Rafael D. Guariento

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.

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.

Stoichiometry of benthic invertebrate nutrient recycling: interspecific variation and the role of body mass

Ecological stoichiometry (ES) and allometry offer frameworks for predicting how nutrient recycling varies within and among animal species. Despite the importance of benthic-derived nutrients in most aquatic systems, predictions based on ES and allometry have been poorly tested among benthic invertebrate consumers. Here, we show that the rates and ratios at which three freshwater benthic invertebrate species (a crustacean, an insect, and a polychaeta) recycled nitrogen (N) and phosphorus (P) can be partially predicted by ES and allometry depending on whether data are analyzed intra- or interspecifically. Mass-specific N and P excretion rates were negatively correlated with invertebrate body size both among and within taxa, supporting allometric predictions. However, mass-specific N and P excretion rates were positively and negatively correlated to invertebrate body N and P, respectively, but only when data were analyzed intraspecifically. As a corollary, the mass-specific N:P excretion ratio was positively related to body N:P ratio. Such a contrasting pattern on excretion-mediated N and P recycling suggests that stoichiometric constraints regarding consumer-resource imbalances for the three species utilized in this study may be stronger for P than for N. Our results indicate that the variation in nutrient recycling, which is mediated by taxonomic constraints on stoichiometry and allometry, may substantially help us to understand the importance of benthic detritivorous species to the functioning of aquatic ecosystems.

Habitat size determine algae biomass in tank-bromeliads

The primary goal of this study was to evaluate the relative importance of habitat physical properties, bottom-up and top-down factors, and their interaction on algae biomass in tank-bromeliads. We sampled algae biomass (chlorophyll-a concentration), micro-metazoan density, mosquito abundance, and several environmental variables, including nutrient concentrations and characteristics of the habitat physical structure, in a survey of 64 tank-bromeliads of four different species (Aechmea nudicaulis, Aechmea lingulata, Neoregelia cruenta, and Vriesea neoglutinosa). We analyzed the complete and individual bromeliad species datasets using an information-theoretic model selection approach (Akaike’s information criterion). Bromeliad species, maximum water volume, and bromeliad diameter comprised the best model for determining chlorophyll-a concentrations for the complete dataset. The maximum water volume also comprised the best model to explain chlorophyll-a concentrations in three of four bromeliad species datasets. Interactions between consumers and nutrient concentration were included in the subsequent models, but they were not statistically significant. Taken together, these results demonstrate that the impact of habitat size on the associated autotrophic biomass occurs possibly via changes in community susceptibility to disturbances, particularly drought. We can conclude that habitat size is more important than resource availability or herbivory on phytotelm autotrophic biomass regulation in these natural microcosms.

Interactive effects of predation risk and conspecific density on the nutrient stoichiometry of prey

Abstract The mere presence of predators (i.e., predation risk) can alter consumer physiology by restricting food intake and inducing stress, which can ultimately affect prey‐mediated ecosystem processes such as nutrient cycling. However, many environmental factors, including conspecific density, can mediate the perception of risk by prey. Prey conspecific density has been defined as a fundamental feature that modulates perceived risk. In this study, we tested the effects of predation risk on prey nutrient stoichiometry (body and excretion). Using a constant predation risk, we also tested the effects of varying conspecific densities on prey responses to predation risk. To answer these questions, we conducted a mesocosm experiment using caged predators (Belostoma sp.), and small bullfrog tadpoles (Lithobates catesbeianus) as prey. We found that L. catesbeianus tadpoles adjust their body nutrient stoichiometry in response to predation risk, which is affected by conspecific density. We also found that the prey exhibited strong morphological responses to predation risk (i.e., an increase in tail muscle mass), which were positively correlated to body nitrogen content. Thus, we pose the notion that in risky situations, adaptive phenotypic responses rather than behavioral ones might partially explain why prey might have a higher nitrogen content under predation risk. In addition, the interactive roles of conspecific density and predation risk, which might result in reduced perceived risk and physiological restrictions in prey, also affected how prey stoichiometry responded to the fear of predation.

The negative effects of temperature increase on bacterial respiration are independent of changes in community composition

Temporal changes in environmental conditions and in bacterial community composition (BCC) regulate bacterial processes and ecosystem services. An increase in temperature accelerates bacterial processes in polar or temperate regions, but this relationship has not been documented for the tropics. Here, we tested the interactive effects of changing the BCC and increasing the water temperature on tropical bacterial respiration (BR). The BCC was manipulated through successional changes of the bacterial community in a filtered water sample from a tropical coastal lagoon. Four succession incubation periods (120, 240, 288 and 336 h) and four different water temperatures (23, 28, 33 and 38(o)C) were tested in a full-factorial design microcosm experiment. Both the BCC and the temperature had significant individual, but not interactive, effects on BR. Temperature increasing consistently decreased BR, while there was no clear pattern of successional effects on BR observed. No BCC tested was able to diminish the negative effects of temperature increases on BR. Our results suggest that the effects of an increasing temperature can negatively affect BR, even in tropical ecosystems with different BCC.

