Blind estuaries during drought

The influences of a sandbar on mangrove trees

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Small blind estuaries are a common phenomenon on the coast of Nicaragua, but unfortunately not much is known on their hydrological characteristics. A blind estuary can be identified by a sandbar that temporarily closes the estuary from sea. These blind estuaries often have very diverse flora and fauna on which they play an important ecological role. Mangrove forests growing in this ecosystem often play a central position, and are valued for a wide range of functions, productivities, uses and values. Unfortunately these forests are also under threat of destruction, which is often caused by human influences. In this research human impact, as well as the impact of drought caused by an El Niño event, have been included to understand the influence of a blind estuary on the well being of the mangrove forest. The study was performed in a blind estuary at the Pacific coast of Nicaragua; located near the village El Ostional. During a fieldwork period of approximately three months, the study area was observed and measurements on predominantly water levels and salinity were conducted. The resulting information was used (1) to classify the separate morphologic, biologic, and social structures in the area, (2) identify the interactions between mangrove well being and hydrological characteristics, and finally (3) do more in-depth analyses on the salt intrusion occurring in the area. The estuary contains two main landforms; the sandbar separating estuary and sea, and the back-barrier floodplain behind it. The sandbar is formed by wave action, and can be ruptured due to a highly seasonal river discharge. The floodplain inundates when accumulation of water behind the sandbar increases. A variety of common mangroves like the Red and White mangrove, and the Buttonwood tree, can be found in the floodplain. The floodplain next to this is home to the Tea mangrove, a very rare and low salt tolerance mangrove tree. Together these mangrove trees form an interior mangrove forest. The inhabitants of El Ostional mainly use the floodplain to shortcut their route to the shore. Their main interest in the estuary is for the fishes it provides. The inhabitants on occasions like to 'help' the sandbar to open, in order to have easy fishing possibilities. Inundation in the floodplain is revealed to be extremely beneficial for the mangrove population. The crab population in the floodplain turned out to play an important ecological role, where their burrows increase permeability. This enhances infiltration into the soil, which increases soil moisture and decreases soil salinity. The blind estuary is, although stressed by drought, able to maintain relatively low salinity levels. This is caused by a steady fresh groundwater inflow. An overall outflow of estuary water occurs through the sandbar, where head differences between sea tide and estuary water level are the main forces that induce a transmission through the sandbar. The resulting relatively fresh water in the estuary permits the growth of the Tea mangrove. A mainly pressure induced muted tide is revealed in the estuary. Together with the fresh water inflow and wind influence, a circulation is established that reveals a well-mixed salt intrusion, which is caused by over-wash events. Would the sandbar be ruptured manually during drought, then the result is an increase in salinity that would be devastating for the Tea mangrove, but supply the estuary and mangroves with more water and provide the connection between sea and estuary necessary for fish reproduction. Salt intrudes the floodplain during inundation, when sea water evaporates and salt is left behind, which accumulates in the soil. The more infrequent the inundation, the higher the salinity in the soil. This only occurs in the floodplain. Salt intrusion in the subsurface of the Ostional area is due to pressure differences between fresh water in the aquifer and sea tide. This induces a mixing zone of which the interface is still near the shore; most of the wells in Ostional maintain potable water salinities. In order to protect the mangrove forest, it is thus important to protect the complete ecosystem, in this case especially the crab population. It further is important to ensure the maintenance of a groundwater inflow, by protecting the upstream forests that enhance infiltration to the groundwater system. Whether the manual rupture of the sandbar during drought is considered problematic depends on the the value given to the Tea mangrove, the other mangrove population, or the nursery function the estuary provides. Further research and monitoring for the Ostional area is necessary to confirm the formulated hypotheses, and in order to keep track of salt intrusion in the estuary and subsurface.