Does elevation modify aquatic export to riparian habitats?

Background/Question/Methods

Stream ecosystems traverse gradients of elevation and are tightly linked to adjacent riparian habitats through reciprocal subsides.  Aquatic insect emergence is an important component of this linkage and is generally assumed to reflect benthic production. However, elevation-specific environmental conditions may also influence exchange rates and modify connectivity across the gradient.  To examine how elevation and concomitant environmental variables affected lotic-riparian connectivity, we studied two reaches of the Rio Tambillos in the Andes Mountains of Argentina.  The study reaches were 20 m, extended 4 m into the riparian habitat, and were located at 2470 and 2630 m elevation.  We assessed aquatic, terrestrial and emergent arthropod assemblages at each site by collecting 12 Surber samples, 18 pitfall traps and 9 sticky traps.  Stream depth and near-bed current were measured for each Surber sample.  Pitfall and sticky traps were arranged along 3, 4-m transects that extended away from the stream and were left in place 5-d prior to collection.  Aquatic and terrestrial temperature was logged over 72-h in the stream and 10 cm above the ground on the shore.  Riparian plants were identified and their density was measured across each transect.  Data were analyzed using ANOVA and multiple regression.

Results/Conclusions

Temperature, riparian vegetation, terrestrial arthropod abundance and, to a lesser extent, the aquatic macroinvertebrate community changed with elevation.  However, effects on aquatic emergence were inconclusive.  The upper reach was cooler than the lower, with respective maximum temperatures of 16.2° and19.2° C for the stream and 44.1° and 49.6°C for the riparian zone.  Both reaches had fewer terrestrial arthropods near the stream and the upper reach had less overall, averaging 39 vs. 62 individuals/trap (P < 0.01).  Benthic macroinvertebrate diversity and total abundance in the stream did not differ between reaches; however, Ephemeroptera were twice as numerous at the upper site, averaging 1333 vs. 646 individuals/m² (P < 0.01).  Benthic assemblages showed unique responses to local physical conditions.  Macroinvertebrate numbers declined with velocity at lower reach (P = 0.005), but showed no relationship to current at the upper.  Similarly, macroinvertebrate diversity was inversely related to depth at in the lower reach, but not at the upper (P = 0.005).  No clear pattern was seen for aquatic emergence.  Although both riparian and terrestrial systems changed with elevation, we could not find a link through aquatic export.  This suggests that longer-term study is needed to distinguish the connection.