Grazer Abundance Research Articles

Cover Image

COVER PHOTO: Tide pools at Corona del Mar State Beach, California, USA, are pictured on the cover. Declines in biodiversity in marine systems are occurring at unprecedented rates and it is essential to evaluate the complexities of the resulting transformations on ecosystems. Bracken et al. used tide pools on the southern California shoreline to experimentally manipulate grazer abundance and quantify mechanisms by which molluscan grazers promote algal abundance. Their results (Article e4439; doi: 10.1002/ecy.4439) suggest that mollusks in the pools enhance the abundance of the algae they consume by recycling nutrients and creating sticky slime trails that promote algal recruitment. Photo credit: Matthew E. S. Bracken. image

Unraveling the multiple facilitative effects of consumers on marine primary producers.

The loss of consumers threatens the integrity of ecological systems, but the mechanisms underlying the effects on communities and ecosystems remain difficult to predict. This is, in part, due to the complex roles that consumers play in those systems. Here, we highlight this complexity by quantifying two mechanisms by which molluscan grazers-typically thought of as consumers of their algal resources-facilitate algae on rocky shores. Initial observations in high-zone tide pools revealed that both water-column ammonium concentrations and photosynthetic biomass were higher in pools containing higher densities of grazers, suggesting that local-scale nutrient recycling by the grazers could be enhancing algal biomass. We assessed this possibility by experimentally manipulating grazer abundances at the level of whole tide pools but controlling access of those grazers to experimental plots within each pool. Contrary to predictions that algal biomass inside grazer exclusions would increase as grazer abundances in the pools increased, we found that algal biomass inside grazer-exclusion fences was unaffected by grazer abundances. Instead, the consumptive effects of grazers that were evident at low grazer abundances transitioned to facilitative effects as experimentally manipulated grazer abundances increased. This finding suggested that these positive interactions were associated with the physical presence of grazers and not just grazers' effects on nutrient availability. Subsequent experiments highlighted the potential role of "slime"-the pedal mucous trails left behind as the mollusks crawl on the substratum-in promoting the recruitment of algae and thereby mediating a spatial subsidy of new organic matter into the system. Furthermore, different grazer groups contributed disproportionately to ammonium excretion (i.e., turban snails) versus slime production (i.e., littorine snails), suggesting a potential role for grazer diversity. Our work highlights the complex ways in which consumers affect their resources, including multiple, complementary mechanisms by which these grazers facilitate the algae they consume.

Insight into the impact of air flow rate on algal-bacterial granules: Reactor performance, hydrodynamics by Computational Fluid Dynamics (CFD) and microbial community analysis

This study evaluated the impact of air flow rates on nitrogen removal and the formation of algal-bacterial granules in domestic wastewater treatment, revealing that sodium acetate addition and aeration shifted the microbial community composition. Two separate feedings and the addition of external organic carbon significantly enhanced inorganic nitrogen removal efficiency (over 90 %), which might have contributed to the increased microbial diversity and the relative abundance of denitrifiers such as Acinetobacter. This study also demonstrates that the higher air flow rate (0.5 LPM) resulted in higher nitrification rates (up to 6.4 mg N/L/h) and produced smaller and denser granules. The higher air flow rate also led to the increasing relative abundances of nitrogen-related genera (such as Nitrospira) and the decreasing relative abundances of cyanobacteria and Chlorella. Computational Fluid Dynamics (CFD) simulations revealed that mechanical mixing predominantly generated shear force. Increasing the air flow rate from 0.2 LPM to 0.5 LPM only yielded a 12 % increment in the volume-averaged strain rate, suggesting diminishing returns from higher air flow rates in terms of shear force enhancement. Moreover, the decrease in total abundance of grazers and pathogens along with the operation, including Chytridiomycetes, Sessilida, and Operculariidae, might result from the shear force and the decrease of Chlorella spp.. This study underscores the critical roles of aeration and carbon source management in optimizing the performance and microbial ecology of algal-bacterial systems in wastewater treatment while minimizing energy inputs.

