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Toxic bloom in Peruvian Altiplano lake

We are investigating an ongoing toxic Anabaena flos aquae bloom in a domestic and industrial water supply lake high (4,500 m) in the Andes Mountains of Southern Peru. We studied most major aspects of the lake¹s ecology 5 and 6 years ago, when the phytoplankton community was dominated by green microalgae. At that time there were signs of stress in the zooplankton community, apparently related to large-scale stocking of rainbow trout by native fishers and groups. The trout to blue green connection is not a direct one, but there are some extensive data supporting our working hypothesis (see below). We are interested in comparing ideas with other workers in the field who are conducting similar studies or are working in nearby regions. There is a paucity of information about other high elevation, low latitude lakes in south America and studies at Lake Titicaca or northern hemisphere lakes are generally not applicable to these smaller, altiplano lakes. The government in Peru is considering accelerating efforts to plant trout in the region, so these efforts are timely to protect water quality and native fish and invertebrates. Please contact me at jackrensel@msn.com if you wish to share information on other lakes in the region or are involved in pertinent lake ecology or HAB studies anywhere else.

Working Hypothesis: Volcanic soils provide a high rate of total phosphorus supply to this lake and dissolved inorganic nitrogen is usually above limiting concentrations most of the time. Grazing by zooplankton appears to be a major control of the phytoplankton population, with some possible light limitation in the wet, summer season. Waters are relatively clear in normal years with Secchi disk readings of about 1.5 to 4 m. The small trout (<20 cm fork length) focus their predation nearly exclusively on Daphnia spp. (water flea, cladocerans zooplankton) in the December to May period after planting. After that, they shift to eating larger amphipods that are in turn supported by the rich macrophyte populations in littoral zones. The Daphnia and some copepods are a major control of the green microalgae, but when grazed too much, the green algae bloom with chlorophyll a levels an order of magnitude greater than normal. This causes the N:P ratio to decline further and DIN levels fall to within known half saturation rates for uptake by many microalgae. At that point, blue-greens slowly take over during the calm, sunny but relatively cool winter season. These blue green blooms are so dense that the macrophytes die off and hence the amphipods, that are food for the larger trout (>20 cm), are severely depleted, affecting the health and condition index of the larger trout. The trout are also reported to taste bad. The bloom has continued for nearly 7 months at this point with no sign of cessation. Several blue green species including Anabaena sp. were seen in the lake in one early 1980 study, but since trout planting started about a decade ago there have been 3 years of nearly monospecific blue green blooms, each apparently worse in duration. It may be possible to mitigate the situation by reducing the number of fish planted, based on our past recommendations, or by pumping N rich groundwater into the lake at appropriate times to prevent the cessation of green algae dominance. There is a private weather station at the lake, so we can directly monitor that effect. There is no water treatment for the human consumers of this lake water other than chlorination, and Anatoxin a has been documented by workers at Wright State University.

Thanks to Wayne Carmichael for laboratory support and John Sowles for capable assistance with field work. Jon M. Lindbergh was an initial co-investigator in this study.

J.E. Jack Rensel Ph.D.
Rensel Associates Aquatic Science Consultants
4209 234th Street N.E.
Arlington WA 98223 USA
email: jackrensel@ msn.com
phone: 360-435-3285
fax: 360-435-7409