Marcel O. Tanaka

Temporal variation of amphipod assemblages associated with Sargassum filipendula (Phaeophyta) and its epiphytes in a subtropical shore

The phytal assemblages change in response to variation in biological and environmental conditions. In the present study, we evaluated the temporal variation of amphipod assemblages associated with a Sargassum filipendula bed in a subtropical shore, in relation to variation of the host alga, its epiphytes and local environmental conditions. Samples of S. filipendula with associated amphipods, water temperature, water movement and suspended solids were obtained monthly from June 2000 to May 2001. We recorded 24 species of amphipods associated with S. filipendula. Species richness varied throughout the year, with maximum values in October 2000 and minimum in April 2001. Total amphipod density gradually increased during the sampling period, with the highest value in March 2001. Amphipod diversity and evenness were both positively influenced by epiphyte load and negatively by temperature, with higher values during summer months. Total density and tube-builder density were also positively influenced by temperature, whereas nestler density was influenced by epiphyte load. Individual amphipod species showed significant density fluctuations over the year. The canonical correspondence analysis performed explained 88.2% of the variation, with a strong correlation of water movement, temperature and suspended solids with the first axis, and a strong effect of epiphyte load on both the first and the second axes. The temporal structural variation of the studied algal bed strongly influenced amphipod diversity and assemblage composition, possibly through direct and indirect effects.

Influence of a forest remnant on macroinvertebrate communities in a degraded tropical stream

Land use changes have resulted in large environmental impacts, and in agricultural landscapes sometimes only forest fragments remain. Riparian forest remnants can positively influence stream water quality, and serve as refuges for aquatic species. We evaluated whether the presence of a riparian forest remnant influenced the structure and composition of macroinvertebrate communities in a rural stream in southeastern Brazil. We sampled three reaches upstream (within abandoned sugarcane cultivation) and nine downstream the remnant edge, until 600 m inside the forested area, using leaf litter bags. The abundances of Elmidae, Chironomidae, and total macroinvertebrates increased along the forest remnant, whereas the abundance of Baetidae, proportion of Ephemeroptera, Plecoptera, and Trichoptera (EPT), rarefied taxonomic richness, and diversity decreased. Taxon richness and EPT abundance did not vary along the forest remnant. Increases in Chironomidae and total abundance within the forest remnant can be related to moderate increases in nutrient concentrations, or to the availability of high quality leaf litter patches. Forest remnants can influence macroinvertebrate communities, although variation both in temperate and tropical studies can be related to local agricultural practices and land use at the watershed scale. Forest remnants are important in maintaining stream water quality in rural landscapes, and deserve attention in watershed management projects.

Shell Morphological Variation of Littoraria angulifera among and within Mangroves in NE Brazil

The shell morphological variation of the periwinkle Littoraria angulifera (Lamarck, 1822) was studied in tropical northeast Brazilian mangroves. This area was selected because mangroves in different stages of regeneration, and thus different tree heights can be found. We evaluated whether differences in solar radiation due to differences in tree height influenced the distribution and shell morphology of L. angulifera, and carried out an experiment to test if individuals collected from mangroves with different tree heights differed in their resistance to desiccation. We also analysed if there were differences in L. angulifera shell length and shape between tidal levels within a mangrove. Finally, we tested if increased habitat complexity due to the presence of oysters could influence L. angulifera shell length and shape in different-statured mangroves. We predicted that the oysters could reduce desiccation stress on periwinkles in small-statured mangroves but have no effect in taller ones. Shell length and shape varied among mangroves, although a large variation within mangroves was also recorded. Shell proportionality (shell length:width ratio) increased with shell length, and this relationship differed among mangroves; however, no differences were found in a subsequent year. Individuals from small-statured mangroves survived longer than those from taller mangroves in the desiccation experiment, with a weak correlation between shell proportionality and loss of mass. The presence of oysters had no apparent influence on shell morphology that could be correlated with reduced desiccation stress. The patterns found varied greatly both at small and large spatial scales, suggesting that future studies should evaluate phenotypic and genetic variation at the same time to properly understand variation in L. angulifera shell morphology.

