tropical streams


BLANCO,JUAN F.;SCATENA,FREDERICK N. 2007. The spatial arrangement of neritina virginea (gastropoda: neritidae) during upstream migration in a split-channel reach.. River Res. Applic. 23: 235-245.

This paper relates differences in flow hydraulics between a main channel (MC) and a side channel (SC) of a river to patterns of upstream migration by Neritina virginea (Neritidae: Gastropoda), a dominant diadromous snail in streams of Puerto Rico (Greater Antilles). Near-bed water velocity, snail density and shell size were measured on a weekly basis between August and December 2000 along cross-sections in a main channel (MC) and an adjacent channel (SC) under a bridge crossing of the Rio Mameyes of Northeastern Puerto Rico. Near-bed velocity and water depth were used to compute Reynolds (Re) and Froude (Fr) numbers, and to classify flows within each channel. During base flow conditions (<2m3 s1), flow was chaotic and supercritical (Fr>1) in the MC, and non-chaotic and subcritical (Fr<1) in the SC. Higher mean densities (>100 indm2) of relatively small snails (mean s.d., 6.3 2.8 mm) were consistently recorded in the MC. Conversely, the SC had lower mean densities(<20 indm2) and significantly larger snails (7.6 2.4 mm). Within the MC, migratory groups preferred near-bed velocities>0.8ms1. Within the SC, they preferred the channel thalweg and depths>30 cm. The spatial arrangement that was observed between and within the channels may be related to food resources, predation pressure or biomechanics. Characteristics of preferred upstream migration pathways of N. virginea must be accounted when building road crossings incoastal streams with diadromous fauna.

Hierarchical contribution of river–ocean connectivity, water chemistry, hydraulics, and substrate to the distribution of diadromous snails in Puerto Rican streams

Blanco, Juan F.; Scatena, Frederick N. 2006. Hierarchical contribution of river-ocean connectivity, water chemistry, hydraulics, and substrate to the distribution of diadromous snails in Puerto Rican streams.. J. N. Am. Benthol. Soc., 25(1) :82-98.

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.

Distribution of migratory fishes and shrimps along multivariate gradients in tropical island streams

Fievet E, Doledec S, Lim P. 2001. Distribution of migratory fishes and
shrimps along multivariate gradients in tropical island streams. Journal
of Fish Biology 59: 390–402.

Among the 16 species of fishes and shrimps studied at 51 sites along several small streams at Basse Terre, Guadeloupe, Lesser Antilles, more than 60% showed a spatial distribution significantly influenced by environmental conditions. These included altitude, basin size, terrestrial vegetation and land use. However, the range of the species habitat was generally high, except for three species limited to the downstream stretches (two fishes and one shrimp). The habitat characteristics of fish and shrimp species tended to differ, but the difference was not significant (P<0·05). On the contrary, the habitat characteristics of amphidromous and catadromous species clearly differed, with a lower occurrence at the most elevated sites of catadromous species than amphidromous species.

Transforming tropical rivers: an environmental perspective on hydropower development in Costa Rica

Anderson E, Pringle C, Rojas M (2006) Transforming tropical rivers: an environmental perspective on hydropower development in Costa Rica. Aquatic Conservation-Marine and Freshwater Ecosystems 16: 679–693

1. Tropical rivers are increasingly being altered by hydropower dams. In Costa Rica, more than 30 hydropower plants were built during the 1990s and more dams are being proposed. Hydropower dams currently provide more than 80% of electricity consumed by the country’s 4 million residents, yet most of Costa Rica’s hydropower potential remains untapped. 2. Ecological consequences of dams in Costa Rica stem primarily from river fragmentation, stream de-watering, and downstream hydrological alterations. Dams affect distribution and abundance of aquatic biota, especially migratory species. Cumulative effects of multiple dams on individual river basins, especially in the northern part of the country, are also of concern but have not been adequately documented. 3. In light of recent hydropower development, we recommend conservation strategies that protect remaining free-flowing rivers, call for assessment of ecological impacts of dams on a broader scale, encourage research on aquatic systems and sustainable hydropower technologies, and promote the development of methods for estimating environmental flows for Costa Rican rivers. Copyright # 2006 John Wiley & Sons, Ltd.

Effects of extreme low flows on freshwater shrimps in a perennial tropical stream

COVICH, A.P.; CROWL, T.A.; SCATENA, F.N. 2003. Effects of extreme low flows on freshwater shrimps in a perennial tropical stream.. Freshwater Biology 48, 1199-1206.

1. Long-term data on rainfall suggests that perennial rainforest streams rarely are subject to drying of riffles or pools in the wet, non-seasonal Caribbean climate of Puerto Rico. Unusually low rainfall in 1994 caused some headwater riffles to dry out completely, resulting in isolated pools, reduced pool volumes and loss of access to microhabitats by benthic invertebrates. 2. From 1992 to 1998, shrimp populations were sampled bimonthly using baited traps in six pools along 1200 m (from 305 to 480 m in altitude) of Quebrada Prieta, a second-order headwater stream in the Luquillo Experimental Forest (Caribbean National Forest). 3. Following contraction of the smaller and shallower pools in the most upstream sectionof the stream, mean densities of the dominant shrimp (Atya lanipes) increased from 22 to 75shrimp m)2 of pool area during the 1994 drought year. A second common species(Xiphocaris elongata) increased from 5 to 14 shrimp m)2. A smaller percentage of adults of both species was gravid during the drought. 4. Following the 1994 drought (1995–1998), densities of both shrimp species and reproductive activity of Atya returned to predrought (1990–1993) levels. However, the reproductive activity of Xiphocaris remained lower than in the predrought period. 5. It is suggested that prolonged droughts, even in tropical rainforest biomes, may significantly alter aquatic communities through localised crowding effects resulting from habitat contraction, and lead to prolonged decreases in reproductive output. Consequently, major alterations in aquatic populations and communities would be predicted by current climate change scenarios of decreased total rainfall and increased variability.
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