Assessing trophic state of an endorheic tropical lake:: the algal growth potential and limiting nutrients

Lake Zirahuen is one of Mexico's most beautiful lakes. It is apparently endorheic. The highly transparent waters with very low algal biomass characterize an oligotrophic condition. Nevertheless, the external nutrient load and the potential for internal nutrient loads in an anoxic hypolimnion predict eutrophy. To determine if the low algal biomass in the lake is due to nutrient control or to inhibition by pollution from copper goods manufacturing in the watershed, we undertook algal bioassays of the lake and inflowing stream waters. The algal growth potential of the stream water was high but was zero for the lake waters. Nutrient additions showed nitrogen and phosphorus colimitation for the lake and phosphorus limitation for the stream waters. The good growth found with added nutrients eliminates copper inhibition as the cause of the low in-lake algal growth. These data suggest that the lake is not endorheic but has significant subsurface flushing of the inflowing nutrients, and also includes internal controls precluding nutrients mobilization into the food web.