Streams in the Luquillo Mountains
Streams in the Luquillo Mountains have steep channels with many boulders and series of pools and riffles. The water is clear, as runoff is chemically diluted and suspended particles are mainly transported during flood events (see article Climate and Hydology in this web page).
The dynamics of stream ecosystems are strongly influence by physical factors, such as the rapid change in elevation from headwaters to coastal plains, the intensity and frequency of rainfall events, and the high temporal variability of stream discharge. In addition, the isolation of Puerto Rico from continental sources of freshwater species results in an aquatic fauna low in diversity.
The stream fauna is dominated by decapods (mainly shrimps and crabs), fishes, and aquatic insects. Shrimps have longitudinal distributions, with Xiphocaris elongata typically more abundant in the headwater pools, where it can co-occur with Atya lanipes, and Macrobrachium dominating low elevation reaches. Studies with stable isotopes indicated that algae, and not detritus, are the most important food resource for shrimps along streams. Shrimps have migratory life cycles, also know as amphidromous life cycles, referring to the fact that adults live in freshwater and the larvae in salt water. Adult populations of shrimps release their larvae into the water column, and the larvae migrate to estuarine environments, where they grow and develop. Juvenile shrimps then migrate back to the headwaters to complete the life cycle.
Fishes are more abundant in lowland reaches, where they can compete and prey upon shrimps. The presence of waterfalls in many streams within the Luquillo Mountains limit the distribution of fishes, but not that of shrimps. As a result, shrimps dominate headwater reaches above waterfalls and fishes and shrimps dominate reaches in areas without waterfalls.
The abundance of fishes and shrimps in combination with frequent disturbance events, are probably responsible for low densities of aquatic insects in the streams. Studies assessing the abundance and composition of insect assemblages among different stream habitats show that insects are more abundant in those habitats where shrimps are absent (e.g. , waterfalls, areas bedrock with a shallow sheet of water flowing over them).
Stream ecosystems within the Luquillo Mountains are vulnerable to hydrologic alterations from outside their boundaries. A water budget developed in 1994 indicates that on an average day more than 50% of stream water draining the Luquillo Mountains is diverted into municipal water supplies. While the forest has some of the last undeveloped water supplies on the island, water withdrawals conflict with other functions that the Forest fulfills, such as maintenance of biodiversity.
Information generated by ecological research within the Luquillo LTER is being applied to stream management within the Forest. Projects include: 1) long-term recording of migratory shrimp and fish populations; 2) in-stream flow habitat requirements of shrimps and fishes; 3) assessment of nocturnal behavior of shrimps and timing of larval shrimp migration to estuaries; 4) upstream migration of shrimps, fish, and snails; 5) effects of low-head dams and water withdrawals on shrimp and fish mortality; 6) effects of large dams on stream ecosystem characteristics; 7) genetics of shrimp populations; 8) impacts of water extraction and sewage release on water quality; and (9) effects of whole stream poisonings on shrimps and fish populations.
Management recommendations from ecological research are being used to mitigate negative environmental effects of stream water withdrawals. For example, the designs of two new water withdrawal systems have been altered by the Puerto Rican Aqueduct and Sewage Authority to minimize mortality of migrating shrimps and fishes. Water intakes also operate when stream flows are high so that base flows are maintained. Equally encouraging is that water withdrawal from some intakes has been prohibited during peak larval shrimp migration (7 to 11 pm) and fish/shrimp ladders are now required. Stream research at Luquillo highlights the critical need to address cumulative long-term effects of hydrologic alterations on public lands and illustrates the synergism that can occur between field managers and scientists in implementing ecosystem management solutions.