterize estuarine systems have been proposed. In this present paper, one of the most important estuaries in North Africa, the Oued Loukkos (Morocco), forms a case-study for proposing a systematic classification of this particular tidal estuary according to the vertical salinity gradient.

Findings: This study, conducted using a CTD, shows that the spatial-temporal distribution of salinity depends on the stage of the tide and the upstream distance from the mouth of the river. In this case, it is also evident that the morphology of the bottom was capable of impacting the distribution of salinity by locally changing the water circulation.

Conclusions: Based on the vertical salinity gradient measurement, the Oued Loukkos represents an estuarine environment with one section near its mouth that can be characterized as a mixed mesotidal estuary and another section upstream which can be characterized as a stratified mesotidal estuary. Between, there is an intermediate zone with a low vertical gradient of salinity, classified as a partially mixed mesotidal estuary. When the effect of terrestrial inputs is low compared to marine inputs, the river bed topography plays a role in the stratification of salinity by either disrupting the vertical stratification of the water or by changing the lateral distribution of salinity. The proposed classification deepens our hydrological knowledge and provides descriptive labels to the Oued Loukkos estuary. It provides a valid starting point for predicting the environmental impact of future recreational, agricultural and commercial activities on the estuary.

Keywords: Characteristics of salinity; Estuary classification; Morocco; Oued Loukkos; Stage of tide.

Figure 1

The Oued Loukkos, Larache, Morocco. Gray bars with arrows indicate the boundaries of the hydrographic survey.

Figure 2

Vertical profiles of temperature and salinity recorded on 19 October during a CTD cast taken near the predicted time of a mid-flood tide at the mouth of the Oued Loukkos.

Figure 3

Surface salinity variation in the Oued Loukkos. a) Surface salinity variation as a function of distance from the ocean; b) Surface salinity variation as a function of stage of tide.

Figure 4

Salinity stratification recorded during a low stage of tide in the far inland reach of the survey.

Figure 5

Conceptual influence of the pit at White Cliff on the Oued Loukkos’ hydrodynamic flow during a flood tide.

Figure 6

Subdivisions of the Oued Loukkos based on the vertical gradient of salinity, with the typical profiles of salinity for each zone. Gray bars with arrows indicate the boundaries of the hydrographic survey.