Shifts In Assemblages Research Articles

Responses of ichthyoplankton assemblages to the recent marine heatwave and previous climate fluctuations in several Northeast Pacific marine ecosystems.

The effects of climate warming on ecosystem dynamics are widespread throughout the world's oceans. In the Northeast Pacific, large-scale climate patterns such as the El Niño/Southern Oscillation and Pacific Decadal Oscillation, and recently unprecedented warm ocean conditions from 2014 to 2016, referred to as a marine heatwave (MHW), resulted in large-scale ecosystem changes. Larval fishes quickly respond to environmental variability and are sensitive indicators of ecosystem change. Categorizing ichthyoplankton dynamics across marine ecosystem in the Northeast Pacific can help elucidate the magnitude of assemblage shifts, and whether responses are synchronous or alternatively governed by local responses to regional oceanographic conditions. We analyzed time-series data of ichthyoplankton abundances from four ecoregions in the Northeast Pacific ranging from subarctic to subtropical: the Gulf of Alaska (1981-2017), British Columbia (2001-2017), Oregon (1998-2017), and the southern California Current (1981-2017). We assessed the impact of the recent (2014-2016) MHW and how ichthyoplankton assemblages responded to past major climate perturbations since 1981 in these ecosystems. Our results indicate that the MHW caused widespread changes in the ichthyoplankton fauna along the coast of the Northeast Pacific Ocean, but impacts differed between marine ecosystems. For example, abundances for most dominant taxa were at all-time lows since the beginning of sampling in the Gulf of Alaska and British Columbia, while in Oregon and the southern California Current species richness increased as did abundances of species associated with warmer waters. Lastly, species associated with cold waters also increased in abundances close to shore in southern California during the MHW, a pattern that was distinctly different from previous El Niño events. We also found several large-scale, synchronized ichthyoplankton assemblage composition shifts during past major climate events. Current climate projections suggest that MHWs will become more intense and thus our findings can help project future changes in larval dynamics, allowing for improved ecosystem management decisions.

Seasonal periphyton response to low-level nutrient exposure in a least disturbed mountain stream, the Buffalo River, Arkansas

Like most streams located in the Ozark Plateaus, the Buffalo River in Arkansas generally has excellent water quality. Water-quality conditions in Big Creek, however, a major tributary of the middle Buffalo River, have been less favorable than that of other Buffalo River tributaries. Concerns regarding the influence of water quality in Big Creek on the Buffalo River magnified in 2013 when a large confined animal feeding operation (CAFO) began operating in the watershed. In response to these concerns, the U.S. Geological Survey compared monthly nutrient concentrations and seasonal periphyton assemblage metrics of a site on Big Creek downstream of the CAFO, two Buffalo River control sites upstream of the confluence with Big Creek, and three Buffalo River test sites downstream of the confluence with Big Creek. In addition to identifying potential nutrient patterns and periphyton responses along a low-level nutrient exposure gradient, the study determined how nutrient contributions from Big Creek (and the CAFO) are affecting ecological conditions and consequent ecosystem services in the Buffalo River. Nutrient and periphyton data exhibited more temporal than spatial variability. Nutrient concentrations were generally highest of all sites at the Big Creek site. Concentrations at the five sites on the Buffalo River were typically low (near laboratory reporting limits), and concentrations at the three test sites rarely exceeded those of the two control sites. An index developed with three ecologically relevant periphyton metrics (oligotrophic taxa and Homoeothrix percent relative abundance and mesotrophic diatoms percent taxa richness) suggested that nutrient uptake at sites downstream of the Big Creek-Buffalo River confluence resulted in subtle shifts in downstream periphyton assemblages. The periphyton index of biological integrity at control sites was slightly and generally more favorable compared to test sites. Even so, when periphyton data were considered in conjunction with both hydrology and water-quality data, the negative consequences of antecedent high flows and associated scouring exceeded the potential positive effects that low-level nutrients had on algal productivity. These findings emphasize the importance of comparing biological and chemical data across extended temporal scales, particularly when working with low-level nutrient gradients.