Whole-lake coarse woody habitat addition facilitates ecosystem regime restructuring in an oligotrophic lake

Preul-Stimetz T, Shaw SL, Sass GG. 2024. Whole-lake coarse woody habitat addition facilitates ecosystem regime restructuring in an oligotrophic lake. Lake Reserv Manag. XXX-XXX. The loss of vital littoral zone and riparian habitat due to lakeshore residential development is a pervasive issue across temperate aquatic ecosystems. Oligotrophic glacial lakes may be particularly vulnerable to reductions in coarse woody habitat (CWH) due to their lack of alternative structural habitat. Although influences of CWH on fish populations have been well documented, questions remain regarding CWH influences on lower trophic levels and water quality. We conducted a whole-lake CWH addition experiment within a BACI framework using 2 lakes, Sanford (treatment) and Escanaba (reference), in the Northern Highland Lake District of Wisconsin. Premanipulation monitoring began in 2015, trees were dropped in 2018, and data collection continued until 2023. We monitored limnological, planktonic, and benthic macroinvertebrate parameters to test whether CWH addition initiated trophic shifts and altered the structure of the aquatic food web. We found that CWH addition minimally affected water quality and changed some characteristics of Sanford Lake at lower trophic levels. Chlorophyll a concentrations in Sanford Lake significantly increased but Secchi depth was unaffected. The abundance of macrophytes increased in both systems; that in Sanford Lake increased more and was linked to the CWH addition. Dissolved oxygen significantly increased at multiple depth strata. Water temperature was unaffected. We observed declines in total zooplankton that were not related to CWH addition, but the decline in relative abundance of large grazers was. Our results provide a mechanistic understanding of the effects of habitat enhancement at an ecosystem scale and can help guide stakeholder and manager expectations following CWH addition.

Long-term data reveal that grazer density mediates climatic stress in salt marshes.

Understanding how climate and local stressors interact is paramount for predicting future ecosystem structure. The effects of multiple stressors are often examined in small-scale and short-term field experiments, limiting understanding of the spatial and temporal generality of the findings. Using a 22-year observational dataset of plant and grazer abundance in a southeastern US salt marsh, we analyzed how changes in drought and grazer density combined to affect plant biomass. We found: (1) increased drought severity and higher snail density both correlated with lower plant biomass; (2) drought and snail effects interacted additively; and, (3) snail effects had a threshold, with additive top-down effects only occurring when snails were present at high densities. These results suggest that the emergence of multiple stressor effects can be density dependent, and they validate short-term experimental evidence that consumers can augment environmental stress. These findings have important implications for predicting future ecosystem structure and managing natural ecosystems.

Salmon farming alters the structure and functioning of Norwegian maerl bed communities

Abstract Effluents resulting from fish farming activities can alter the structure and functioning of benthic ecosystems. In Norway, recent technological advances within the salmon industry have facilitated the expansion of aquaculture into shallower locations, leading to additional pressures upon coastal habitats. Maerl/rhodolith beds are important bioengineers in coastal waters—supporting highly diverse associated macrofaunal communities—and are likely to be particularly at risk. This study aims to investigate the environmental changes induced by fish farms in their vicinity and their impact on the structure and functioning of maerl bed macrofaunal communities. High dissolved nutrients and organic enrichment levels were observed close to the cages, associated with lower live maerl cover. Overall, the most substantial impact of salmon farms on macrofaunal communities was observed within 100 m from the cages. Nevertheless, changes in the structure and functioning of assemblages were evidenced up to 300 m. In the vicinity of the cages, communities were characterized by the dominance of endobenthic subsurface deposit feeders and opportunistic species, notably the polychaetes Chaetozone sp., Capitella sp. and Scoloplos armiger. Conversely, sites situated farther away from the cages exhibited higher abundances of epibenthic taxa, including crawlers, grazers and species sensitive to organic enrichment. Sites unaffected by the presence of fish farms were described by high abundances of the ophiuroid Ophiura robusta, the polychaete Proclea graffi and ostracods, reflecting more favourable environmental conditions. Such changes in macrofaunal communities highlight the vulnerability of maerl beds to the impacts of aquaculture, thus emphasizing the importance of rigorous site selection when establishing new fish farming facilities.

