Diadromous faunas dominate most tropical coastal streams and rivers, but the factors controlling their distribution are not well understood. Our study documents abiotic variables controlling the distribution and abundance of the diadromous snail Neritina virginea (Gastropoda:Neritidae) in the Caribbean island of Puerto Rico. An intensive survey of N. virginea density and shell size, and channel substrate, velocity, and depth was conducted at microhabitat, habitat, and reach scales of a coastal plain reach of the Río Mameyes between August and December 2000. In addition, the inland extent of distribution (stream-network scale) and presence (regional scale) of N. virginea were surveyed in 32 coastal rivers around the island during summer 2001 and 2003. At the microhabitat scale, snail density and microhabitat electivity were greater in patches consisting of a mix of boulders and cobbles than in other types of substrate. At the habitat scale, snail density increased with depth. At the reach scale, snail density increased with fast and turbulent flows (riffle > pools > pond), whereas snail size showed the opposite pattern. At the regional scale, populations were present in 13 of 32 streams. Populations of N. virginea were not found in rivers that were disconnected from the ocean for most of the year because of channel dewatering, formation of sediment bars at their mouths, and low mean monthly discharge (Q=0.69 m3/s). In contrast, rivers with N. virginea populations had a permanent (Q=4.04 m3/s) or seasonal (Q=2.88 m3/s) connection to the ocean over the year. At the regional scale, the inland distribution of populations was not correlated with stream gradient, but was negatively correlated with concentrations of SiO2, P, and acid neutralizing capacity of the water. Populations colonized montane reaches in only 5 rivers, all of which were forested and protected. Our study highlights the importance of taking a hierarchical approach in managing tropical coastal rivers, and the usefulness of neritid snails as biological indicators of the physical and chemical integrity of rivers.

Inaccessible locations and inhospitable working conditions have resulted in a lack of detailed hydrological knowledge in the tropics. I have established a new field observatory located near the town of Gamboa, Panama, adjacent to the mid-point of the Panama Canal. This observatory allowed detailed measurements of the hydrologic cycle at the first-order catchment scale. My investigations reveal that competing runoff production mechanisms are responsible for the observed streamflow. Both of these mechanisms are strongly dependent upon rainfall rate and secondarily upon cumulative rainfall volume. I have identified a small but hydrologically significant riparian zone that is decoupled from the hillslope. In contrast to the remainder of the catchment this riparian zone shows signs of frequent overland flow. Upslope regions occasionally produce overland flow during high-intensity rainfall. Parallel to the small scale studies in Gamboa, sub-watersheds of the 414 km2 upper Rio Chagres were instrumented. These gages showed that the Rio Chagres rarely reacts uniformly; instead, the observed runoff is usually the result of a limited portion of the watershed. Based on inspiration from field research a new conceptual model, Sac3, was developed. Sac3 was successfully calibrated and verified on several study watersheds. Sac3 was also used to explore the relationship deep groundwater storages had on seasonal changes in baseflow runoff behavior.

Small dams for hydropower have caused widespread alteration of Central American rivers, yet much of recent development has gone undocumented by scientists and conservationists. We examined the ecological effects of a small hydropower plant (Don˜a Julia Hydroelectric Center) on two low-order streams (the Puerto Viejo River and Quebradon stream) draining a mountainous area of Costa Rica. Operation of the Don˜a Julia plant has dewatered these streams, reducing discharge to
10% of average annual flow. This study compared fish assemblage composition and aquatic habitat upstream and downstream of diversion dams on two streams and along a
4 km dewatered reach of the Puerto Viejo River in an attempt to evaluate current instream flow recommendations for regulated Costa Rican streams. Our results indicated that fish assemblages directly upstream and downstream of the dam on the third order Puerto Viejo River were dissimilar, suggesting that the small dam (<15m high) hindered movement of fishes. Along the
4 km dewatered reach of the Puerto Viejo River, species count increased with downstream distance from the dam. However, estimated species richness and overall fish abundance were not significantly correlated with downstream distance from the dam. Our results suggested that effects of stream dewatering may be most pronounced for a subset of species with more complex reproductive requirements, classified as equilibrium-type species based on their life-history. In the absence of changes to current operations, we expect that fish assemblages in the Puerto Viejo River will be increasingly dominated by opportunistic-type, colonizing fish species. Operations of many other small hydropower plants in Costa Rica and other parts of Central America mirror those of Don˜a Julia; the methods and results of this study may be applicable to some of those projects. Copyright # 2006 John Wiley & Sons, Ltd.

The small watershed approach is well-suited but underutilized in mercury research. We applied the small watershed approach to investigate total mercury (THg) and methylmercury (MeHg) dynamics in streamwater at the five diverse forested headwater catchments of the US Geological Survey Water, Energy, and Biogeochemical Budgets (WEBB) program. At all sites, baseflow THg was generally less than 1 ng L1 and MeHg was less than 0.2 ng L1. THg and MeHg concentrations increased with streamflow, so export was primarily episodic. At three sites, THg and MeHg concentration and export were dominated by the particulate fraction in association with POC at high flows, with maximum THg (MeHg) concentrations of 94 (2.56) ng L1 at Sleepers River, Vermont; 112 (0.75) ng L1 at Rio Icacos, Puerto Rico; and 55 (0.80) ng L1 at Panola Mt., Georgia. Filtered (<0.7 mm) THg increased more modestly with flow in association with the hydrophobic acid fraction (HPOA) of DOC, with maximum filtered THg concentrations near 5 ng L1 at both Sleepers and Icacos. At Andrews Creek, Colorado, THg export was also episodic but was dominated by filtered THg, as POC concentrations were low. MeHg typically tracked THg so that each site had a fairly constant MeHg/THg ratio, which ranged from near zero at Andrews to 15% at the low-relief, groundwater-dominated Allequash Creek,Wisconsin. Allequash was the only site with filtered MeHg consistently above detection, and the filtered fraction dominated both THg and MeHg. Relative to inputs in wet deposition, watershed retention of THg (minus any subsequent volatilization) was 96.6% at Allequash, 60% at Sleepers, and 83% at Andrews. Icacos had a net export of THg, possibly due to historic gold mining or frequent disturbance from landslides. Quantification and interpretation of Hg dynamics was facilitated by the small watershed approach with emphasis on event sampling.  2008 Elsevier Ltd. All rights reserved.