Francisco de Assis Esteves

Photoproduction of dissolved inorganic carbon in temperate and tropical lakes – dependence on wavelength band and dissolved organic carbon concentration

We have evaluated photoeffects of UV-B, UV-A and PAR radiation on dissolved organic matter (DOM). Photochemical production of dissolved inorganic carbon (DIC) was measured in sterile lake water from Sweden and Brazil after 6 hours of sun exposure. Tubes were exposed to four solar radiation regimes: Full-radiation, Full-radiation minus UV-B, Full-radiation minus UV-B and UV-A (PAR) and darkness.In both areas, lakes with most DOC (varying between 3 and 40 mg C l-1) were highly humic, resulting in high UV-B attenuation coefficients (Kd = 5–466 m-1). Under Full-radiation, photooxidative DIC-production varied from 0.09 to 1.7 mg C l-1per 6 h, without UV-B from 0.07 to 1.4 mg C l-1 and with PAR only from 0.02 to 0.7 mg C l-1. UV-B radiation explains a minor part (17%) of the photoooxidative DIC-production, while UV-A and PAR have larger effects (39% and 44%, respectively). Photooxidation was proportional to DOC-content and DIC-production was positively related to decrease in DOC and to loss of absorbance at 250 nm. There was no significant difference in DOC and radiation normalized DIC-production between Swedish and Brazilian lakes. The UV-B dose during incubations was approximately 3 times higher in Brazil compared to Sweden, while UV-A and PAR doses were similar. We conclude that DOC from tropical and temperate freshwaters do not seem to differ with respect to sensitivity to photooxidation.

Neotropical coastal lagoons: an appraisal of their biodiversity, functioning, threats and conservation management

Neotropical coastal lagoons (NCL) are human-dominated ecosystems. Their distribution along densely populated coastal areas of developing countries makes these systems among the most threatened in the world. Here, we summarize some aspects of the causes and consequences of NCL biodiversity, their functioning, their importance to the surrounding populations, their fragility, and their responses to local and global anthropogenic impacts and the challenges that Neotropical countries face in conserving these systems. Although still scarce and geographically concentrated, a growing body of studies has shown that NCLs are physiographically diversified systems, which harbor a considerable and particular proportion of the Neotropical inland aquatic biodiversity. Despite the fact that coastal lagoons are ecotones that are intricately connected to surrounding environments, they develop mechanisms for structural and functional regulation, which confer to these systems higher productivity and carrying capacities than surrounding ecosystems. Such traits attract residential developments and subsidize local traditional populations with important economic and aesthetic ecosystem revenues such as fisheries and scenic beauty. However, the disorganized human occupation around NCLs are causing profound impacts such as eutrophication, salinization, exotic species introduction, as well as other effects, which are ultimately imposing major habitat degradations and biodiversity extirpations in NCLs. We argue that interdisciplinary conservation strategies, which integrate scientific expertise, government officials, private companies and the general public, are the most likely to overcome the geographic and economic obstacles to NCL conservation.

Nutrient limitation of bacterial production in clear water Amazonian ecosystems

The aim of this research was to determine the main limiting nutrient (carbon, nitrogen or phosphorus) to bacterial production in different clear water Amazonian ecosystems during the high water period, when there is influence of the flooded land, mainly as sources of organic matter. Five stations were sampled in three clear water ecosystems: Trombetas River, Lake Batata and Caranã Stream. We estimated in each station the nutrient concentration, bacterial production and bacterial abundance. The experiment was set up with GF/F filtered water from all stations together with additions of glucose (400 μM C), KNO3 (15 μM N) and KH2PO4 (5 μM P) in accordance with each treatment (C, N, P ,CN, CP, NP, CNP and no amends). Bacterial production was estimated after 24 h of incubation. We observed that the values of bacterial production after additions of phosphate alone (P treatment) were 2- to 6-fold greater than the values measured in control flasks. Additions of nitrate (N treatment) and glucose alone (C treatment) had no effect on the bacterial production in four out of five ecosystems studied. However, additions of glucose with phosphate (CP treatment) strongly stimulated bacterial production in all ecosystems studied, including treatments with phosphate addition only. We conclude that phosphorus is the main limiting nutrient to bacterioplankton production in these clear water Amazonian ecosystems during the high water period. In addition, we conclude that, together with phosphorus, additions of glucose stimulated the bacterial production mainly due to the low quality of the carbon pool present in these ecosystems.

