A high‐throughput method to quantify feeding rates in aquatic organisms: A case study with Daphnia

1. Food ingestion is one of the most basic features of all organisms. However, obtaining precise—and high‐throughput—estimates of feeding rates remains challenging, particularly for small, aquatic herbivores such as zooplankton, snails, and tadpoles. These animals typically consume low volumes of food that are time‐consuming to accurately measure.
2. We extend a standard high‐throughput fluorometry technique, which uses a microplate reader and 96‐well plates, as a practical tool for studies in ecology, evolution, and disease biology. We outline technical and methodological details to optimize quantification of individual feeding rates, improve accuracy, and minimize sampling error.
3. This high‐throughput assay offers several advantages over previous methods, including i) substantially reduced time allotments per sample to facilitate larger, more efficient experiments; ii) technical replicates; and iii) conversion of in vivo measurements to units (mL^‐1 hr^‐1 ind^‐1) which enables broad‐scale comparisons across an array of taxa and studies.
4. To evaluate the accuracy and feasibility of our approach, we use the zooplankton, Daphnia dentifera, as a case study. Our results indicate that this procedure accurately quantifies feeding rates and highlights differences among seven genotypes.
5. The method detailed here has broad applicability to a diverse array of aquatic taxa, their resources, environmental contaminants (e.g., plastics), and infectious agents. We discuss simple extensions to quantify epidemiologically relevant traits, such as pathogen exposure and transmission rates, for infectious agents with oral or trophic transmission.

     

The seasonal dynamics of Copidodiaptomus numidicus (Gurney, 1909) and Thermocyclops dybowskii (Lande, 1890) in Castelo-do-Bode Reservoir

We investigated the population dynamics of Copidodiaptomus numidicus (Copepoda: Calanoida) and Thermocyclops dybowskii (Copepoda: Cyclopoida) in Castelo-do-Bode Reservoir (Portugal). Both species in the reservoir were regulated mainly by food availability during the summers of 1993 and 1994. C. numidicus was always more abundant than T. dybowskii in the reservoir. C. numidicus produced two generations during the sampling period of both years whereas T. dybowskii produced two and three generations in 1993 and 1994, respectively. Because of high temperatures and low rainfall in 1994, the reservoir was more eutrophic than in 1993. Higher clutch size and a higher percentage of ovigerous females suggest that both species were benefiting from better feeding conditions in 1994. Despite this, population growth was reduced in 1994 relatively to 1993. Vertebrate predation and predation by copepods seem to have been minor factors in explaining this decline. However, the presence of Mesostoma sp. in the reservoir may have contributed to the reduction of both copepod populations during specific periods in 1994 although the calanoid was more affected than the cyclopoid by this predation.     

Reproductive activity and physiological status of the calanoid copepods Calanus helgolandicus and Calanoides carinatus under food-limiting conditions

The reproductive activity and the physiological state of the calanoid copepods Calanus helgolandicus and Calanoides carinatus were investigated off the coast of NW Spain during autumn to evaluate the effect of short food resources on both populations. Phytoplankton biomass was low, and neither phytoplankton size distribution nor composition was suitable to support high reproductive rates. Accordingly, egg production rates (EPR) were much lower than maximum rates for both species, pointing to food limitation. The reproductive index (RI), which represents the proportion of females with mature gonads, was < 50% at each of the three zones into which the sampling area was divided (coast, shelf and ocean). Potential recruitment rates were very low except at some nearshore stations, where the highest concentrations of chlorophyll-a (Chl-a), diatoms, dinoflagellates and large cells were found. EPR of C. helgolandicus and C. carinatus were correlated with phytoplankton biomass and unaffected by temperature. Phytoplankton carbon ingestion explained ca. 50% of the variability in EPR for both species. At most of the stations, herbivory was insufficient to cover the carbon requirements for reproduction and respiration, so females probably fed on heterotrophic prey to meet their demands. However, given the low fecundity observed, this omnivorous diet did not seem to be optimum for reproduction, and a severe food limitation is thus suggested. Furthermore, the high C/N values measured point to a notable lipid storage, but given the low EPR found, lipid reserves were probably invested into female maintenance rather than into gonad maturation. C. helgolandicus and C. carinatus populations did not mirror phytoplankton biomass distribution, but they correlated well when considering only copepodites V (CV). The CV could be preparing for the overwintering, storing lipid reserves to ensure a successful diapause, and they could also be advected by the poleward current detected during the study. Females showed a diel feeding rhythm, with highest ingestion rates during night. From our results, it follows that C. helgolandicus and C. carinatus females did not perform diel vertical migrations. We suggest that this behaviour is likely due to the food-limiting conditions, which make it more advantageous to remain at the surface during daytime.     

