On June 28th and 29th, 2007, thousands of dead fish appeared on the surface of Weems Creek, a Severn tributary at the northern edge of the city of Annapolis. In response to residents reports, the Maryland Department of the Environment sent a team to investigate this event. They estimated the number of dead fish at 15-20,000, mostly menhaden, but also including white and yellow perch as well as sunfish and carp. Most of the fish appeared to have died in the tidal creek above the Rowe Boulevard bridge.
Because low levels of dissolved oxygen in the water are the most common causes of fish kills in this area, measurements were immediately made of this critical parameter. They showed that this portion of Weems Creek had critically low dissolved oxygen levels, even near the surface. These levels were low enough to cause fish to die. Low oxygen levels have been found by the Severn Riverkeeper monitoring program throughout the Severn during the summers of 2006 and 2007. However, such “hypoxia” is normally found near the bottom, and fish can avoid these areas by staying nearer the surface where oxygen from the atmosphere replenishes the supply.
Low oxygen levels are a widespread summer problem in lakes, estuaries and salt water. They are caused by the excessive growth of single
celled plants known as phytoplankton in response to plant nutrients nitrogen
and phosphorous in the water. Completing their life cycle, phytoplankton
die, fall the bottom, and are then consumed by decay bacteria that deplete
the oxygen locally. If vertical water circulation is restricted as sometimes
happens in Severn creeks, oxygen can drop to critically low levels even
near the surface. When this happens fish have no escape and will die.
An alternative cause for fish kills is toxic chemicals, which some may suspect in this case because Rte 50 and the Annapolis Mall are in the Weems Creek watershed. While toxics are responsible for relatively few Chesapeake fish kills, this possibility was addressed by collecting some dead fish and having them examined by fish pathologists. The results were not compatible with toxics as the principle cause of the kill. However, toxics cannot be ruled out as a complicating factor, particularly toxins originating from Karlodinium dinoflagellates. This form of phyotplankton was found in the Severn earlier in the year, and has the potential to produce a toxin that damages fish gills, thus sensitizing fish to the effects of low oxygen.
In any case, it is clear that the basic cause of this fish kill is the overgrowth of phytoplankton due to excess nitrogen and phosphorous in the Severn River. The dead zones seen elsewhere in the Severn are another consequence of this nutrient pollution. Join the Severn Riverkeeper in working to restrict the entry of nitrogen and phosphorous into our waterways.