Proposed research is designed to meet criteria for
NSF's LTREB (Long-term Research in Environmental Biology) Program.
The primary
objective of this project is to understand the link between surface-subsurface
water interactions and ecosystem processes in
tropical streams of lowland Costa Rica. Our focus is on how nutrient-rich
groundwater affects microbially-mediated decomposition
processes in streams where decomposing organic mater is the primary
food source for higher trophic levels.
Research will occur at La Selva Biological Reserve,
Costa Rica. Here, solute-rich groundwater emerges at the gradient break,
where
the foothills of the central mountain range merge with the coastal
plain. Solute-rich groundwater has been geothermally-modified and
is
associated with underlying volcanic activity which has altered the
chemistry of receiving streams throughout Central America.
Geothermally-modified groundwaters have high levels of P (up to 400
mg SRP L-1) and other solutes (Ca, Mg, SO4) but are not elevated in
temperature. Spatial patterns in stream solute chemistry are
determined by geomorphic features of the volcanic landscape that
include: upland lavas drained by P-poor streams; a gradient break (~50
m.a.s.l.), at or near where P-rich springs emerge; and lowland
alluvial areas drained by streams that are both P-rich and P-poor.
High P levels in solute-rich streams appear to enhance leaf
decomposition rates via increased microbial metabolism and insect consumption.
We will examine how landscape patterns in stream
solute chemistry (resulting from variation in solute-rich grounwater inputs)
affect
patterns in growth and secondary production of stream-dwelling insects.
First, we will test the hypothesis that landscape-scale
variation in insect secondary production is affected by interstream
variation in the proportion of geothermally modified ground-water:
solute-poor streams above the gradient break will have significantly
lower secondary production than solute-rich streams below the
gradient break. Second, in order to isolate effects of P (i.e.
in the absence of other geothermally introduced solutes), we will use an
ongoing long-term (>2 yrs) whole-stream P-enrichment study to evaluate
P effects on insect growth rates, insect secondary
production, rates of leaf decomposition, and microbial activity associated
with leaf decomposition. We will also test the hypothesis
that these aforementioned processes will become N-limited in P-rich
conditions through whole-stream N amendments in combination with P.
Finally, we will use natural and experimental P-gradients in streams
to compare and contrast nutrient cycling processes.
Proposed studies will be the first to determine
long-term effects of nutrient enrichment in a detrital-based stream in
the wet neotropics.
In addition, we will continue to build the only 'long-term' (1988-present)
data set on stream solute chemistry that we are aware
of for primary lowland rainforest of Central America. Stream
solute chemistry and ecosystem process-oriented data are of fundamental
importance to our understanding and management of tropical forests
and in predicting effects of regional (and potentially global)
environmental change on these threatened ecosystems. Our long-term
program has provided, and will continue to provide, critical
information for other ecosystem studies, along with numerous opportunities
for undergraduate and graduate student research.