High Grazing Rates Research Articles

Contrasting responses of different mixotrophic protists to light and nutrient availability

AbstractMixotrophic protists are important members of aquatic microbial food webs where they can dominate bacterivory and strongly impact energy and nutrient flow. While light and nutrient availability are known to impact grazing rates by mixotrophs in laboratory studies, little is known about how changes in resource availability affect mixotrophic organisms in natural communities. Here, we performed a short‐term grazing assay using fluorescently labeled bacteria in a phosphorus‐limited subalpine mountain lake to test how the availability of light and nutrients affect mixotroph grazing rates. High grazing rates by a Pseudopedinella‐like dictyochophyte were unaffected by nutrient or light manipulation. In contrast, grazing rates by pigmented cryptophytes significantly decreased upon phosphorus addition, light deprivation, and a combination of both. We compare these findings with the seasonal analysis of a 4‐yr time‐series dataset of 18S rRNA gene amplicon abundances. While cryptophytes dominated in autumn and winter and were negatively related with water column stability, Pseudopedinella amplicon abundances showed a consistent spring–summer peak and were positively related with chlorophyll a concentrations. These results demonstrate the diversity in nutritional strategies among co‐occurring mixotrophic protists, which cannot be resolved by quantification of bulk grazing rates. Moreover, these different strategies result in distinct seasonal dynamics and coexistence of different mixotrophs, suggesting their respective trajectories into future aquatic ecosystems will also differ.

Modeling the metabolic evolution of mixotrophic phytoplankton in response to rising ocean surface temperatures

BackgroundClimate change is expected to lead to warming in ocean surface temperatures which will have unequal effects on the rates of photosynthesis and heterotrophy. As a result of this changing metabolic landscape, directional phenotypic evolution will occur, with implications that cascade up to the ecosystem level. While mixotrophic phytoplankton, organisms that combine photosynthesis and heterotrophy to meet their energetic and nutritional needs, are expected to become more heterotrophic with warmer temperatures due to heterotrophy increasing at a faster rate than photosynthesis, it is unclear how evolution will influence how these organisms respond to warmer temperatures. In this study, we used adaptive dynamics to model the consequences of temperature-mediated increases in metabolic rates for the evolution of mixotrophic phytoplankton, focusing specifically on phagotrophic mixotrophs.ResultsWe find that mixotrophs tend to evolve to become more reliant on phagotrophy as temperatures rise, leading to reduced prey abundance through higher grazing rates. However, if prey abundance becomes too low, evolution favors greater reliance on photosynthesis. These responses depend upon the trade-off that mixotrophs experience between investing in photosynthesis and phagotrophy. Mixotrophs with a convex trade-off maintain mixotrophy over the greatest range of temperatures; evolution in these “generalist” mixotrophs was found to exacerbate carbon cycle impacts, with evolving mixotrophs exhibiting increased sensitivity to rising temperature.ConclusionsOur results show that mixotrophs may respond more strongly to climate change than predicted by phenotypic plasticity alone due to evolutionary shifts in metabolic investment. However, the type of metabolic trade-off experienced by mixotrophs as well as ecological feedback on prey abundance may ultimately limit the extent of evolutionary change along the heterotrophy-phototrophy spectrum.

Growth, biomass and productivity of the seagrass Thalassia hemprichii at Ashmore Reef, Australia

Ashmore Reef, located approximately 630 km north of Broome, Australia, is notable for its contribution to overall seagrass cover in the Timor Province bioregion, providing valuable ecosystem services including critical habitat for feeding and breeding green turtles (Chelonia mydas) and a small geographically isolated population of dugong (Dugong dugon). Yet, seagrass surveys at Ashmore Reef have not been completed since 2005. Seagrass was surveyed in 2019, with only two (Thalassia hemprichii and Halophila ovalis) of the five previously recorded species observed. Monitoring survey sites were established to determine density, biomass and productivity of the dominant species, T. hemprichii. Overall mean total biomass was 116.9 DW m−2 with an above- and below-ground biomass ratio of 1:2.5. Mean shoot density was 407 shoots m−2 (range 88.9–600 shoots m−2) with a mean canopy height of 42.8 mm (range 10–80 mm), and average blade length of 37.6 mm ( ± 1.1 SE) and blade width of 4.5 mm ( ± 0.2). Mean leaf growth was calculated as 2.2–2.4 mm day−1 and shoot growth 3.2–3.5 mm day−1. Canopy height increased by 5.3–5.7 % day−1 with a turnover rate of 17.5–21.3 days. Mean biomass of emergent leaves was 8 g DW m−2 with a grazing rate of 0.4 g DW m−2 day −1 or 123 kg DW ha−1 month−1. This study suggests that T. hemprichii at Ashmore Reef is productive and may support high turtle grazing rates.