The impacts of non-native watercress in Oklahoma spring ecosystems

Watercress (Nasturtium officinale) has spread widely from Europe and commonly occurs in Oklahoma (USA) springs. Watercress is usually an emergent plant and affects water flow patterns and may provide habitat for biota. Although watercress is not considered an invasive species, its impacts in springs have not been reported. With a goal to describe possible impacts of watercress in springs, 14 karst-associated springs (12 with watercress) were surveyed for sediment characteristics, macroinvertebrates, and diatoms in July 2021. The effects of watercress were evident. Sediment particle size was unaffected by the presence of watercress but sediment organic matter was higher under watercress beds than outside of beds. Although there was no difference in total benthic macroinvertebrate density or richness, higher organic matter was associated with slightly higher abundances of detritivorous and predatory macroinvertebrates (SIMPER). Submerged portions of watercress had significantly much lower diatom density than other spring substrates, with diatom composition similar to other plants but different from that of rocks. Self-shading or possible allelopathy may cause the low diatom density. The significantly lower macroinvertebrate density within watercress mats relative to that of other plants may result from a reduced food source because the plant’s low diatom density. Only predatory damselflies were more common in watercress than in other plants, which had more abundant grazers. Although watercress can increase heterogeneity in sediments and is sometimes valued as an edible plant, watercress supports low algal and macroinvertebrate densities, such that extensive growth of watercress can have an overall negative impact on spring ecosystems.

Modeling grazer-mediated effects of demographic and material connectivity on giant kelp metapopulation dynamics

From dispersal-based metapopulations to meta-ecosystems that arise from flows of non-living materials, spatial connectivity is a major driver of population dynamics. One potentially important process is material transport between populations also linked by individual dispersal. Here, I explored material and demographic connectivity in metapopulations of giant kelp Macrocystis pyrifera, a foundation species that produces both detritus and reproductive spores. Kelp detritus (drift) subsidizes grazers, helping maintain the kelp forest ecosystem state. Drift could potentially be exchanged among kelp patches, but this is less studied than spore dispersal. Therefore, I built an ordinary differential equation (ODE) model to investigate conditions under which drift and/or spore connectivity promotes the kelp forest state. I fit statistical models (generalized linear mixed models, GLMMs) to observational data and used the GLMM’s predictions to validate the ODE model. My results suggest kelp patch dynamics are best explained by connectivity of both drift and spores, and that the impacts of these forms of connectivity depend on local grazer (urchin) abundance. Both models predicted greater kelp persistence in well-connected patches across a range of urchin densities. These effects were largely driven by drift, which reduced grazing in recipient patches and thereby enhanced spore recruitment. While testing these predictions will require greater empirical quantification of interpatch drift transport, my findings indicate drift connectivity may be an important spatial process in kelp forest systems. More broadly, this work highlights the role of meta-ecosystem dynamics within a single ecosystem type, reinforcing the need to expand traditional metapopulation perspectives to consider multiple forms of spatial connectivity.