Climate and land-use as the main drivers of recent environmental change in a mid-altitude mountain lake, Romanian Carpathians.

Recent decades have been marked by unprecendented environmental changes which threaten the integrity of freshwater systems and their ecological value. Although most of these changes can be attributed to human activities, disentagling natural and anthropogenic drivers remains a challenge. In this study, surface sediments from Lake Ighiel, a mid-altitude site in the Carpathian Mts (Romania) were investigated following high-resolution sedimentological, geochemical, environmental magnetic and diatom analyses supported by historical cartographic and documentary evidence. Our results suggest that between 1920 and 1960 the study area experienced no significant anthropogenic impact. An excellent correspondence is observed between lake proxy responses (e.g., growth of submerged macrophytes, high detrital input, shifts in diatom assemblages) and parameters tracking natural hydroclimate variability (e.g., temperature, NAO). This highlights a dominant natural hydroclimatic control on the lacustrine system. From 1960 however, the depositional regime shifted markedly from laminated to homogenous clays; since then geochemical and magnetic data document a trend of significant (and on-going) subsurface erosion across the catchment. This is paralleled by a shift in lake ecosystem conditions denoting a strong response to an intensified anthropogenic impact, mainly through forestry. An increase in detrital input and marked changes in the diatom community are observed over the last three decades, alongside accelerated sedimentation rates following enhanced grazing and deforestation in the catchment. Recent shifts in diatom assemblages may also reflect forcing from atmospheric nitrogen (N) deposition, a key recent drive of diatom community turnover in mountain lakes. In general, enhanced human pressure alongside intermittent hydroclimate forcing drastically altered the landscape around Lake Ighiel and thus, the sedimentation regime and the ecosystem’s health. However, paleoenvironmental signals tracking natural hydroclimate variability are also clearly discernible in the proxy data. Our work illustrates the complex link between the drivers of catchment-scale impacts on one hand, and lake proxy responses on the other, highlighting the importance of an integrated historical and palaeolimnological approach to better assess lake system changes.

Forest ditch maintenance impoverishes the fauna of aquatic invertebrates: Opportunities for mitigation

One of the main factors causing biodiversity loss in wetlands is drainage, nevertheless, even drained areas may provide habitat for aquatic fauna in the form of drainage ditches. Assemblages in ditches are regularly disturbed by ditch maintenance, but the extent of these disturbances and mitigation possibilities are poorly documented. We conducted an experimental study in three commercially managed forest plots in eastern Estonia, aiming to find out how ditch network maintenance (DNM) affects the diversity and assemblages of aquatic macroinvertebrates in ditches and remnant pools, and whether this effect can be alleviated by constructing mitigation waterbodies. For comparison we also collected data from natural pools in three undrained forest plots. Before DNM, ditches supported greater number of higher taxa compared to remnant and natural pools and more strictly aquatic taxa, whereas natural pools in undrained plots supported more Trichoptera shredders. After DNM, the diversity in remnant pools decreased. Moreover, majority of the pools dried out, which resulted in further reduction of the richness and abundance of macroinvertebrates. In ditches the diversity dropped immediately after DNM, but recovered in two to three years. Nevertheless, plot-scale richness and abundance did not completely recover. Assemblage shift in ditches took place right after DNM and remained distinct after the four year survey period. Mitigation pools provided habitat for several taxa (especially Odonata) uncommon in other waterbodies in drained and undrained plots. Our results show that DNM in forests substantially impoverishes habitat availability and reduces the abundance and diversity of aquatic macroinvertebrates. We recommend retaining uncleaned sections in ditches and constructing mitigation pools as tools for supporting wetland biodiversity in drained forests.