Rising temperature more strongly promotes low-abundance Paramecium to remove Microcystis and degrade microcystins

Driven by global warming and eutrophication, Microcystis blooms have posed a severe threat to freshwater ecosystems, especially their derived pollutants cause serious harm to aquatic organisms, thus it is urgent to develop an effective strategy to eliminate nuisance Microcystis. Some protozoa can efficiently graze on toxic Microcystis aeruginosa and degrade cyanotoxins, and play a vital role in regulating harmful cyanobacteria. In the process of protozoa feeding on harmful algae, both temperature and protozoa population density are critical factors that affect the consequences of harmful M. aeruginosa population dynamics. In this study, we first found that Paramecium multimicronucleatum has strong ability to feed on M. aeruginosa, and then studied the interactive effects between temperature and initial density of P. multimicronucleatum on controlling M. aeruginosa. Results showed that increasing temperature accelerated the elimination of M. aeruginosa by P. multimicronucleatum, e.g. the time for M. aeruginosa elimination at 32 °C was shortened to 3.5–4 days. The higher temperatures (26, 29, and 32 °C) were more conducive to improve the efficiency of controlling M. aeruginosa by P. multimicronucleatum with low initial density (10 inds mL−1). Furthermore, P. multimicronucleatum can rapidly degrade microcystins, and the degradation ratio approximately 100% at 32 °C after 6 days. This is the first study to discover that P. multimicronucleatum can high efficiently graze on M. aeruginosa and has a much higher grazing rate (3.5–5.5 × 104Microcystis Paramecium−1 d−1) than other protozoa. These findings contribute to the establishment of a new feasible method for the biological control of M. aeruginosa, and provide a theoretical guidance for the practical application of P. multimicronucleatum in the removal of M. aeruginosa.

Intrinsic timescales of variability in a marine plankton model

Phytoplankton are known to exhibit temporal variability in biomass and community composition. While physically driven sources of variability have been studied extensively, ecosystems often exhibit complicated intrinsic dynamics that are not as well understood. As a first step towards assessing the contribution of this intrinsic variability to the total variability in the ocean, we examine the temporal scales of intrinsic variability in a marine plankton model suitable for use in climate model projections. Our rationale is that a better understanding of the time scales over which intrinsic variability manifests could help in the attribution of observed variability. Our model includes multiple phytoplankton, dissolved inorganic nutrients, and zooplankton and supports two oscillatory mechanisms: “R-oscillations”, corresponding to patterns of species succession and associated with changes in resources, and “Z-oscillations”, corresponding to changes in total phytoplankton biomass due to predator–prey interactions.Over a wide range of model parameters, we found that while Z-oscillations typically occurred on time scales not exceeding 60 days, R-oscillations ranged from roughly 100 to 900 days under predation-free conditions, and R-oscillations occurred on longer time scales when interacting with Z-oscillations. Thus the two kinds of oscillations can be easily distinguished. At high grazing rates, we identified aperiodic cases where the dominant period never resolved, with distinct regimes emerging over decadal (or longer) time scales. These chaotic regime shifts are likely highly dependent on the model parameters and structure. More work must be done to understand how these oscillations interact with physical forcings.

Occurrence of enhanced herbivory in the microbial food web of a tropical estuary during southwest monsoon

