Past vegetation changes in the Brazilian Pantanal arboreal–grassy savanna ecotone by using carbon isotopes in the soil organic matter

Measurements of the organic carbon inventory, its stable isotopic composition and radiocarbon content were used to deduce vegetation history from two soil profiles in arboreal and grassy savanna ecotones in the Brazilian Pantanal. The Pantanal is a large floodplain area with grass-dominated lowlands subject to seasonal flooding, and arboreal savanna uplands which are only rarely flooded. Organic carbon inventories were lower in the grassy savanna site than in the upland arboreal savanna site, with carbon decreasing exponentially with depth from the surface in both profiles. Changes in ¹³C of soil organic matter (SOM) with depth differed markedly between the two sites. Differences in surface SOM ¹³C values reflect the change from C₃ to C₄ plants between the sites, as confirmed by measurements of ¹³C of vegetation and the soil surface along a transect between the upland closed-canopy forest and lowland grassy savanna. Changes of ¹³C in SOM with depth at both sites are larger than the 3–4 per mil increases expected from fractionation associated with organic matter decomposition. We interpret these as recording past changes in the relative abundance of C₃ and C₄ plants at these sites. Mass balances with ¹⁴C and ¹³C suggest that past vegetational changes from C₃ to C₄ plants in the grassy savanna, and in the deeper part of the arboreal savanna, occurred between 4600 and 11 400 BP, when major climatic changes were also observed in several places of the South American Continent. The change from C₄ to C₃, observed only in the upper part of the arboreal savanna, was much more recent (1400 BP), and was probably caused by a local change in the flooding regime.     

Response of tidal freshwater macrobenthos to sediment disturbance

The macrobenthic fauna in the tidal freshwater James River, Virginia, USA, exhibited a high degree of resilience and limited temporal response to sediment disturbance caused by large quantities of low bulk density (< 1.3 g cm^–3) fluid mud. The fluid mud was produced by hydraulic dredge deepening of a ship channel with disposal of dredged sediments onto a nearby shoal. The response of tidal freshwater communities to fluid mud was limited to mainly quantitative changes in abundance of dominant taxa (Limnodrilus spp., L. hoffmeisteri, Ilyodrilus templetoni, Corbicula fluminea, Coelotanypus scapularis) and was directly related to the thickness of fluid mud layers. Disturbance effects were short lived and for tubificids most obvious in areas with > 0.3 m fluid mud. In areas that received < 0.3 m fluid mud, acute effects were limited to chironomids and small (< 10 mm) C. fluminea. The fauna colonizing the areas disturbed by fluid mud was the same as that inhabiting the shoal prior to disturbance. There was no indication of a successional sequence, as reported for other freshwater and marine habitats. Three weeks after the disturbance ended, all but a few insect taxa had recolonized. Changes in community structure from fluid mud disturbance were slight with total taxa best characterizing the disturbance. The insensitivity of community structure measures reflects the high resiliency of macrobenthic communities to physical stresses in tidal freshwater systems.     

Equiproportional temperature-duration responses and thermal influences on distribution and species switching in the copepods Metadiaptomus meridianus and Tropodiaptomus spectabilis

