The Science: Freshwater Flow

This post is the first of many that will outline the science that helps to explain why our coastal habitats and fisheries are in such dire straights. Let’s begin with freshwater flow. Changes in the amount, timing, and location of freshwater flows into estuaries cause significant ecological changes. And we’re just talking about water, not about pollution in the water. Changes in freshwater flows – even the cleanest water – causes damage. This is because large changes in the salinity (salt content) of the water impacts the organisms that live in the water. Freshwater has a salinity of zero. Ocean salinity is typically around 35. Most organisms are adapted to live in a specific salinity range. For example, if large mouth bass are placed into water that is too salty, they will die. Similarly, put a tuna in a freshwater lake and it will die. These are obvious extremes, but the same principle is at play for other organisms.
Take oysters for example. They do best in middle-salinity water. They grow fine in salt water, but so do their predators and diseases. If they are exposed to full freshwater for more than a couple of weeks, they’ll die. So the sweet spot for oysters is the idle salinity that we find in healthy estuaries.
Sea grasses are similar to oysters when it comes to salinity. Turtle Grass, for example does great in full ocean salinity, and also can handle middle salinity. But expose it to full freshwater for more than a couple of weeks and it dies.
Now think about all of the small organisms (many of which are prey for game fish) that live in oyster reefs and sea grass beds. They also have specific salinities that they can handle. So drastic changes in salinity, like we get from large freshwater discharges, kill them too – they are typically to small to make a run for it or are attached or live in the bottom.
What has happened in Florida is that the natural freshwater plumbing has been changed. Ditches and canals and dams and pumps have been constructed to move freshwater from its natural paths. This has resulted in drastic changes in
how that water flows into estuaries. Way back when this was started, they didn’t know any better. Now we do. There are major consequences to those changes in Florida’s natural plumbing, and most of those consequences are in the estuaries.
By changing where freshwater flows into an estuary, where fresh and salt water mix changes, which greatly changes where organisms can live. For example, areas that used to have sea grass might now be barren bottom because there is too much freshwater.
By changing the amount of freshwater entering the estuary, the entire ecosystem can change. For example, stop all of the freshwater coming into an estuary, and the ecosystem becomes marine, with only marine organisms. Or dump too much freshwater into an estuary, and it could become more of a freshwater system with only freshwater organisms. Worse yet, alternate periods of zero freshwater with periods of full freshwater. The no-freshwater periods kill all of the freshwater organisms, the full freshwater periods kill all of the marine organisms. This is what has been happening in areas impacted by the Lake Okeechobee discharges, and happens on a smaller scale in portions of the Indian River
Lagoon.
In a place like Florida, where we have distinct wet and dry seasons, the life cycles of a lot of organisms revolve around these seasons. Some are better adapted to dry seasons, others to wet seasons. So when the timing of freshwater entering the estuaries changes away from the normal wet and dry cycles, this can greatly impacts things like growth, survival, and reproduction of the fish we fish for as well as the prey that these game fish eat.

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2 thoughts on “The Science: Freshwater Flow

  1. Ryan Troy Reply

    Thanks for making the effort to post such meaningful articles! I am happy to send more readers your way. Keep up the good work !
    -Ryan

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