Structural variation in the brown alga Sargassum cymosum and its effects on associated amphipod assemblages.

The presence of diverse biological substrates adds complexity to coastal landscapes and increases the number of ecological niches that can be used by the mobile epifauna. Studies on the influence of structural complexity have focused mainly on algal host species, but there is little information about the influence of intraspecific structural variation on the associated mobile epifauna. In this work, we examined whether intraspecific variation in the brown alga Sargassum cymosum influenced the structure of amphipod assemblages on two shores with different wave exposure. At least 15 fronds were randomly sampled at Fortaleza and Perequê-Mirim beaches, on the Atlantic coast of São Paulo state, southeastern Brazil, and 12 variables were measured for each alga. The amphipods were identified and counted. The greatest structural variation in S. cymosum occurred within shores, whereas the differences between shores were mainly related to algal size. These characteristics influenced amphipod assemblages differently on each shore, with the greatest effects being associated with variables related to morphological complexity, such as holdfast size, the number and size of branches, and the extent of cover by sessile colonial animals. These findings show that monospecific algal banks are not homogeneous, and that morphological differences and interactions with other biological substrates can influence the mobile epifaunal assemblages.

Factors influencing temporal variation of a Sargassum filipendula (Phaeophyta: Fucales) bed in a subtropical shore

In the present study, we evaluate the influence of biotic and abiotic factors on temporal fluctuations of Sargassum filipendula in a subtropical shore. Monthly algal samples, abiotic components, amphipod grazer density, and epiphyte biomass were obtained from a Sargassum bed in south-eastern Brazil. Density of S. filipendula fronds decreased during the sampling period, whereas dry mass was more constant, although with a noticeable reduction in the warmer months. Hypnea musciformis was the most frequent epiphyte on S. filipendula, occurring in all sampling periods, although with significant temporal variation. Sargassum filipendula density and dry mass were both influenced by epiphyte dry mass, temperature, and amphipod grazers. Sargassum filipendula biomass negatively influenced total epiphyte biomass, whereas H. musciformis biomass was positively influenced by phosphate, nitrite, and S. filipendula density and negatively influenced by S. filipendula dry mass and amphipod grazer abundance. Algal temporal fluctuations can be related to local abiotic and biotic factors, but the variation observed for S. filipendula and its epiphytes suggest that these factors have quite distinct effects for these algae.

Avaliação de efeitos de espécies de mangue na distribuição de Melampus coffeus (Gastropoda, Ellobiidae) no Ceará, nordeste do Brasil

Melampus coffeus (Linnaeus, 1758) is a pulmonate, macrodetritivore gastropod, important in energy flows in neotropical mangroves, but its distribution in different spatial scales is still unknown in Brazil. The objective of this study was to evaluate the distribution of M. coffeus: 1) among the mangrove trees Rhizophora mangle Rhizophoraceae and Laguncularia racemosa Combretaceae; 2) in relation to the position of R. mangle prop roots and 3) among mangroves with different tree heights. Further, its behavior along one tidal cycle was also described. The densities of M. coffeus did not differ between R. mangle and L. racemosa, but the largest snails were found on R. mangle. The distribution of snails found on the sediment differed relative to the position of the prop roots, with the highest density in the center and largest size in the edge of the studied areas. However, this distribution was not related with the availability of food resources provided by decayed leaves. Higher densities were also recorded in intermediate and low-stature mangroves when compared with taller ones, but no differences in snails sizes among mangroves were found. During the high tide, M. coffeus individuals climb nearby trees. These results are possibly related to the availability of resources and shelter supplied by R. mangle. Thus, studies on the distribution of M. coffeus in diverse conditions are essential, because mangroves are being intensively disturbed.