Co‐restoring keystone predators and foundation species to recover a coastal wetland

Abstract Achieving global targets for ecosystem restoration necessitates the development of effective restoration approaches. Current restoration approaches focus primarily on mitigating abiotic stresses or supplementing propagules to restore foundation species that support associated species by ‘engineering’ local habitats. Although keystone predators can strongly affect foundation species in natural ecosystems via trophic cascades and are often diminished in degraded ecosystems, they are traditionally neglected in efforts to restore foundation species. Here, we investigated how a keystone predator (Scylla serrata) might affect the restoration of a foundational plant (Scirpus mariqueter) in a degraded salt marsh. In a series of restoration experiments, we examined the effects of proximity to existing Scylla burrows, as well reintroducing Scylla, on the establishment of planted Scirpus in restoration areas. Furthermore, we tested if Scylla affected Scirpus via non‐consumptive effects in experiments that controlled Scylla's consumption of grazers. We found that planted Scirpus suffered less grazing and achieved higher density in areas closer to existing Scylla burrows where Scylla predation on grazers was stronger. Consistently, reintroduction of Scylla reduced grazing on Scirpus and increased its density by 360%. These positive effects of Scylla on Scirpus were driven by consumptive effects that reduced the abundance of grazers and by non‐consumptive effects that inhibited the herbivory by grazers. Synthesis and applications. These results demonstrate that incorporation of keystone predators, including both their consumptive and non‐consumptive effects, into restoration designs can greatly enhance the restoration of foundation species. Our study thus highlights the value of co‐restoring keystone predators and foundation species. Given that predators are depleted in many ecosystems, this co‐restoration approach may be broadly applicable.

Urban shading and artificial light at night alter natural light regimes and affect marine intertidal assemblages

Urban development in many coastal cities has resulted in altered natural light regimes, with many coastal habitats being artificially shaded during the daytime by built structures such as seawalls and piers, while artificial light emitted from buildings and associated infrastructure creates pollution at night. As a result, these habitats may experience changes to community structure and impacts on key ecological processes such as grazing. This study investigated how changes to light regimes affect the abundance of grazers on natural and artificial intertidal habitats in Sydney Harbour, Australia. We also examined whether differences in patterns of responses to shading or artificial light at night (ALAN) varied across different areas within the Harbour, characterised by different overall levels of urbanisation. As predicted, light intensity was greater during the daytime on rocky shores than seawalls at the more urbanised sites of the harbour. We found a negative relationship between the abundance of grazers and increasing light during the daytime on rocky shores (inner harbour) and seawalls (outer harbour). We found similar patterns at night on rocky shores, with a negative relationship between the abundance of grazers and light. However, on seawalls, grazer abundances increased with increasing night-time lux levels, but this was mainly driven by one site. Overall, we found the opposite patterns for algal cover. Our findings corroborate those of previous studies that found that urbanisation can significantly affect natural light cycles, with consequences to ecological communities.

Physiological responses of cooccurring intertidal limpets (Cellana spp.) to acute and repeated heat stress

Limpets help maintain bare space and act as sentinel species of thermal stress in intertidal reef ecosystems. However, little is known about the physiological responses of New Zealand's widely distributed Cellana spp. limpets to thermal stress. We tested if there were differences in thermal tolerance in summer and winter and heat shock protein concentrations among four Cellana species that occupy different shore elevations. For one highly abundant species (C. denticulata), we tested whether thermal tolerance was affected by limpet size in summer and winter and whether thermal tolerance and oxygen consumption in air and water were affected by repeated heat stress. Limpet species from higher shore elevations did not always have higher breakpoint temperatures than those from lower elevations, potentially due to differences in homing behaviour. Similarly, there was no effect of size, season, or repeated heat stress on thermal tolerance. HSP70 concentrations were highest in C. denticulata, but were not affected by temperature (19–34 °C), indicating that limpets maintain high concentrations of constitutive HSP70. Limpets exposed to repeated heat stress in air had higher heart rates and oxygen consumption than those maintained at ambient temperature (19 °C). However, when both treatments were flooded with 18 °C seawater, heart rates were similar, but oxygen consumption was greater in the ambient treatment, indicating that non-stressed limpets may incur an oxygen debt during aerial exposure. This study provides new understanding about the effect of acute and repeated heat stress on the physiology of these conspicuous and abundant grazing invertebrates, which will help better predict their distributions in response to climate change.