Summer temperature development 18,000–14,000 cal. BP recorded by a new chironomid record from Burgäschisee, Swiss Plateau

The termination of the Last Ice Age after the Last Glacial maximum (LGM) represents a dynamic period in the history of the circum-north Atlantic region. So far, there are few reliably dated climatic reconstructions covering the Lateglacial period prior to 14,700 cal. BP in Central and Northern Europe. We present a new chironomid record for the period 18,000–14,000 cal. BP, from Burgäschisee, Switzerland. Chironomid assemblages immediately following glacier retreat were dominated by taxa indicative of cold, oligotrophic conditions such as Sergentia coracina -type and Micropsectra radialis -type. A gradual transition to assemblages with moderate abundances of taxa indicative of warmer climatic conditions such as Dicrotendipes nevosus -type and Tanytarsus glabrescens -type started after ca. 16,100 cal. BP. This initial and gradual chironomid assemblage shift culminated in a more pronounced and rapid inferred temperature change at the Oldest Dryas/Bølling transition at ca. 14,700 cal. BP, where further types indicative of warm conditions such as Tanytarsus lactescens -type first occurred and replaced chironomids indicative of colder conditions such as Paracladius and Protanypus. We estimated past July air temperature changes from the chironomid assemblages by applying to the record a chironomid-temperature transfer function that is based on chironomid distribution data from 274 lakes in Switzerland and Norway. The resulting reconstruction, which features a sample-specific root mean square error of prediction of 1.36–1.46 °C, indicates temperatures around 9 °C at the beginning of the record. An initial gradual warming phase initiating at ca. 16,100 cal. BP is recorded reaching values around 10 °C for the period 16,100–15,500 cal. BP. Temperatures continue to increase reaching values around 12 °C for the period preceding the Bølling warming, when temperatures rose rapidly to values around 15 °C. The early temperature rise to values of 10–12 °C prior to the Bølling warming agrees with widespread vegetation changes recently reported for this region based on palaeobotanical analyses, which indicate a shift from herbaceous tundra to shrub tundra with low density tree birch stands with open canopies. Together, these results suggest an earlier Lateglacial temperature increase in southwest Central Europe than expected based on earlier palaeobotanical reconstructions, although with a less pronounced warming than has been reported for ca. 16,000 cal. BP from south of the Alps. This early Lateglacial warming agrees with glacier reconstructions which suggest several step-wise reductions of glacier extent in this period as well as with evidence from other palaeotemperature reconstructions and suggests that not only Southern Europe but significant parts of Europe north of the Alps may have been characterized by early Lateglacial warming well before the rapid warming at ca. 14,700 cal. BP.

Inferences on the role of coral bleaching or seasonality on cross-habitat movement of nekton assemblages in adjacent coral reef, seagrass, and mangrove habitats

We investigated shifts in nekton assemblages in coral reef, seagrass, and mangrove habitats that coincided with a coral bleaching event in June 2016. The study was conducted in May 2016 (prebleaching), July 2016 (bleaching), October 2016 (4-mo postbleaching), and February 2017 (7-mo postbleaching) in relatively undisturbed coastal areas in Busuanga, Palawan, western Philippines. We used triangular trap nets to capture nekton samples from each habitat. In coral reef and mangrove habitats, there were increases in nekton abundance and biomass from the prebleaching to the bleaching period. After the bleaching event, however, there were reductions in nekton abundance and biomass at the coral reefs until 7-mo postbleaching. Species composition changed at all sites where shifts in dominant species, habitat affinity, and trophic category were observed. The postbleaching increase in nekton abundance in mangroves coincided with the decreased nekton abundance in the coral reef, suggesting a cross-habitat movement, likely due to the reduced suitability and/or food in the bleached coral reef. The changes in the nekton assemblage may also have been due to seasonal fluctuations in environmental parameters, especially salinity. Our study presents evidence of the possible role of coral bleaching or seasonal changes on cross-habitat movements of nekton assemblages, which can be inferred as an indicator of disturbance. The presence of adjacent vegetated habitats may provide refuge for the affected nekton assemblage of the coral reef.

