Why Our Water Turns Green

The process of eutrophication in Lake Dunlap has been greatly accelerated by releases from the north and south Kuehler NBU waste water treatment plants (WWTP). Eutrophication is the process where surface waters such as lakes, rivers, and reservoirs become more productive from increases in the nutrients supplied to those waters. This increase in productivity is usually reflected by an increase in plant biomass (more plants, and in the case of Lake Dunlap more of the microscopic algae that we refer to as phytoplankton- those algae that are in suspension in the water and turn the water a pea green when present in high enough amounts). While the idea of something becoming more productive sounds positive, eutrophication is usually a process which degrades the water quality in a number of important aspects, including: 1) the increased phytoplankton reduce the clarity of the water, an important aesthetic value of a river or lake; 2) often the increase in algae and/or large macroscopic plants leads to a depletion of oxygen in the water when this plant material sinks to the bottom of the lake and decomposes- this can sometimes cause fish kills or impact the fish and other organisms found there; 3) the phytoplankton commonly associated with eutrophic waters is often referred to as "nuisance algae"- these are algae which tend to look bad and may commonly release chemicals that cause taste and odor problems in the water, and under extreme conditions may be toxic to cattle and can be associated with diarrhea or allergies in humans. Eutrophication is usually caused by the increase in the nutrients phosphorus and/or nitrogen (adding these to water has a similar effect as when someone fertilizes their lawn), which are both in high concentration in the effluent from the New Braunfels WWTPs. Nitrogen release from the WWTPs is not particularly harmful because background concentrations of nitrogen entering the reservoir, mostly from the Comal River, are moderately high already. On the other hand, phosphorus concentrations entering he reservoir from the upstream Guadalupe River are fairly low. Concentrations of phosphorus increase dramatically in Lake Dunlap just downstream of the creek that these two WWTPs empty into. Research carried out in the summer of 2001 show that phytoplankton is greatly increased downstream towards the dam in Lake Dunlap, with the resultant decrease in water clarity (on a typical summer day the water is a lovely aquagreen color down near I-35 and you can see 6 to 10 feet down into the water, while near the dam the water tends to be pea green and water clarity is less than 3 feet). Research from this summer also shows that the inflowing river water is phosphorus-limited, and therefore the addition of phosphorus to this water stimulates the growth of algae.