Quantitative assessment of aerosolized cyanobacterial toxins at two New Zealand lakes

The cyanobacterial toxins, nodularin and microcystin, are highly efficient inhibitors of cellular protein phosphatases. Toxicity primarily evolves following ingestion of cyanobacterial material or toxins and results in liver and renal pathology. Ingestion is the main route of exposure in the World Health Organizations current risk assessment of nodularin and microcystins. Nasally applied microcystin appears to have a 10-fold higher availability and toxicity than orally ingested toxins, suggesting that aerosolized toxins could represent a major risk for human populations close to lakes with cyanobacterial blooms. In this study, nodularin and microcystin levels in aerosols were assessed using high and low volume air samplers for 4, 12 and 24 h periods at lakes Forsyth and Rotorua (South Island, New Zealand). These lakes were experiencing blooms of Nodularia spumigena and Microcystis sp., respectively. Using the high volume samplers up to 16.2 pg m(-3) of nodularin and 1.8 pg m(-3) of microcystins were detected in the air. Aerosolized nodularin and microcystins do not appear to represent an acute or chronic hazard to humans. The latter was concluded based on calculations using average human air intakes, the highest nodularin or microcystin concentrations measured in the air in this study, and assuming inhalatory toxicities comparable to toxicological data obtained following intraperitoneal applications in mice. However, as the toxin concentrations in the air were calculated over extended sampling periods, peak values may be underestimated. Aerosolized toxins should be considered when developing risk assessments particularly for lakeside populations and recreational users where inhalation of cyanotoxins may be a secondary exposure source to a primary oral exposure.     

Lethal and nonlethal predator effects on an herbivore guild mediated by system productivity

Indirect effects propagated through intervening species in a food web have important effects on community properties. Traditionally, these indirect effects have been conceptualized as mediated through density changes of the intervening species, but it is becoming increasingly apparent that those mediated through trait (phenotypic) responses also can be very important. Because density- and trait-mediated indirect effects have different properties, it is critical that we understand the mechanisms of transmission in order to predict how they will interact, and when or where they will be important. In this study, we examined the mechanisms and consequences of the lethal (density-mediated) and nonlethal (trait-mediated) effects of a larval odonate predator on a guild of four herbivore species (a larval anuran and three species of snails) and their resources. We also manipulated system productivity in order to explore the effects of environmental context on the transmission of these two types of indirect effects. We show that trait-mediated effects arising from the predator can be very strong relative to density-mediated effects on both the competing herbivores and the species composition and production of their resources. A number of these indirect effects are shown to be contingent on productivity of the system. We further present evidence that trait- and density-mediated indirect effects originating from a predator may be transmitted independently through different routes in a food web, particularly when spatial responses of the transmitting prey are involved. Finally, effects on prey growth due to trait responses to the predator varied from negative to positive in predictable ways as a function of time and indirect effects on the larger food web. These results indicate the important role that trait-mediated indirect effects can play in trophic cascades and keystone predator interactions, and we discuss how the mechanisms involved can be incorporated in theory.     

Consumer-resource body-size relationships in natural food webs

It has been suggested that differences in body size between consumer and resource species may have important implications for interaction strengths, population dynamics, and eventually food web structure, function, and evolution. Still, the general distribution of consumer-'resource body-size ratios in real ecosystems, and whether they vary systematically among habitats or broad taxonomic groups, is poorly understood. Using a unique global database on consumer and resource body sizes, we show that the mean body-size ratios of aquatic herbivorous and detritivorous consumers are several orders of magnitude larger than those of carnivorous predators. Carnivorous predator-prey body-size ratios vary across different habitats and predator and prey types (invertebrates, ectotherm, and endotherm vertebrates). Predator-prey body-size ratios are on average significantly higher (1) in freshwater habitats than in marine or terrestrial habitats, (2) for vertebrate than for invertebrate predators, and (3) for invertebrate than for ectotherm vertebrate prey. If recent studies that relate body-size ratios to interaction strengths are general, our results suggest that mean consumer-resource interaction strengths may vary systematically across different habitat categories and consumer types.