Oceanographic structuring of the mucous-mesh grazer community in the Humboldt Current off Peru

The Humboldt Upwelling System (HUS) supports high levels of primary production and has the largest single-stock fishery worldwide. The high fish production is suggested to be related to high trophic transfer efficiency in the HUS. Mucous-mesh grazers (pelagic tunicates and gastropods) are mostly of low nutritious value and might reduce trophic transfer efficiency when they are locally abundant. Unfortunately, little is known about the spatial dynamics of mucous-mesh grazers from Peruvian waters, limiting our understanding of their potential ecological role(s). We provide a spatial assessment of mucous-mesh grazer abundance from the Peruvian shelf in austral summer 2018/2019 along 6 cross-shelf transects spanning from 8.5 to 16°S latitude. The community was dominated by appendicularians and doliolids. Salps occurred in high abundance but infrequently, and pelagic gastropods were mostly restricted to the north. At low latitudes, the abundance of mucous-mesh grazers was higher than some key species of crustacean mesozooplankton. Transects in this region had stronger Ekman transport, higher temperature, lower surface turbidity and a broader oxygenated upper water layer compared to higher-latitude transects. Small-scale lateral intrusions of upwelled water were potentially associated with high abundances of doliolids at specific stations. The high abundance and estimated ingestion rates of mucous-mesh grazers in the northern HUS suggest that a large flux of carbon from lower trophic levels is shunted to tunicates in recently upwelled water masses. The data provide important information on the ecology of mucous-mesh grazers and stress the relevance to increase research effort on investigating their functioning in upwelling systems.

Periphyton and benthivorous fish affect charophyte abundance and indicate hidden nutrient loading in oligo‐ and mesotrophic temperate hardwater lakes

Abstract Charophytes are a key group of submerged macrophytes in temperate hardwater lakes, but have declined or been replaced by angiosperms as a result of eutrophication for more than 100 years. At present, this process is continuing in many European lakes despite an overall reduced nutrient loading. In eutrophic lakes, light attenuation by periphyton is a major factor responsible for declining angiosperm abundance. For charophyte declines, the impact of benthivorous and herbivorous fish has been discussed often, yet periphyton shading has been largely neglected. In our study, we investigated 11 temperate oligo‐mesotrophic (total phosphorus [P] concentrations 9–22 μg/L) hardwater lakes along a gradient of charophyte abundances. Charophyte abundance was mapped using underwater cameras or scuba diving, and coverage was estimated in five classes. Periphyton biomass, nutrient stoichiometry and invertebrate grazer abundance were measured twice on artificial substrates exposed inside and outside fish exclosures for 4 weeks in July and August. Fish biomass was sampled once using multi‐mesh gill netting. We analysed the relationships between water chemistry (concentrations of nutrients and dissolved organic carbon), periphyton biomass and nutrient stoichiometry, periphytic invertebrate grazers and fish biomass to test (a) whether periphyton biomass was controlled bottom‐up by nutrient availability or top‐down by an omnivorous fish–invertebrate–grazer cascade and (b) whether charophyte abundance was affected by periphyton biomass, benthivorous and herbivorous fish. Multiple linear regressions revealed that charophyte abundance was significantly predicted by the biomasses of periphyton and benthivorous bream (Abramis brama L.), whereas herbivorous rudd (Scardinius erythrophthalmus L.) had no significant effect in the investigated lakes. Bream biomass and its proportion in the total fish biomass were linearly related to lake total P concentrations. Periphyton biomass in the tested lakes reached 0.1–21 g dry weight/m2 and primarily was controlled bottom‐up by P availability as indicated from stoichiometry and positive correlations with grazer abundance. Top‐down control was found in only three lakes, which had significantly lower periphyton biomass inside fish exclosures as compared to controls. Our data imply that periphyton shading and negative fish impacts on charophytes increase with P loading. Low charophyte abundances at moderate P concentrations in the lake water indicate a high sensitivity of charophytes and/or unnoticed nutrient loading due to nutrient retention by charophyte stands. Management measures thus must focus on reducing nutrient loading and/or benthivorous fish biomass at early stages of impact to preserve charophyte stands in oligo‐mesotrophic hardwater lakes.