The several faces of fear: ecological consequences of predation risk in a lagoon model system

OBJETIVO: O objetivo deste estudo foi avaliar o papel do risco de predacao na ocorrencia de cascatas troficas em cadeias bentonicas e detectar se as consequencias ecologicas do risco de predacao podem reverberar em padroes observados em diferentes escalas hierarquicas, como o tamanho de presas, a eficiencia de crescimento de presas e padroes de reciclagem de nutrientes. METODOS: O sistema modelo utilizado no presente experimento consistiu em uma cadeia linear simples contendo um predador, um consumidor e perifiton como o recurso basal. Durante 2 semanas nos manipulamos o risco de predacao utilizando predadores enjaulados, incapazes de matar suas presas, em doze mesocosmos independentes, simulando as condicoes naturais lagunares. RESULTADOS: Nossos resultados mostraram que o risco de predacao pode ser responsavel pela ocorrencia de cascatas troficas e que a forca da cascata e proporcional a intensidade do risco. O risco de predacao tambem pode ser influenciar negativamente a biomassa e a taxa de crescimento de presas, bem como afetar as taxas de reciclagem de nutrientes por alterar as taxas de excrecao de nutrientes. Atraves de uma formulacao matematica simples, nos procuramos mostrar que resultados experimentais no nivel de individuos podem ser generalizados para populacoes naturais se restricoes evolutivas ao fitness de presas podem ser reproduzidos em condicoes experimentais. CONCLUSOES: Nossos resultado corroboram para integrar dinâmicas ecossistemicas ao comportamento animal, ressaltando que nao apenas mecanismos ascendentes mas tambem mecanismos descendentes sao responsaveis pela determinacao de propriedades ecossistemicas. Nos sugerimos, em ultima analise, que o forrageamento adaptativo de presas pode servir para integrar a Ecologia Evolutiva e a Ecologia de Ecossistemas, resultando no desenvolvimento de uma teoria mais preditiva e robusta a respeito do funcionamento de ecossistemas aquaticos.

Effects of food web structure and resource subsidies on the patterns and mechanisms of temporal coherence in a tropical coastal lagoon: an experimental mesocosm approach

OBJETIVOS: Estudos sobre os padroes e mecanismos de coerencia temporal de variaveis ecologicas entre lagos tem se tornado um tema importante na limnologia. Ate o momento, nenhum estudo testou se e como a oferta de recursos e a configuracao da teia trofica afetam os padroes e mecanismos da coerencia temporal de variaveis limnologicas. Nos conduzimos um experimento de mesocosmos em campo durante 11 semanas para testar as seguintes hipoteses: (i) a adicao de nutrientes reduz a coerencia temporal de variaveis ecossistemicas; (ii) a predacao por peixes potencializa a coerencia temporal de variaveis ecossistemicas e (iii) a coerencia temporal e mais forte para variaveis fisicas (transparencia da agua), intermediaria para variaveis quimicas (concentracao de oxigenio dissolvido [OD]) e fraca para variaveis biologicas (biomassa zooplanctonica total). METODOS: Nos manipulamos a presenca de peixe e a adicao de nutrientes inorgânicos (N e P) em um desenho fatorial 2 × 2 em dezesseis mesocosmos instalados em uma lagoa costeira tropical. A coerencia foi estimada por correlacoes de Pearson par-a-par das trajetorias temporais de cada variavel resposta entre os mesocosmos de um mesmo tratamento. RESULTADOS: A presenca de peixes aumentou significativamente apenas a coerencia temporal da biomassa zooplanctonica, e, contrario as nossas expectativas, a adicao de nutrientes aumentou a coerencia temporal da [OD]. A intensidade dos efeitos da presenca de peixe e da adicao de nutrientes sobre a coerencia temporal foi afetada pela identidade da variavel monitorada, mas nao em um padrao consistente. No entanto, a interacao da presenca de peixe e adicao de nutrientes nao afetaram a coerencia temporal de nenhuma variavel monitorada. CONCLUSOES: Nossos resultados indicam que a predacao de peixes e a disponibilidade de recursos podem afetar significativamente padroes de coerencia temporal, mas tais efeitos dependerao mais de efeitos diretos do fator local sobre a variavel do que da identidade da propria variavel. Concluimos que a eutrofizacao e a sobrepesca podem interferir no acoplamento da dinâmica espaco-temporal de algumas variaveis limnologicas.