Fish assemblage variation between geologically defined regions and across a longitudinal gradient in the Monkey River Basin, Belize.
Abstract: Linkages between geology and fish assemblages have been inferred in
many regions throughout the world, but no studies have yet investigated
whether fish assemblages differ across geologies in Mesoamerica. The
goals of our study were to: 1) compare physicochemical conditions and
fish-assemblage structure across 2 geologic types in headwaters of the
Monkey River Basin, Belize, and 2) describe basin-scale patterns in
fish community composition and structure for the benefit of
conservation efforts. We censused headwater-pool fishes by direct
observation, and assessed habitat size, structure, and water chemistry
to compare habitat and fish richness, diversity, evenness, and density
between streams in the variably metamorphosed sedimentary geologic type
typical of 80% of Belize's Maya Mountains (the Santa Rosa Group), and
an anomalous extrusive geologic formation in the same area (the Bladen
Volcanic Member). We also collected species-presence data from 20 sites
throughout the basin for analyses of compositional patterns from the
headwaters to the top of the estuary. Thirty-nine fish species in 21
families were observed. Poeciliids were numerically dominant, making up
39% of individuals captured, followed by characins (25%), and cichlids
(20%). Cichlidae was the most species-rich family (7 spp.), followed by
Poeciliidae (6 spp.). Habitat size and water chemistry differed
strongly between geologic types, but habitat diversity did not. Major
fish-assemblage differences also were not obvious between geologies,
despite a marked difference in the presence of the aquatic macrophyte, Marathrum oxycarpumJoturuspichardi, Agonostomusmonticola, and Gobiomorusdormitor)
that use freshwater and marine habitats. Some of these species probably
rely on a natural flow regime and longitudinal connectivity for
reproduction and dispersal of young, and natural flow regime and
longitudinal connectivity are important factors for maintenance of
functional linkages between the uplands and the coast in the
ridge-to-reef corridor where the Monkey River is located. Therefore, we
suggest that the viability of migratory fish populations may be a good
biological indicator of upland-to-estuary connectivity important both
to fishes and coastal ecosystem function. We recommend follow-up
studies to substantiate the relative strengths of relationships between
community structure and abiotic factors in contrasting geologies and to
examine potential bottom−up responses of stream biota to the higher
nutrient levels that were observed in stream waters draining the Santa
Rosa Group geologic type.
(Podostemaceae), which covered 37% of the stream bottom in
high-nutrient streams draining the Santa Rosa Group, and did not occur
in the low-P streams draining the Bladen Volcanic Member. Correlation
analyses suggested that distance from the sea and amount of cover
within pools are important to fish-assemblage structure, but that
differing abiotic factors may influence assemblage structure within
each geologic type. The fauna showed weak compositional zonation into 3
groups (headwaters, coastal plain, and nearshore). Nearly 20% of the
fish species collected have migratory life cycles (including Joturuspichardi, Agonostomusmonticola, and Gobiomorusdormitor)
that use freshwater and marine habitats. Some of these species probably
rely on a natural flow regime and longitudinal connectivity for
reproduction and dispersal of young, and natural flow regime and
longitudinal connectivity are important factors for maintenance of
functional linkages between the uplands and the coast in the
ridge-to-reef corridor where the Monkey River is located. Therefore, we
suggest that the viability of migratory fish populations may be a good
biological indicator of upland-to-estuary connectivity important both
to fishes and coastal ecosystem function. We recommend follow-up
studies to substantiate the relative strengths of relationships between
community structure and abiotic factors in contrasting geologies and to
examine potential bottom−up responses of stream biota to the higher
nutrient levels that were observed in stream waters draining the Santa
Rosa Group geologic type.
Esselman, P. C., M. C. Freeman, and C. M. Pringle. 2006. Fish assemblage variation between geologically defined regions and across a longitudinal gradient in the Monkey River Basin, Belize. Journal of the North American Benthological Society. 25:140-154.
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