Indirect effects of cleaner fish Labroides dimidiatus on fish grazing per reef area and benthic community structure

Grazing fishes farm algae, and consume algae, detritus and sediment and consequently differentially modify benthic communities. Manipulations of cleaner fish Labroides dimidiatus on reefs show that cleaners affect fish abundance differently according to grazer functional group. Accordingly, whether reefs are grazed differently, with consequences for the benthos (fouling material tile−1), was tested using reefs kept free of L. dimidiatus for 10 years compared with undisturbed control reefs. We recorded on video the grazing density (bites tile−1 h−1 reef−1) on settlement tiles and the natural benthos (roving fishes only), according to territorial algal farmer (Pomacentridae) and roving grazer (Acanthuridae, Labridae, Siganidae) functional groups, and measured the accumulation of fouling material tile−1 after 10 months. Grazing density on tiles (dominated by ‘indeterminate’ farmers, and roving ‘sediment-removing’ detritivore Ctenochaetus striatus) and the natural benthos (dominated by Ct. striatus and other grazers) was not measurably affected by cleaner presence. The composition of fouling material (dominated by detritus > turf algae > sediment > other) and organic and inorganic dry weight of material tile−1 were also not measurably affected by cleaner presence. This points to resilience of the benthic community to loss of cleaners. The likely complex interactions between cleaner fish presence, grazer abundance and mobility, and the often-opposite effects of territorial farmers and roving grazers on the benthos underscore the challenge in determining indirect effects of cleaners on benthic community structure. However, a lack of cleaners has negative ramifications for fish populations and physiology and thus their loss remains problematic for client fishes.

Novel shaker bottle cultivation method for the long spined sea urchin (Diadema antillarum; Philippi, 1845) results in high larval survival and settlement rates

The long spined sea urchin Diadema antillarum was an abundant grazer on Caribbean coral reefs, until 1983–1984, when densities were reduced by ∼98% during a region wide die-off. Since then, there has been very little natural recovery of the species and interest is growing in applying aquaculture as a tool for population enhancement. In this study we optimized a new shaker bottle cultivation method for D. antillarum. The method was tested in a series of experiments by culturing D. antillarum from egg to juvenile in the Netherlands as well as the USA. Larvae were cultured in standard 1-L glass reagent bottles, suspended by gentle constant movement on an orbital shaking table and fed with either the microalgae Rhodomonas lens or Rhodomonas salina. Effects on larval growth and survival were evaluated for different microalgal feeding concentrations, larval densities, and culture temperatures. Larval density and growth were measured twice a week over a period of up to 56 days.Larvae grew significantly faster on a higher feeding concentration up to 90,000 Rhodomonas sp. cells mL−1, twice weekly, compared to 30,000 and 60,000 cells mL−1. A density of 1 larvae mL−1 resulted in the highest body size and survival compared to densities of 2 or 4 larvae mL−1. Overall survival from larva to settled juvenile urchin increased from 8 to 10% settlement to 32–33% when the initial density was lowered further from 1.2 to 0.4 larvae mL−1. Growth, survival, competency and settlement did not differ between larval cultures kept at 25 °C or 28 °C.We believe that this novel method for culturing D. antillarum larvae, once scaled-up and validated to pilot scale, could provide juveniles for restocking of urchin-depleted reefs that suffer from algae overgrowth.