Diel density variation of amphipods associated with Sargassum beds from two shores of Ubatuba, Southeastern, Brazil

As densidades da epifauna movel associada a macrofitas variam muito ao longo do dia, sendo a atividade de predadores visualmente orientados um dos principais fatores apontados para explicar este padrao. Neste estudo, comparamos as densidades de anfipodos associados a uma alga parda comum, Sargassum stenophyllum (Phaeophyceae), em praias contrastando em turbidez da agua. Era esperada a ocorrencia de variacao ao longo do dia numa praia com aguas claras (Pereque) e nenhuma variacao na praia com aguas turvas (Lamberto). A densidade de anfipodos variou ao longo do dia em ambas as praias, sem indicacao de maior variacao em Pereque. A maioria das especies apresentou dois picos de densidade: um a noite e outro a tarde. Estes ocorreram proximos aos periodos de mare alta, sugerindo que o ritmo de mare poderia influenciar mais as densidades de anfipodos que a atividade de predadores. Assim, mais estudos sao necessarios para se compreender os fatores que influenciam a variacao de curto prazo das comunidades epifaunais.

Aspectos estruturais da comunidade arbórea em remanescentes de floresta estacional decidual, em Corumbá, MS, Brasil

(Structural features of the arboreal community in remnants of dry forest in Corumba, MS, Brazil). The structure of the arboreal community of two types of dry forest was compared in an altitudinal gradient of lowland (FEDTB) and submontane (FEDSM) dry forests, in Corumba, MS, Midwest Brazil. Tree species with circumference at breast height ³ 15 cm were sampled, using the point centered quarter method. In the FEDTB 80 points were demarcated, and in FEDSM 78 points were distributed in the following altitudes: 180 m (18 points), 220, 260 and 300 m (20 points each). In FEDTB 34 species were sampled; Calycophyllum multiflorum Griseb., Ceiba pubiflora (A. St.-Hil.) K. Schum. and Anadenanthera colubrina (Vell.) Brenan have the highest VI values. In FEDSM, 33 species were sampled, and Anandenanthera colubrina, Ceiba pubiflora and Acosmium cardenasii H. S. Irwin & Arroyo were the most important. The distribution pattern of the species varied along the altitudinal gradient. In both forests Shannons diversity and equitability indexes were 2.9 and 0.8, respectively, and the most representative families were Fabaceae (8 species) and Rubiaceae (4 species). The studied forests have low density of individuals in relation to other studies, except for the track at 300 m. About 22.46% of the individuals of arboreal community had basal ramifications and were in the initial phases of succession. The canopy varied from 6 to 12 m, and emergent individuals of up to 18 m. These remnants represent a source of biodiversity for the Pantanal, being an important part of the natural forest corridors of the region.

Influence of discharge patterns on temporal variation of macroinvertebrate communities in forested and deforested streams in a tropical agricultural landscape

Deforestation of riparian forests can directly influence stream macroinvertebrate communities, through changes in resource availability, habitat quality, and hydrological alterations. Here we evaluated whether mean discharge and discharge coefficient of variation (CV), estimated over a 15-month study, influenced forested and deforested stream macroinvertebrate communities. Higher diversity and mean discharge were recorded in deforested streams, and community composition was strongly related to mean discharge. Streams formed a gradient of both mean discharges and discharge CV, which both influenced patterns of diversity. Discharge CV negatively influenced diversity in a nonlinear model, whereas both mean discharge and discharge CV influenced differentiation of macroinvertebrate communities among five sampling occasions. Similarity among sampling times within each stream estimated by the Morisita–Horn index decreased with increasing mean discharge, whereas response to CV was quadratic, with higher similarity at intermediate values and lower similarity at more extreme values. These results suggest that stability of stream macroinvertebrate communities display different responses to mean discharge and discharge CV, and that hydrological alterations can interact with land use changes to determine community dynamics at local scales.

Does environmental disturbance also influence within-stream beta diversity of macroinvertebrate assemblages in tropical streams?

ABSTRACT Riparian deforestation is a major threat to the ecological integrity of streams and aquatic biodiversity, influencing microhabitat availability and susceptibility to disturbances. Here we tested if riparian deforestation of tropical streams influenced beta diversity of macroinvertebrate assemblages, by comparing indices that weighted differentially rare and dominant taxa, and testing if nestedness in community composition increased in deforested streams. Within-stream beta diversity was higher in deforested than forested streams, mainly due to taxon loss and higher dominance. In disturbed streams, higher sedimentation in pool mesohabitats resulted in larger differences in community composition, whereas mesohabitats in forested streams were more stable.