The prominence of and biases in biodiversity and ecosystem functioning research

The sub-discipline of biodiversity and ecosystem functioning (BEF) has emerged as a central topic in contemporary ecological research. However, to date no study has evaluated the prominence and publication biases in BEF research. Herein we report the results of a careful quantitative assessment of BEF research published in five core general ecology journals from 1990 to 2007 to determine the position of BEF research within ecology, identify patterns of research effort within BEF research, and discuss their probable proximal and historical causes. The relative importance of BEF publications increased exponentially during the period analyzed and was significantly greater than the average growth of ecological literature, affirming the prominence of BEF as a current paradigm in ecology. However, BEF research exhibited a strong bias toward experimental studies on terrestrial plant communities, with significantly lower effort devoted to the functional aspects of biodiversity in aquatic systems, multiple trophic level systems, and animal or microbial communities. Such trends may be explained by a combination of methodological adequacy and historic epistemological differences in ecological thinking. We suggest that BEF researchers should direct more effort toward the study of aquatic systems and animal communities, emphasize long-term and trophically complex experiments, such as those with multi-trophic microbial communities, employ larger-scale field observational studies and increase the use of integrative and theoretical studies. Many technical and analytical methodologies that are already employed in ecological research, such as stable isotopes, paleobiology, remote sensing, and model selection criteria, can facilitate these aims. Overcoming the above-mentioned shortcomings of current BEF research will greatly improve our ability to predict how biodiversity loss will affect ecosystem processes and services in natural ecosystems.

Synergy of Fresh and Accumulated Organic Matter to Bacterial Growth

The main goal of this research was to evaluate whether the mixture of fresh labile dissolved organic matter (DOM) and accumulated refractory DOM influences bacterial production, respiration, and growth efficiency (BGE) in aquatic ecosystems. Bacterial batch cultures were set up using DOM leached from aquatic macrophytes as the fresh DOM pool and DOM accumulated from a tropical humic lagoon. Two sets of experiments were performed and bacterial growth was followed in cultures composed of each carbon substrate (first experiment) and by carbon substrates combined (second experiment), with and without the addition of nitrogen and phosphorus. In both experiments, bacterial production, respiration, and BGE were always higher in cultures with N and P additions, indicating a consistent inorganic nutrient limitation. Bacterial production, respiration, and BGE were higher in cultures set up with leachate DOM than in cultures set up with humic DOM, indicating that the quality of the organic matter pool influenced the bacterial growth. Bacterial production and respiration were higher in the mixture of substrates (second experiment) than expected by bacterial production and respiration in single substrate cultures (first experiment). We suggest that the differences in the concentration of some compounds between DOM sources, the co-metabolism on carbon compound decomposition, and the higher diversity of molecules possibly support a greater bacterial diversity which might explain the higher bacterial growth observed. Finally, our results indicate that the mixture of fresh labile and accumulated refractory DOM that naturally occurs in aquatic ecosystems could accelerate the bacterial growth and bacterial DOM removal.

Ecological determinism increases with organism size

After much debate, there is an emerging consensus that the composition of many ecological communities is determined both by species traits, as proposed by niche theory, as well as by chance events. A critical question for ecology is, therefore, which attributes of species predict the dominance of deterministic or stochastic processes. We outline two hypotheses by which organism size could determine which processes structure ecological communities, and we test these hypotheses by comparing the community structure in bromeliad phytotelmata of three groups of organisms (bacteria, zooplankton, and macroinvertebrates) that encompass a 10 000-fold gradient in body size, but live in the same habitat. Bacteria had no habitat associations, as would be expected from trait-neutral stochastic processes, but still showed exclusion among species pairs, as would be expected from niche-based processes. Macroinvertebrates had strong habitat and species associations, indicating niche-based processes. Zooplankton, with body size between bacteria and macroinvertebrates, showed intermediate habitat associations. We concluded that a key niche process, habitat filtering, strengthened with organism size, possibly because larger organisms are both less plastic in their fundamental niches and more able to be selective in dispersal. These results suggest that the relative importance of deterministic and stochastic processes may be predictable from organism size.