Simulated green turtle grazing alters effects of environmental drivers on seagrass growth dynamics across seasons

AbstractSeagrasses form productive marine ecosystems that serve as important foraging grounds for grazers. Meadow productivity is vulnerable to environmental change, however, because environmental factors often strongly regulate seagrass growth. Understanding effects of grazing and environmental driver interactions on growth dynamics is therefore needed to ensure the long‐term sustainability of seagrass meadow foraging habitats. We simulated natural green turtle (Chelonia mydas) grazing by experimentally clipping seagrass for 16 months in a Thalassia testudinum meadow and measured how responses in linear growth, production, the production‐to‐biomass ratio (P : B; compensatory growth), and leaf area index differed between clipped and unclipped seagrass in response to in situ changes in temperature and salinity. While increasing temperature and salinity had positive and negative effects, respectively, on growth rates, clipping did not alter the relationship between these abiotic drivers and seagrass growth. Simulated grazing did, however, alter effects of temperature on seagrass P : B ratio and leaf area index dynamics. Each increased significantly with temperature; however, P : B ratios only increased in experimentally clipped seagrass, whereas leaf area index only increased in unclipped seagrass. These results suggest that, given temperature‐stimulated growth, grazed seagrass prioritizes increasing biomass production, whereas ungrazed seagrass prioritizes increasing photosynthetic surface area. In addition, our results demonstrate that the strength of the compensatory growth response to grazing in T. testudinum is seasonally dependent, highlighting the importance of biotic‐abiotic interactions in driving growth dynamics. In a future with increasing grazer abundance and climate‐driven stressors, understanding these types of interactions will be critical for long‐term sustainability of seagrass ecosystems.

Protistan Communities Within the Galápagos Archipelago With an Emphasis on Micrograzers

The Galápagos Archipelago is a globally significant biodiversity hotspot. However, compared to the relatively well-known megafauna, the distribution and ecological significance of marine protists in this system are poorly understood. To gain an understanding of the protistan assemblages across trophic modes, an intensive oceanographic survey was conducted in the Galápagos Marine Reserve (GMR) in October of 2018. The Equatorial Undercurrent (EUC)-influenced region had higher chlorophyll-a (Chl-a) concentrations than those of the eastern regions of the archipelago, along with higher abundances of protistan grazers. Specifically, proportions of autotrophic and potentially mixotrophic dinoflagellates were higher in the EUC, whereas in the eastern regions, heterotrophic dinoflagellates and chlorophytes dominated. Taxonomic composition and biochemical indicators suggested proportions of micrograzers and their associated heterotrophic biomass was higher in the oligotrophic, low Chl-a regions in the east. We also report observations from a dinoflagellate bloom in the western archipelago, which was heavily influenced by upwelling of the EUC. The red tide-forming dinoflagellate Scrippsiella lachrymosa was highly detected through light microscopy and DNA amplicon sequencing. In addition, the heterotrophic dinoflagellate Polykrikos kofoidii was detected and, based on cell densities observed in this study and grazing rates obtained from the literature, estimated to potentially graze up to 62% of S. lachrymosa bloom population. Our findings thus provide new insights into the composition of micrograzers and their potential roles in structuring protistan communities in the Galápagos Archipelago.

Impacts of piscicide-induced fish removal on resource use and trophic diversity of lake invertebrates

Chemical eradication of non-native species has become a widely used method to mitigate the potential negative impacts of altered competitive or predatory dynamics on biodiversity and natural ecosystem processes. However, the responses of non-target species can vary from rapid full recovery to delayed or absent recolonization, and little is known about the potential shifts in resource use and trophic diversity of native species following chemical treatments. We used a before-after-control-impact approach to study the effects of rotenone piscicide treatment on abundance and trophic niche of benthic invertebrates in three untreated and three treated lakes in central Norway, the latter group hosting non-native roach (Rutilus rutilus) and pike (Esox lucius) prior to rotenone treatment. Based on community composition data, the relative abundance of invertebrate grazers and collectors decreased while that of predators increased following fish removal in the treated lakes. The stable isotope data indicated minor shifts in resource use of, and trophic diversity among, benthic invertebrate communities. While the predatory dragonfly larvae (Odonata) and grazer snails (Lymnaeidae) showed increased δ13C values indicating increased reliance on littoral benthic algae, the collector mayfly larvae (Leptophlebia) showed decreased δ13C values following fish removal in treated lakes. Grazer snails also showed a shift to a lower trophic position, while the predatory dragonflies and collector mayflies showed no changes in δ15N values following fish removal. The community-level isotopic niches of benthic invertebrates showed no consistent changes, although the sample-size corrected and Bayesian estimates of standard ellipse areas (SEAC and SEAB) slightly increased in two of the three treated lakes due to an increased range in δ15N. In conclusion, our study findings indicate some changes in species assemblages but minor shifts in the resource use and trophic diversity of benthic invertebrate communities following fish removal in rotenone treated lakes.

