26.10.2021
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Experimental sand bar breaches as a tool for increasing dissolved oxygen concentration in the Alexander estuary bottom water.

The aim of this experiment was to test whether artificial sand bar breaches increase the dissolved oxygen concentration of the bottom water of the Alexander estuary. The idea was to increase sea-river connectivity and let more oxygenated sea water flow into the estuary.

introduction

After the first year of research and monitoring of the estuary we identified that the dissolved oxygen concentration of the estuary bottom water was very low and that anoxic conditions prevailed for approximately six months of the year, mostly during summer and autumn (see Figures 1 and 2). Oxygen breathing organisms such as fish, crabs, and clams will have a hard time surviving in this environment.


Figure 1. Dissolved oxygen concentration as sampled at station A4 (Michmoret Bridge), January 2014 to March 2015. X axis – time of sampling; Y axis – water depth. The color gradient on the right represents dissolved oxygen concentration. Black dots represent sampling points.

 

Freshwater rich with nutrients and organic matter enter the estuary from the river (estuary head, upstream) while seawater poor with nutrients and organic matter penetrate from the estuary mouth (downstream). The organic load that comes from upstream leads to high oxygen demand and to reduction in dissolved oxygen concentration in the estuarine water.

We also noticed that periodical penetration of sea water into the estuary "ventilate" the bottom water (Figure 2). In the Alexander estuary this penetration is usually limited due to a sand bar built across the river mouth.

Figure 2. Salinity (right Y axis, red) and dissolved oxygen concentrations (left Y axis, blue) in the bottom water of the Alexander estuary 500 meters upstream from the coastline (Michmoret Bridge). Data collection was based on logging sensors that were deployed in the water, with sampling done once every five minute. Black points represent events in which a significant rise in salinity (>0.1 PSU per hour) and in dissolved oxygen (>1 µMol/L per hour) was recorded.

The experiment

To test the efficiency of artificial sand bar breaches as a tool to increase levels of dissolved oxygen, we conducted a 36- hour experiment in which levels of oxygen concentration and salinity were measured continuously. Ten hours after the experiment began the Alexander sand bar was artificially breached using a power shovel, thus allowing oxidized seawater into the estuary. The effect of this water exchange on the levels of salinity and dissolved oxygen was measured for the following 26 hours. We also deployed aditional sensors about 500m and 2km upstream from the coastline (Michmoret bridge) to record longer distance changes in levels of dissolved oxygen, salinity, light and water.

The findings

Immediately after the sill's breach, we found an increase in the levels of oxygen and salinity in the bottom water up to a distance of ~2 km upstream from the sand bar (see Figure 3). The saltier and totaly oxigenated water gradually penetrated the estuary. High biological oxygen demand (BOD) in the estuary water, together with the reconstruction of the sand bar (blocking sea-estuary connectivity) led to a fast decline in dissolved oxygen during and after the experiment. Two days after breaching the dissolved oxygen concentration returned to anoxic level.

Figure 3. Salinity (top) and dissolved oxygen (bottom) 500 meters upstream from the coastline (Michmoret Bridge) after the sand bar was breached. Data collection relied on data loggers deployed in the water under the bridge. The vertical red line denotes when the sand bar was breached.

Conclusions:

  1. Sporadic sand bar breaches are not a sufficient solution for the low dissolved oxygen problem.
  2. To increase oxygen concentrations in the estuary, we must reduce oxygen demand. Oxygen demand can be decreased by lowering the organic and nutrient loads from anthropogenic sources.
  3. The effect of repeated artificial sand bar breaches on dissolved oxygen concentrations in the estuary should be investigated together with the effect of the increase in water salinity.

 

This research was conducted in collaboration with the Israeli Nature Reserve Authority.

We wish to thank the students from the Israeli School of Marine Sciences, Ruppin Academic Center who participated in this project: Maayan Neder, Zur Meiri, Roni Zafriri, and Tom Reich.

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Experimental sand bar breaches as a tool for increasing dissolved oxygen concentration in the Alexander estuary bottom water.
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