The relationship between DOC and planktonic bacteria in tropical coastal lagoons

We evaluated the relationship between DOC concentration and bacterial production, abundance, and biomass in four tropical coastal lagoons in the state of Rio de Janeiro, Brazil. We also evaluated the main nutrients limiting bacterial production in these lagoons, the bioavailability of bulk DOC to planktonic bacteria, and the Bacte- rial Growth Efficiency (BGE) of natural assemblages of bacteria; as well as how enrichment with inorganic nutrients affects BGE and DOC utilization by bacteria. Based on the DOC concentration and bacterial production, abundance, and biomass, sampled monthly, there was no correlation between DOC concentration and planktonic bacteria. Phosphorus is the main nutrient limiting bacterial production in these eco- systems; therefore, bacteria can be expected to be more dependent on phosphorus than on carbon concentration. Despite the high concentrations of carbon, addition of glu- cose (labile carbon) together with phosphorus stimulated bacterial production in two of the four lagoons, indicating that the quality of bulk DOC also limits bacteria there. Bacteria, in natural conditions, consumed from 2.3 to 8.0% of bulk DOC in the la- goons, and showed BGEs from 11 to 14%. Therefore, a large proportion of the DOC entering these ecosystems might accumulate. There was an increase in DOC consump- tion by bacteria and BGE after addition of nutrients. However, only a minor fraction of DOC (6.6 to 17.4%) was consumed by bacteria. We concluded that low phosphorus concentration and low quality of bulk DOC are the main bottom-up factors limiting bacterial growth in these tropical coastal lagoons.

Natural diet of three species of shrimp in a tropical coastal lagoon

The gut content of 495 specimens of Farfantepenaeus brasiliensis, 131 of F. paulensis (Penaeidae) and 102 of Macrobrachium acanthurus (Palaemonidae) were analyzed to establish the composition of their diets F. brasiliensis had as the most important feeding items in its diet larvae of Chironomidae, Polychaeta and Heleobia australis (Mollusca). For F. paulensis, the most important items were the same as for F. brasiliensis, but the order of importance of H. australis and Polychaeta was inverted. M. acanthurus had detritus as the most important item, followed by Chironomidae larvae, Odonata nymphs, and fragments of the macroalgae Chara. The results showed that the three species were omnivorous, with a varied diet including both components of macrofauna of benthos and associated to the macroalgae Chara and plant fragments and detritus.

Primary production and mortality of Eleocharis interstincta in response to water level fluctuations

This study evaluated the net aboveground primary production (NAPP) of Eleocharis interstincta in response to water level fluctuations. Sampling was carried out every 2 weeks from June 1997 to June 1998 in the littoral region of the Jurubatiba (coastal) lagoon (Rio de Janeiro), where three 0.0625 m 2 study plots were selected for cohort observation. Three quadrats from the same area were also harvested. In the laboratory, stems were oven-dried at 70 ◦ C for 3 days to reach constant weight, their heights measured and their weights taken to determine biomass. The water level fluctuated seasonally, with two periods when the water failed to cover the macrophyte stand—the end of winter (natural dry out) and in summer (artificial breaching of the sandbar). Cohort average height and mean water level in the stand were positively correlated (r = 0.91, P< 0.01). The relative growth rate of E. interstincta was not influenced by water level fluctuations, since different growth rates were observed even when the water level was stable ( P< 0.05). Stem mortality and herbivory occurred throughout the year and accounted for 14.8 and 1.8% of net annual aboveground primary production (NAAPP), respectively. NAPP varied from 0.5 to 1.8 g dry weight m −2 per day, and there was a significant correlation with average water level ( P< 0.05), indicating that water level has an important role in E. interstincta dynamics.

Temporal coherence among tropical coastal lagoons: a search for patterns and mechanisms.

Temporal coherence (i.e., the degree of synchronicity of a given variable among ecological units within a predefined space) has been shown for several limnological features among temperate lakes, allowing predictions about the structure and function of ecosystems. However, there is little evidence of temporal coherence among tropical aquatic systems, where the climatic variability among seasons is less pronounced. Here, we used data from long-term monitoring of physical, chemical and biological variables to test the degree of temporal coherence among 18 tropical coastal lagoons. The water temperature and chlorophyll-a concentration had the highest and lowest temporal coherence among the lagoons, respectively, whereas the salinity and water colour had intermediate temporal coherence. The regional climactic factors were the main factors responsible for the coherence patterns in the water temperature and water colour, whereas the landscape position and morphometric characteristics explained much of the variation of the salinity and water colour among the lagoons. These results indicate that both local (lagoon morphometry) and regional (precipitation, air temperature) factors regulate the physical and chemical conditions of coastal lagoons by adjusting the terrestrial and marine subsidies at a landscape-scale. On the other hand, the chlorophyll-a concentration appears to be primarily regulated by specific local conditions resulting in a weak temporal coherence among the ecosystems. We concluded that temporal coherence in tropical ecosystems is possible, at least for some environmental features, and should be evaluated for other tropical ecosystems. Our results also reinforce that aquatic ecosystems should be studied more broadly to accomplish a full understanding of their structure and function.