SPATIAL VARIATION IN BASIC CHEMISTRY OF STREAMS DRAINING A VOLCANIC LANDSCAPE ON COSTA-RICA CARIBBEAN SLOPE

Spatial variability in selected chemical, physical and biological parameters was examined in waters draining relatively pristine tropical forests spanning elevations from 35 to 2600 meters above sea level in a volcanic landscape on Costa Rica's Caribbean slope. Waters were sampled within three different vegetative life zones and two transition zones. Water temperatures ranged from 24-25-degrees-C in streams draining lower elevations (35-250 m) in tropical wet forest, to 10-degrees-C in a crater lake at 2600 m in montane forest. Ambient phosphorus levels (60-300-mu-g SRP L-1; 66-405-mu-g TP L-1) were high at sites within six pristine drainages at elevations between 35-350 m, while other undistrubed streams within and above this range in elevation were low (typically < 30.0-mu-g SRP L-1). High ambient phosphorus levels within a given stream were not diagnostic of riparian swamp forest. Phosphorus levels (but not nitrate) were highly correlated with conductivity, Cl, Na, Ca, Mg and SO4. Results indicate two major stream types: 1) phosphorus-poor streams characterized by low levels of dissolved solids reflecting local weathering processes; and 2) phosphorus-rich streams characterized by relatively high Cl, SO4, Na, Mg, Ca and other dissolved solids, reflecting dissolution of basaltic rock at distant sources and/or input of volcanic brines. Phosphorus-poor streams were located within the entire elevation range, while phosphorus-rich streams were predominately located at the terminus of Pleistocene lava flows at low elevations. Results indicate that deep groundwater inputs, rich in phosphorus and other dissolved solids, surface from basaltic aquifers at breaks in landform along faults and/or where the foothills of the central mountain range merge with the coastal plain.