The temperature-dependence of development was studied in two ecologically divergent freshwater calanoids, Metadiaptomus meridianus (Douwe) and Tropodiaptomus spectabilis (Kiefer). Egg durations were determined between 10 and 35 °C, and food satiated post-embryonic development times between 12 and 32 °C. All responses were basically inverse monotonic functions of temperature, adequately described by Blehrádek's equation. M. meridianus generally developed faster than T. spectabilis. Its egg development was faster at all temperatures, and while its naupliar durations were shorter only up to ± 15 °C, its overall post-embryonic development was more rapid up to ± 24 °C in females and ± 28 °C in males. Relatively speaking, however, T. spectabilis is clearly more warm-adapted than M. meridianus. The respective distributions (warm subtropical lowlands vs cooler uplands) of these copepods in the southern African subcontinent, as well as reversible switches between these species observed in two Natal impoundments are consistent with their contrasting thermal responses, although additional considerations apply in respect of the species alternations. T. spectabilis was replaced by M. meridianus in L. Midmar in spring 1981 and 1989, and in L. Albert Falls in spring 1990. Reciprocal replacements occurred in Midmar in autumn 1984, and in Albert Falls in late summer 1991. Both spring switches in Midmar coincided with cool spring temperatures, although the consequent shifts in growth rate advantage predicted from the measured temperature-duration responses only partly explain the switches in this warm-temperate reservoir. Parasitism of T. spectabilis by an ellobiopsid was observed during both switching events in Midmar, and perhaps augmented the change, although its effects on the host are indeterminate. Both species showed exactly parallel temporal changes in fecundity during the recent switches in both reservoirs, indicating closely similar trophic niches in the adults at least, and mitigating the possibility of trophic influences as determinants of the replacement. A dramatic but inexplicable increase (around 50% at 20 °C) in the development time of T. spectabilis was noted between 1988 and 1990, and perhaps contributed too.The protracted historical dominance of T. spectabilis in thermally suboptimal conditions in Midmar is ascribed to a general competitive superiority presumed from its K-selected attributes, in contrast to the r-selection evident in M. meridianus. This alternation between species with contrasting life styles is of fundamental ecological interest. Studies on Albert Falls, commenced in 1989, suggest an even greater competitive superiority of T. spectabilis, in keeping with the warmer conditions in this larger sister reservoir below Midmar.     

Oligochaetes (Naididae,Tubificidae, Enchytraeidae and Alluroididae) of Guyana,Peru and Ecuador

More than 41 species in 23 genera of the microdrile oligochaete families Tubificidae, Naididae, Opistocystidae, and Enchytraeidae and the freshwater megadrile family Alluroididae have been identified in recent collections made in Peru, Guyana and Ecuador. Just less than 70% of our species records are new for one or more of these countries and one is a new, albeit tentative, generic record for the South American continent. About 16 species new to science remain to be described. One of these is only the second reported species of Brinkhurstia (Alluroididae) and possesses unusual, single, very elongate penial setae. All of our species records are pertinent to tests of different hypotheses about historical and phylogenetic relationships among organisms of northern and southern South America and North America. The species, including new ones, with limited distributions are of particular significance to such hypotheses.     

Regulation of phytoplankton dynamics in a Laurentian Great Lakes estuary

The composition and dynamics of phytoplankton populations were examined in Old Woman Creek estuary, Lake Erie (USA). The centric bacillariophytes,Cyclotella atomus Hust.,Cyclotella meneghiniana Kütz., andAulacoseira alpigena (Grun.) Krammer, and the cryptophytes,Cryptomonas erosa Ehren. andRhodomonas minuta var. nannoplanctonica Skuja, dominated the phytoplankton most of the year. Chlorophytes, euglenophytes, and cyanophytes were observed less frequently. Estuarine and Lake Erie phytoplankton were considered distinct populations; lake taxa were largely confined to the estuary mouth and present only in low biomass. Maxima and minima of estuarine phytoplankton coincided with meteorological and hydrological forcing in the form of rainfall and subsequent storm-water inflows, respectively. Distinct population dynamics between the upper and lower estuary following storm events were attributed to the presence/absence of refugia serving as a source for repopulation by opportunistic taxa, fluctuating light conditions in the water column resulting from influx of particulate matter and resuspension of bottom sediments, and nutrient inputs associated with surface runoff and sub-surface interflow. Additionally, agricultural herbicides introduced by storm-water inflows potentially may affect and/or control the growth and physiological responses of individual taxa.     