The effects of manipulation of sedimentary iron and organic matter on sediment biogeochemistry and seagrasses in a subtropical carbonate environment

The microbial metabolism of organic matter (OM) in seagrass beds can create sulfidic conditions detrimental to seagrass growth; iron (Fe) potentially has ameliorating effects through titration of the sulfides and the precipitation of iron-sulfide minerals into the sediment. In this study, the biogeochemical effects of Fe availability and its interplay with sulfur and OM on sulfide toxicity, phosphorous (P) availability, seagrass growth and community structure were tested. The availability of Fe and OM was manipulated in a 2 × 2 factorial experiment arranged in a Latin square, with four replicates per treatment. The treatments included the addition of Fe, the addition of OM, the addition of both Fe and OM as well as no addition. The experiment was conducted in an oligotrophic, iron-deficient seagrass bed. Fe had an 84.5% retention efficiency in the sediments with the concentration of Fe increasing in the seagrass leaves over the course of the experiment. Porewater chemistry was significantly altered with a dramatic decrease in sulfide levels in Fe addition plots while sulfide levels increased in the OM addition treatments. Phosphorus increased in seagrass leaves collected in the Fe addition plots. Decreased sulfide stress was evidenced by heavier δ³⁴S in leaves and rhizomes from plots to which Fe was added. The OM addition negatively affected seagrass growth but increased P availability; the reduced sulfide stress in Fe added plots resulted in elevated productivity. Fe availability may be an important determinant of the impact that OM has on seagrass vitality in carbonate sediments vegetated with seagrasses.     

Global patterns of diversity among the specialist birds of riverine landscapes

1. Despite the growing view that biodiversity provides a unifying theme in river ecology, global perspectives on richness in riverine landscapes are limited. As a result, there is little theory or quantitative data on features that might have influenced global patterns in riverine richness, nor are there clear indications of which riverine landscapes are important to conservation at the global scale. As conspicuous elements of the vertebrate fauna of riverine landscapes, we mapped the global distributions of all of the world's specialist riverine birds and assessed their richness in relation to latitude, altitude, primary productivity and geomorphological complexity (surface configuration). 2. Specialist riverine birds, typical of high‐energy riverine landscapes and dependent wholly or partly on production from river ecosystems, occur in 16 families. They are represented by an estimated 60 species divided equally between the passerines and non‐passerines. Major radiation has occurred among different families on different continents, indicating that birds have evolved several times into the niches provided by riverine landscapes. 3. Continental richness varies from four species in Europe to 28 in Asia, with richness on the latter continent disproportionately larger than would be expected from a random distribution with respect to land area. Richness is greatest in mountainous regions at latitudes of 20–40°N in the riverine landscapes of the Himalayan mountains, where 13 species overlap in range. 4. Family, genus and species richness in specialist riverine birds all increase significantly with productivity and surface configuration (i.e. relief). However, family richness was the best single predictor of the numbers of species or genera. In keeping with the effect of surface configuration, river‐bird richness peaks globally at 1300–1400 m altitude, and most species occur typically on small, fast rivers where they feed predominantly on invertebrates. Increased lengths of such streams in areas of high relief and rainfall might have been responsible for species–area effects. 5. We propose the hypothesis that the diversity in channel forms and habitats in riverine landscapes, in addition to high temperature and primary productivity, have been prerequisites to the development of global patterns in the richness of specialist riverine organisms. We advocate tests of this hypothesis in other taxonomic groups. We draw attention, however, to the challenges of categorically defining riverine organisms in such tests because (i) rivers grade into many other habitat types across several different ecotones and (ii) `terrestrialisation' processes in riverine landscapes means that they offer habitat for organisms whose evolutionary origins are not exclusively riverine.     

Frequency-dependent conspecific attraction to food patches

In many ecological situations, resources are difficult to find but become more apparent to nearby searchers after one of their numbers discovers and begins to exploit them. If the discoverer cannot monopolize the resources, then others may benefit from joining the discoverer and sharing their discovery. Existing theories for this type of conspecific attraction have often used very simple rules for how the decision to join a discovered resource patch should be influenced by the number of individuals already exploiting that patch. We use a mechanistic, spatially explicit model to demonstrate that individuals should not necessarily simply join patches more often as the number of individuals exploiting the patch increases, because those patches are likely to be exhausted soon or joining them will intensify future local competition. Furthermore, we show that this decision should be sensitive to the nature of the resource patches, with individuals being more responsive to discoveries in general and more tolerant of larger numbers of existing exploiters on a patch when patches are resource-rich and challenging to locate alone. As such, we argue that this greater focus on underlying joining mechanisms suggests that conspecific attraction is a more sophisticated and flexible tactic than currently appreciated.     

Microhabitat nest‐site selection by ducks in the boreal forest

The boreal forest is one of the North America’s most important breeding areas for ducks, but information about the nesting ecology of ducks in the region is limited. We collected microhabitat data related to vegetation structure and composition at 157 duck nests and paired random locations in Alberta’s boreal forest region from 2016 to 2018. We identified fine‐scale vegetation features selected by ducks for all nests, between nesting guilds, and among five species using conditional logistic regression. Ducks in the boreal forest selected nest sites with greater overhead and graminoid cover, but less forb cover than random sites. Characteristics of the nest sites of upland‐ and overwater‐nesting guilds differed, with species nesting in upland habitat selecting nests that provided greater shrub cover and less lateral concealment and species nesting over water selecting nests with less shrub cover. We examined the characteristics of nest sites of American Wigeon (Mareca americana), Blue‐winged Teal (Spatula discors), Green‐winged Teal (Anas crecca), Mallards (Anas platyrhynchos), and Ring‐necked Ducks (Aythya collaris), and found differences among species that may facilitate species coexistence at a regional scale. Our results suggest that females of species nesting in upland habitat selected nest sites that optimized concealment from aerial predators while also allowing detection of and escape from terrestrial predators. Consequently, alteration in the composition and heterogeneity of vegetation and predator communities caused by climate change and industrial development in the boreal forest of Canada may affect the nest‐site selection strategies of boreal ducks.