Tropical monsoonal estuaries have a significant role in global fishery as they act as rich feeding and breeding grounds for many commercial species. Food web dynamic studies in these coastal ecosystems are mainly focused on the classic food chain which assumes that microbial food webs contribute more towards the recycling of organic carbon and nutrients rather than production. But recently it is observed that phyto-microbial food web which is largely dependent on pico-autotrophs can contribute significantly towards the higher trophic level. Based on these observations we have analysed phyto-microbial pathway of food web in a highly productive tropical monsoonal estuary. We assessed the grazer-prey interaction in the microbial food web of Cochin estuary during spring intermonsoon and southwest monsoon seasons. Our results show that during spring intermonsoon the food web is dependent more on heterotrophic bacteria, but in southwest monsoon the dependency of grazers switches over to autotrophic picoplankton. We also analysed the growth rate, grazing rate and carbon contribution of autotrophic picoplankton experimentally during both seasons and found that autotrophic picoplankton show higher growth rate during southwest monsoon. The higher grazing rate and clearance rate of autotrophic picoplankton recorded during southwest monsoon indicate the efficient consumption of autotrophic picoplankton crop during the season. We could also observe a change in autotrophic picoplankton community structure between the seasons with the dominance of picocyanobacteria during spring intermonsoon and that of picoeukaryotes during southwest monsoon. Thus we infer that high freshwater influx into the estuary during monsoon results in high turbid and low saline (mesohaline to oligohaline) environment which increases the growth rate of low light adapted autotrophic picoplankton populations and in turn enhances herbivory (phyto-microbial pathway) in the microbial food web. Apart from protists, autotrophic picoplankton were found to be widely consumed by many larval forms of mesozooplankton, gastropods, bivalves, rotifers and large sized filter feeders either as aggregates or as unicellular forms. Thus monsoon enhanced herbivory can have a significant role in the growth and survival of larval forms recruited into the estuary during monsoon season and in turn the fishery.

Implications of Pyrosoma atlanticum range expansion on phytoplankton standing stocks in the Northern California Current

Coinciding with the marine heat wave in 2014 and a strong El Niño in 2016, blooms of the tropical-temperate pelagic colonial tunicate, Pyrosoma atlanticum, expanded into the Northern California Current (NCC). Given the high densities of P. atlanticum colonies observed in the NCC, and their potential for high grazing rates, we hypothesized that the species could have a substantial negative effect on the phytoplankton standing stock in the Northeastern Pacific Ocean. In the late winter, spring and early autumn of 2018, we measured abundances and length frequencies of P. atlanticum colonies off the coast of Oregon and estimated their spring grazing impact on the local phytoplankton standing stock. Abundances of P. atlanticum peaked in late winter (0.7 colonies m−3), dropped off in spring (0.42 colonies m−3), and declined to near-absent in early autumn (0.07 colonies m−3). We observed a doubling in the median size of P. atlanticum colonies from late winter to spring, followed by a decline in size in early autumn. With their high individual colony gut pigment contents and bloom densities, the overall spring grazing impact of P. atlanticum was substantial, reaching a maximum of 22% of the available phytoplankton standing stock.

Possible control of acute outbreaks of a marine fungal pathogen by nominally herbivorous tropical reef fish

Primary producers in terrestrial and marine systems can be affected by fungal pathogens threatening the provision of critical ecosystem services. Crustose coralline algae (CCA) are ecologically important members of tropical reef systems and are impacted by coralline fungal disease (CFD) which manifests as overgrowth of the CCA crust by fungal lesions causing partial to complete mortality of the CCA host. No natural controls for CFD have been identified, but nominally herbivorous fish could play a role by consuming pathogenic fungi. We documented preferential grazing on fungal lesions by adults of six common reef-dwelling species of herbivorous Acanthuridae and Labridae, (surgeonfish and parrotfish) which collectively demonstrated an ~ 80-fold higher grazing rate on fungal lesions relative to their proportionate benthic coverage, and a preference for lesions over other palatable substrata (e.g. live scleractinian coral, CCA, or algae). Furthermore, we recorded a ~ 600% increase in live CFD lesion size over an approximately 2-week period when grazing by herbivorous fish was experimentally excluded suggesting that herbivorous reef fish could control CFD progression by directly reducing biomass of the fungal pathogen. Removal rates may be sufficient to allow CCA to recover from infection and explain historically observed natural waning behaviour after an outbreak. Thus, in addition to their well-known role as determinants of macroalgal overgrowth of reefs, herbivorous fish could thus also be important in control of diseases affecting crustose coralline algae that stabilize the foundation of coral reef substrata.

