From Sea to Shining Estuary
I was going to start out with a marine-inspired rendition of America the Beautiful but I think the pun is already clear. So I will begin instead with this map of Port Aransas, courtesy of Google maps.
I have marked on this map in orange to show the starting point of the trip I took on the Katy- the University of Texas Marine Science Institute’s research vessel. If you are interested in learning more about the trip and the research vessel, check out my post: On the Katy
Also for your reference, marked in bright blue, is the direction of the open ocean. The dark greenish blue area shown on the map is part of the estuary ecosystem.
What makes the area shown so different from the ocean that it gets its own name? Estuaries are a unique middle ground between river and marine ecosystems. As defined clearly by NOAA, an estuary is where fresh and saltwater mix. Estuaries are home to a wide range of interesting species, due to the low energy of the system and the uniquely brackish water. In Port Aransas, the estuary ecosystem is formed by two major rivers—the Aransas River and the Mission River—hitting the ocean. We explored differences between this ecosystem and the more typical marine ecosystem while aboard the research vessel Katy.
Case Study #1: WATER
An estuary is composed of many different zones, with the end of the river called the head of the estuary, and the end just before the ocean called the mouth. The conditions between these two regions are diverse, with a significantgradient of salinity levels and variable water speeds. Estuary areas are often in protected bays or inlets, and as such water speeds are relatively slow.
Case Study #2: MUD
Similar to the water salinity gradient, there is also a gradient in the amount of ground cover over the range of an estuary system. At the estuary head, larger pebbles and stone from the riverbed tend to make up most of the ground cover. As this sediment gets pushed along with the current and weathered, it gradually becomes smaller and smaller particles increasing the turbidity of the water (amount of suspended solids) and forming the mud that we pulled up during our mud grab on the R/V Katy. This high turbidity and loose muddy bottom limits organisms that need a sturdy base to grow on, but makes the perfect home for the worms, crabs and brittle stars we found in our sample. As you move towards the mouth of the estuary, you approach the sandy or rocky bottom typical of an ocean floor.
Case Study #3: FISH
The species that live in an estuary are specifically adapted to the variable salinity waters and the high volume of suspended solids. While there are not as many species adapted for this as opposed to straight freshwater or marine systems, estuaries do play an important role in biodiversity—they are home to a lot of young species. There are many species that lay eggs in freshwater but live in saltwater, and vice versa. Estuaries serve as an important transition zone between these two parts of an organism’s life. Many of the fish species we pulled up in our trawlin the estuary area were smaller and younger fish, including a baby squid. The instructors said it is also common to pull up baby crabs or other young marine species.
Estuaries are often protected areas to ensure the safety of the unique flora and fauna and to monitor water quality. The Mission-Aransas National Estuarine Research Reserve is a great place to learn more about this unique ecosystem and how it is being protected.