Boeckella diamantina n.sp. (Calanoida,Centropagidae), from a high Andean lake in Mendoza,Argentina

A new species of Boeckella from limnetic samples of Laguna del Diamante, a high lake in the Andes (34°10 S) is described and illustrated. The species is defined by the characters of the male fifth leg: the right two segmented endopod bears four peculiar, short, claw-like spines, the left endopod is a simple finger-like projection. This species is related to B. gibbosa, also a species from the Andes and B. vallentini from Malvinas (Falkland Islands) and other subantarctic islands. It is distinguished from them by diagnostic features of the fifth legs of the male and abdominal structure and fifth legs of the female. Some current views on the features used in the taxonomy of the genus Boeckella are discussed.     

Dry-season spawning in a cyprinid fish of southern India

Synopsis A population of the S. Indian cyprinid fishBarbus melanampyx was sampled monthly through 24 months. Seasonal cycles of the gonado-somatic index (GSI), ovarian stages, male breeding tubercles, spawning behaviour and population structure were assessed. These fish breed strictly seasonally during the main dry period: December/January through April. Comparison with other Barbus species of the same general region led to the conclusion that the patterns of reproductive investment ofB. melanampyx are similar to those of perennial species, and different from those of wet-season spawners. The reasons for this rather unexpected result were found in the more constant conditions prevailing during a dry season as compared to the monsoon. It was argued thatB. melanampyx and the species spawning perennially are in effect small-brood spawners, rather than partial spawners.     

Effects of decreased atmospheric deposition on the sulfur budgets of two Dutch moorland pools

The chemical composition of surface waters of two Dutch moorland pools and of incident precipitation, was monitored from 1982 to 1990. For this period, sulfur and water budgets were calculated using a hydrochemical model developed for well-mixed non-stratifying lakes. Total atmospheric deposition of S decreased significantly after 1986 at both locations. A model describing the sulfur budget in terms of input, output and reduction/oxidation processes predicted a fast decrease of pool water SO₄ ²⁻ concentrations after a decrease of atmospheric input. However, SO₄ ²⁻ concentrations in the surface water was lowered only slightly or remained constant. Apparently a source within the lake caused the unexpectedly high SO₄ ²⁻ concentrations. The possible supply of SO₄ ²⁻ from the sediment through regulation by (K-)Al-SO₄ containing minerals or desorption of SO₄ ²⁻ from positively charged surfaces in the sediment was evaluated. Solubility calculations of pore water with respect to alunite, basaluminite and jurbanite indicated that SO₄ ²⁻ concentration was not regulated by these minerals. It is suggested here (1) that desorption of SO₄ ²⁻ from peaty sediments may account for the estimated SO₄ ²⁻ supply provided that the adsorption complex is periodically recharged by partial oxidation of the upper bottom sediments and (2) that because of exposure of a part of the pool bottom to the atmosphere during dry summers and subsequent oxidation of reduced S, the amount of SO₄ ²⁻ may be provided which complements the decreasing depositional SO₄ ²⁻ input. In future research these two mechanisms need to be investigated.     

Biomass structure and dry matter dynamics of subtropical alluvial and exotic Ligustrum forests at the Río de la Plata,Argentina

Biomass, litterfall, litter standing crop, and decomposition was studied in a native subtropical alluvial forest locally known as Selva Marginal (SM) and an exotic Ligustrum lucidum forest (LF) at the Reserva Integral de Punta Lara, Buenos Aires Province, 34°47S and 58°1W. The alluvial forest site was at the southern limit of distribution of subtropical forests in South America. The Ligustrum forest was invading disturbed areas. Total biomass was 147.7 Mg/ha (86% aboveground and 14% belowground) in the SM, and 71.4 Mg/ha (93% and 7%, respectively) in the LF. Litterfall was 10.3 Mg/ha·yr and 13.8 Mg/ha·yr respectively. Annual leaf decomposition rate was greater for Ligustrum (k=4.07) than for SM species (k=1.48). The mean residence time of aboveground biomass was 12 yr for the SM and 5 yr for the LF. The k₁ values (litterfall/standing crop) were 1.9 and 2.0 for SM and LF respectively. The influence of coastal road and wall in the hydroperiod, native forested wetland ecosystem survival and exotic forest invasion is discussed.