Effects of the cleaner fish Labroides dimidiatus on grazing fishes and coral reef benthos

Territorial and roving grazing fishes farm, and feed on, algae, sediment, or detritus, thus exerting different influences on benthic community structure, and are common clients of cleaner fish. Whether cleaners affect grazing-fish diversity and abundance, and indirectly the benthos, was tested using reefs maintained free of the bluestreak cleaner wrasseLabroides dimidiatusfor 8.5 yr (removals) compared with controls. We quantified fish abundance per grazing functional group, foraging rates of roving grazers, cleaning rates of roving grazers byL. dimidiatus, reef benthos composition, and fouling material on settlement tiles. Abundances of ‘intensive’ and ‘extensive’ territorial farmers, non-farmers, parrotfishes andAcanthurusspp. were lower on removal than control reefs, but this was not the case for ‘indeterminate’ farmers andCtenochaetus striatus. Foraging rates ofAcanthurusspp. andC. striatuswere unaffected by cleaner presence or cleaning duration. This suggests some robustness of the grazers’ foraging behaviour to loss of cleaners.Acanthurusspp. foraged predominantly on sediment and detritus, whereasC. striatusand parrotfishes grazed over algal turfs. Nevertheless, benthic community structure and amount of organic and inorganic material that accumulated over 3.5 mo on tiles were not affected by cleaner presence. Thus, despite greater abundances of many roving grazers, and consequently higher grazing rates being linked to the presence of cleaners, the benthos was not detectably affected by cleaners. This reveals that the positive effect of cleaners on fish abundance is not associated with a subsequent change in the benthos as predicted. Rather, it suggests a resilience of benthic community structure to cleaner-fish loss, possibly related to multiple antagonistic effects of different grazer functional groups. However, losing cleaners remains a problem for reefs, as the lack of cleaning has adverse consequences for fish physiology and populations.

Can extension programs improve grazing management in rangelands: a case study in Australia

A key challenge in reducing sediment moving from grazing lands into the Great Barrier Reef in Australia is to encourage beef cattle producers to improve management practices. Excessive grazing pressures cause land degradation, leading to both increased sediment runoff and lower future profits. Although higher grazing rates may be possible (and profitable) in better seasons, slow rates of adjustment to poorer seasons can lead to overgrazing and negative impacts on land condition. For policymakers the challenge is to find mechanisms that encourage or signal producers to be more precise in their management and avoid overstocking. Some of the most common options include extension programs, grant programs that use financial incentives, and regulation. In this paper we outline a conceptual framework that shows why extension may be a more powerful driver of management change than incentive programs, and then test this through an evaluation of a case study program conducted with beef cattle producers in catchments of the Great Barrier Reef, Australia. The pathway involving landholders to implement management change was through improved efficiency and productivity, as these are the issues that drive ongoing participation in broader environmental programs. The results present multiple lines of evidence to infer positive outcomes of an extension program in terms of changed management practices, which may be expected to generate improved productivity and better water quality outcomes. These can be grouped into three key areas. First, outcomes show positive improvement relative to the Grazing Water Quality Risk framework for the Great Barrier Reef catchments, which is designed to assess the links between land management and water quality. This indicates that resource condition is likely to improve and sediment emissions should be reduced over time. A second outcome is increased landholder engagement and improved understanding of their business and engagement in future programs, which should underpin ongoing adoption. A third outcome is improved management of risk and developing the skills to do this through data collection and monitoring, which should improve management responses in drought years.

Grazing of the copepod Cyclops vicinus on toxic Microcystis aeruginosa: potential for controlling cyanobacterial blooms and transfer of toxins

Abstract Grazing of zooplankton on phytoplankton may contribute to a reduction of harmful cyanobacteria in eutrophic waters. However, the feeding capacity and interaction between zooplankton and toxic cyanobacteria vary among grazer species. In this study, laboratory feeding experiments were designed to measure the grazing rate of the copepod Cyclops vicinus on Microcystis aeruginosa and the potential microcystin (MC) accumulation in the grazer. Copepods were fed a mixed diet of the edible green alga Ankistrodesmus falcatus and toxic M. aeruginosa for 10 days. The results showed that C. vicinus efficiently ingested toxic Microcystis cells with high grazing rates, varying during the feeding period (68.9–606.3 Microcystis cells animal-1 d-1) along with Microcystis cell density. Microcystis cells exhibited a remarkable induction in MC production under grazing conditions with concentrations 1.67–12.5 times higher than those in control cultures. Furthermore, C. vicinus was found to accumulate MCs in its body with concentrations increasing during the experiment (0.05–3.21 μg MC animal-1). Further in situ studies are needed to investigate the ability of Cyclops and other copepods to assimilate and detoxify MCs at environmentally relevant concentrations before deciding on the biocontrol of Microcystis blooms by copepods.