Fire, herbivores and the management of temperate Eucalyptus savanna in Tasmania: Introducing the Beaufront fire – mammalian herbivore field experiment

SummaryThe temperate Eucalyptus savannas in the Midlands of Tasmania are ancient ecosystems where fire and grazing are intrinsic ecological disturbances. The arrival of Aboriginal people into Tasmania some 40,000 years ago altered natural fire regimes, and since the end of the last ice age, their skilful patch burning increased the grass cover and the abundance of large grazers in the Midlands savannas. This ancient socio‐ecological tradition abruptly ended following European invasion in the early 19th century, which resulted in the rapid establishment of pastoralism, causing profound adverse changes to the ecological integrity of the temperate savannas. These changes include widespread tree clearing, extinction of native biota, establishment of domestic and feral mammalian herbivores, the introduction of exotic plants, broadscale application of chemical fertilisers and more recently irrigation. The Midlands retains a small fraction of the original vegetation, which typically occurs in small fragments on private land. These have been colonised by non‐native plants and animals, and experience altered fire regimes. There is a growing awareness that to effectively manage temperate savanna fragments may require the intentional coupling of fire and herbivory. We describe the establishment of a field experiment designed to test four broad hypotheses: a) herbivore off‐take increases after fire; b) smaller burned areas experience more intense herbivory than larger ones; c) non‐native herbaceous plants are more tolerant of herbivory, whereas native herbaceous species are more tolerant of fire; and d) Eucalyptus seedlings are most likely to reach maturity in areas which are both burned and protected from herbivores. A novel aspect of the fire‐herbivore experiment was that the Tasmanian Aboriginal community were engaged with and were contracted to conduct the burning. The findings of this landscape ecology experiment will inform the management of remnant temperate Eucalyptus savannas.

Mesozooplankton Community Dynamics and Grazing Potential Across Algal Bloom Cycles in a Subtropical Estuary

Mesozooplankton, as abundant grazers of microalgae in coastal systems, have the potential to prevent or mitigate harmful algal blooms (HABs) and their effects. The Indian River Lagoon (IRL) is a subtropical estuary in eastern Florida (United States) where repeated blooms, dominated by the toxic dinoflagellatePyrodinium bahamense, the brown tide speciesAureoumbra lagunensis, pico/nano planktonic cyanobacteria and other nano-eukaryotes, have highlighted the need to better understand fluctuations in the grazing potential of mesozooplankton populations across bloom cycles. Mesozooplankton and abiotic environmental data were collected at five sites in the northern IRL system at 6-week intervals from November 2013 through June 2016. A total of 107 taxa from 14 phyla were detected. Communities varied across sites, dates and between bloom and non-bloom periods, with densities up to 338 individuals L–1. Eight taxa comprising 85–94% of the total population at each site were identified as primary potential grazers, including barnacle nauplii, cladocerans, adult copepods, gastropod veligers, larvaceans, and polychaete metatrochophores. Although abundant, the estimated grazing potential of the primary taxa, calculated from their measured densities and previously published grazing rates, suggest that mesozooplankton lack the capacity to suppress phytoplankton once they reach bloom levels. These findings illustrate the utility of monitoring data and underscore the importance of systematically evaluating algal bloom controls with a consideration for the dynamic conditions of each unique ecosystem.