THE EFFECT OF ORGANIC POLLUTION ON THE MACROINVERTEBRATE FAUNA OF ECUADORIAN HIGHLAND STREAMS

The effect of organic pollution on macroinvertebrate communities was s tudied in five small streams located at 2600 to 3100 meters above sea level in the Andes of Ecuador. Sampling of invertebrates and measureme nts of physico-chemical parameters were performed at the end of the ra iny season and at the end of the dry season at upstream unpolluted sit es and at adjacent downstream polluted sites. At all upstream sites, m ore taxa were collected in the dry season, while at the polluted downs tream sites, more taxa were collected in the wet season. Also values o f two biotic indices (BMWP and ASPT) tended to increase at the upstrea m sites and decrease at the downstream sites in the dry season. Thus, the effect of organic pollution was most pronounced during the dry sea son. In addition, both biotic indices were highly correlated to minimu m oxygen saturation and phosphate concentration in the dry season, whi le correlations were much weaker in the wet season. Overall, the shift in faunal composition with organic pollution resembled that described from temperate streams at higher latitudes. However, the main shift i n the tropical highland fauna occurred abruptly at about 80 % oxygen a ir saturation, but because of the low partial pressure of oxygen at an altitude of 3000 meters, this corresponds to no more than 56 % of the oxygen partial pressure of air saturated water at sea level. I propos e that tropical highland streams are more sensitive to further lowerin g of oxygen levels through organic pollution than their temperate coun terparts.