High Grazing Rates on Cryptophyte Algae in Chesapeake Bay

Cryptophyte algae are globally distributed photosynthetic flagellates found in freshwater, estuarine, and neritic ecosystems. While cryptophytes can be highly abundant and are consumed by a wide variety of protistan predators, few studies have sought to quantify in situ grazing rates on their populations. Here we show that autumnal grazing rates on in situ communities of cryptophyte algae in Chesapeake Bay are high throughout the system, while growth rates, particularly in the lower bay, were low. Analysis of the genetic diversity of cryptophyte populations within dilution experiments suggests that microzooplankton may be selectively grazing the fastest-growing members of the population, which were generally Teleaulax spp. We also demonstrate that potential grazing rates of ciliates and dinoflagellates on fluorescently labeled (FL) Rhodomonas salina, Storeatula major, and Teleaulax amphioxeia can be high (up to 149 prey predator-1 d-1), and that a Gyrodinium sp. and Mesodinium rubrum could be selective grazers. Potential grazing was highest for heterotrophic dinoflagellates, but due to its abundance, M. rubrum also had a high overall impact. This study reveals that cryptophyte algae in Chesapeake Bay can experience extremely high grazing pressure from phagotrophic protists, and that this grazing likely shapes their community diversity.

Trophic ecology of glass sponge reefs in the Strait of Georgia, British Columbia

Sponges link the microbial loop with benthic communities by feeding on bacteria. Glass sponge reefs on the continental shelf of western Canada have extremely high grazing rates, consuming seven times more particulate carbon than can be supplied by vertical flux alone. Unlike many sponges, the reef building species Aphrocallistes vastus has no microbial symbionts and removes little dissolved organic carbon. To determine how reef sponges therefore get enough food to sustain such substantial grazing we measured stable carbon and nitrogen isotope signatures of water, sediment and sponge tissues. To ensure samples were temporally associated, we also studied the duration particles were retained in tissues in controlled feeding studies using microscopic beads and 13C-labeled bacteria. Although fecal pellets were expelled from sponges within 24 hours of feeding, intact bacteria were still found in tissues and sponge tissues retained elevated 13C levels for at least 14 days. These independent lines of evidence suggest that carbon in reef sponge tissues may reflect food consumed from days to weeks earlier. Stable isotope analysis suggests that heterotrophic bacteria ingested by the sponges comes from a confluence of trophic subsidies: from terrestrial and oceanic sources, and also potentially on sediment-borne bacteria resuspended by tidal currents.

Evolution of a sediment-starved, over-stabilised dunefield: Kenfig Burrows, South Wales, UK

The past few decades have seen a progressive reduction in the extent of mobile dune systems and early successional stage habitats in many parts of NW Europe. The evidence suggests that a number of factors have contributed to this process, but their relative importance remains uncertain. This paper examines the nature and possible causes of geomorphological and vegetation changes at Kenfig Burrows, South Wales, a site of European nature conservation importance. This dune system is interpreted to have evolved from a sand barrier system which formerly existed to seaward of the present shoreline and which moved landwards and broke down during the later Holocene, driven by rising sea level and periods of more frequent storms. Most of the inland sand invasion occurred during the Little Ice Age, and large-scale sand-blowing continued until the early twentieth century, by which time the shoreline in the area had reached a state of quasi-equilibrium and a trend towards stabilization began, encouraged by exhaustion of marine sand supply and a change towards warmer, wetter and less windy conditions. Stabilization after the 1940–50s was also favoured by other factors, including reduced grazing by livestock and rabbits, reduced physical disturbance, increased nitrogen deposition, and sand dune management measures. By the 1980s the dunes were almost entirely stabilized. Natural reactivation of the surface sand is unlikely given the existing balance between the factors which favour dune mobility (frequent occurrence of sand-moving winds and high sand supply), and those which encourage vegetation growth and dune stability (high precipitation, high temperatures, low wind speeds, high rates of nitrogen deposition and low grazing / disturbance pressure). Since 2012 trials have been underway to determine if it is possible to increase the mobility of the dune system through intervention measures, including turf stripping and creation of artificial ‘notches’ to increase local wind speeds and sand-transport, but it will be several years before the effectiveness of these measures is known.

Can nutrient pathways and biotic interactions control eutrophication in riverine ecosystems? Evidence from a model driven mesocosm experiment

Ecological theory predicts that the relative importance of benthic to planktonic primary production usually changes along the rivers' continuum from a predomination of benthic algae in lower stream orders to a predomination of planktonic algae at higher orders. Underlying mechanisms driving the interaction between algae in these habitats, its controlling factors and consequences for riverine ecosystems are, however, only partly understood. We present a mechanistic analysis of the governing ecological processes using a simplified, numerical model and examine how abiotic factors and biotic interactions influence benthic and planktonic algae by changing resource competition. We compare the outcome of the model with the results of a factorial mesocosm experiment mimicking the parameter spaces of the model. The results show a remarkable similarity with regard to the temporal development of benthic and pelagic algal biomass and shifting dominance patterns. In particular we analyse the effects of the pathways of nutrient supply (upwelling from the hyporheic zone, direct supply to the surface water, or via both pathways) and grazing in a gradient of river depths. Our results show that detachment of benthic algae, sinking of planktonic algae and the pathway of nutrient supply are key processes determining the respective algal biomass distributions particularly in shallow and intermediate deep systems. Increasing nutrient supply increases algal biomasses, but does not change the general pattern of the interactions. Decreasing light supply decreases the dominance of planktonic algae, but increases dissolved nutrients. At intermediate to high grazing rates algal biomass can be controlled by grazers, but however, at high grazing rates, dissolved nutrients accumulate in the surface water. Our results indicate that nutrient pathways, resource competition and internal control by grazing need to be considered explicitly for the understanding and explanation of eutrophication phenomena in riverine ecosystems. As a consequence, ecologically effective eutrophication management of running water systems has to go beyond the control of nutrient emissions or the achievement of limiting threshold values in the receiving waters, but requires the consideration of the nutrient pathways (surface water versus groundwater) and the shifting biological controls from lower to higher order stream ecosystems.

Application of dual stable isotopes in investigating the utilization of two wild dominant filamentous algae as food sources for Daphnia magna

ABSTRACTIn this study, we evaluated the effects of two dominant microfilamentous algae (i.e. Melosira granulata and Oscillatoria sp.), collected from the West Lake, on growth and metabolism of Daphnia magna. Our experiment utilized 13C and 15N dual labeling to calculate the carbon and nitrogen isotopic turnover rates and half-life times in D. magna. The two labeled types of filaments were offered to D. magna as sole food sources or as paired mixtures with the unlabeled Scenedesmus obliquus. Labeled S. obliquus served as the control. Combined results showed that D. magna had a higher grazing rate on Oscillatoria sp. than on M. granulate and a small percentage of unlabeled S. obliquus addition could improve the grazing rate in both filamentous algae, especially for Oscillatoria sp., which had the highest carbon and nitrogen isotopic turnover rates and the lowest half times, even superior to the sole S. obliquus treatment. Our study revealed that D. magna could utilize the two dominant filamentous algae as a food source for their growth and metabolism, and a small percentage addition of S. obliquus could ameliorate the negative impact of these two filamentous algae on D. magna.

Ungulate control of grassland production: grazing intensity and ungulate species composition in Yellowstone Park

AbstractKnowledge of the envelope of grazing conditions within which grassland plant and soil processes are sustainable is important for the ecologically sound management of grazed grassland. Here we examined how a recent shift from elk to bison dominance of the northern ungulate community in Yellowstone National Park (YNP), and an associated increase in the duration that grassland was grazed, affected grazing intensity and aboveground net primary production (ANPP). Mean grazing intensity (GI, percentage ANPP removed) and stimulation (grazed ANPP minus ungrazed [exclosed] ANPP) were compared among three studies, two when elk (1988–1989, 1999–2001) and one when bison (2012–2014) dominated the northern YNP ungulate community. We also manipulated GI with small exclosures established for different lengths of time after the start of the growing season to directly determine the effect of the combination of grazing duration (GD) and GI on stimulation at a dry grassland and a mesic grassland. GI was greater under a bison‐dominant grazing regime (49%) compared with that measured during the two earlier periods when elk were the dominant ungulate species (31%, 13%). Plotting stimulation on GI for sites sampled across all three studies revealed a unimodal relationship, with peak stimulation of 34% occurring at a GI of 40%. Manipulating GI indicated that the greater GI and longer GD of grazing under a bison‐dominant regime reduced stimulation at a mesic grassland and negated it completely at a dry grassland. These findings revealed that a shift in the grazing ungulate community composition and an associated change in the migratory behavior of the dominant species impacted YNP grassland processes. Results also showed that grassland ANPP was resilient to the relatively high rates of prolonged grazing by the bison‐dominant community and did not reduce ANPP below paired, ungrazed conditions. However, YNP grassland should be continued to be monitored if such high rates of herbivory continue.

Dimethylsulfoniopropionate (DMSP) Increases Survival of Larval Sablefish, Anoplopoma fimbria.

High concentrations of dimethylsulfoniopropionate (DMSP), a chemical compound released by lysed phytoplankton, may indicate high rates of grazing by zooplankton and may thus be a foraging cue for planktivorous fishes. Previous studies have shown that some planktivorous fishes and birds aggregate or alter locomotory behavior in response to this chemical cue, which is likely adaptive because it helps them locate prey. These behavioral responses have been demonstrated in juveniles and adults, but no studies have tested for effects on larval fish. Larvae suffer from high mortality rates and are vulnerable to starvation. While larvae are generally thought to be visual predators, they actually have poor vision and cryptic prey. Thus, larval fish should benefit from a chemical cue that provides information on prey abundance. We reared larval sablefish, Anoplopoma fimbria, for one week and supplemented feedings with varying concentrations of DMSP to test the hypothesis that DMSP affects larval survival. Ecologically relevant DMSP concentrations increased larval survival by up to 70 %, which has implications for production in aquaculture and recruitment in nature. These results provide a new tool for increasing larval production in aquaculture and also suggest that larvae may use DMSP as an olfactory cue. The release of DMSP may be a previously unappreciated mechanism through which phytoplankton affect larval survival and recruitment.

Meso‐ and microzooplankton grazing in the Amazon River plume and western tropical North Atlantic

AbstractLargely due to size differences, mesozooplankton are important exporters of carbon and prey for larger organisms, while microzooplankton are important recyclers of nutrients, dominant grazers of phytoplankton, and a key link in the microbial loop. We investigated the relative importance of meso‐ and microzooplankton grazing in the western tropical North Atlantic Ocean (WTNA) and Amazon River plume. Sampling as part of the ANACONDAS project occurred in spring (May–June) 2010 during the peak outflow of the Amazon River and in fall (September–October) 2011 during the plume seasonal retroflection. Mesozooplankton grazing rates decreased with increasing salinity in both seasons, but during the fall both day and nighttime grazing rates were significantly negatively correlated with salinity. Mesozooplankton grazing was highest in plume‐influenced surface waters (0–25 m), and usually dominated by smaller size classes (0.2–0.5 mm and 0.5–1.0 mm). Microzooplankton grazing accounted for approximately 68% of bulk phytoplankton growth across all stations. Comparison of meso‐ and microzooplankton grazing suggests a transition in food web dynamics from a mesozooplankton dominated “export” structure in the plume transitioning to a microzooplankton dominated “retention” structure at mesohaline and oceanic stations above sea surface salinity of 33. Comparison between the seasons suggests a seasonal planktonic succession of low mesozooplankton grazing during the spring peak discharge followed by higher grazing rates and impact by mesozooplankton during the fall retroflection. These results provide important baseline information required for examining effects of climate change on the planktonic food web of the WTNA and for use in biogeochemical models of the region.

Regional variation in the structure and function of parrotfishes on Arabian reefs

Parrotfishes (f. Labridae) are a unique and ubiquitous group of herbivorous reef fishes. We compared the distribution and ecosystem function (grazing and erosion) of parrotfishes across 75 reefs in Arabia. Our results revealed marked regional differences in the abundance, and taxonomic and functional composition of parrotfishes between the Red Sea, Arabian Sea, and Arabian Gulf. High densities and diversity of parrotfishes, and high rates of grazing (210%year−1) and erosion (1.57kgm−2year−1) characterised Red Sea reefs. Despite Arabian Sea and Red Sea reefs having broadly comparable abundances of parrotfishes, estimates of grazing (150%year−1) and erosion (0.43kgm−2year−1) were markedly lower in the Arabian Sea. Parrotfishes were extremely rare within the southern Arabian Gulf, and as such rates of grazing and erosion were negligible. This regional variation in abundance and functional composition of parrotfishes appears to be related to local environmental conditions.