Increasing Cervidae populations have variable impacts on habitat suitability for threatened forest plant and lichen species

Large herbivores play a key role in temperate and boreal forest ecosystems. Cervidae (deer) population densities and community structure have undergone drastic changes in many parts of the world over the past decades, often with deer populations increasing. Many studies show impacts of Cervidae on multiple ecosystem properties, including vegetation and biodiversity, at local spatial scales. At larger spatial scales, however, impacts of changing Cervidae populations on forest ecosystems are less known. Although both abiotic and biotic dimensions contribute to shaping species’ niches, abiotic variables are generally given prominence when modelling species habitats and ranges. This is despite biotic changes, including changes in trophic structure, being an important component of global environmental change. In this study, we examined the potential contribution of Cervidae densities to the habitat suitability for rare plant and lichen species across the temperate and boreal forests of Norway, where cervid densities have increased over the past 60 years. We also examined how these changes in herbivore communities may have shaped habitat suitability for rare lichens and plants and discuss the results in light of continuing shifts in herbivore assemblages. We ran habitat suitability models for 47 species of rare plants and lichens, which were selected based on herbivory reported as a criterion for placement on the national red list for species. Climate (temperature and precipitation), forest (forest type and productivity), soil pH and Cervidae densities (moose Alces alces, red deer Cervus elaphus and roe deer Capreolus capreolus) were used as independent variables. Densities of one or more of the three Cervidae species were inferred to be associated with the distribution of 14 (ten lichen, one bryophyte and three vascular plant species) of these 47 species. We found a range of habitat suitability associations with Cervidae densities, including positive, negative and hump-backed responses. Increases in Cervidae densities over the past 60 years may have led to different spatial trends in habitat suitability across the 14 species. Our results suggest that Cervidae densities are associated with the distribution of rare forest plant and lichen species differently at large spatial scales; experimental studies should test the causality of these associations. If causal, this implies that Cervidae management should find a balance between high and low densities to conserve several plant and lichen species. The preponderance of epiphytic lichens species, for which habitat suitability was associated with Cervidae densities, calls for field studies to focus on Cervidae impacts on forest lichens.

Landscape-scale forest loss as a catalyst of population and biodiversity change.

Global biodiversity assessments have highlighted land-use change as a key driver of biodiversity change. However, there is little empirical evidence of how habitat transformations such as forest loss and gain are reshaping biodiversity over time. We quantified how change in forest cover has influenced temporal shifts in populations and ecological assemblages from 6090 globally distributed time series across six taxonomic groups. We found that local-scale increases and decreases in abundance, species richness, and temporal species replacement (turnover) were intensified by as much as 48% after forest loss. Temporal lags in population- and assemblage-level shifts after forest loss extended up to 50 years and increased with species' generation time. Our findings that forest loss catalyzes population and biodiversity change emphasize the complex biotic consequences of land-use change.

Post-wildfire denudation assessed from compositional features of river sediments (Central Portugal)

The wildfires of 2017 were responsible for vegetation clearance in a wide area of central Portugal, leaving the weathering profiles more exposed to erosive action. In the present research, suspended loads from a set of catchment areas affected by these fires with diverse geological and orographic features were periodically sampled to evaluate spatial and seasonal variability in sediment production. Bulk mineralogy reflected the geology of the source areas, but displayed high variability even for a single sampling site, which can be partially linked to the grain-size control on sediment composition. Clay mineralogy provided a better picture of the denudation at catchment scale. The predominance of detrital illite with low I5/I10 (ratio of the intensities at 5 Å (002) and 10 Å (001)) indicated very shallow weathering profiles. Chlorite was usually the second most common mineral, more common in metasedimentary-derived sediments, whilst kaolinite was detected only in less steep catchment areas with a predominance of granitic basement rocks or where previous-cycle depositional units are also present. Year-long shifts in clay assemblages were attributed to different spatial patterns of denudation. A depletion of kaolinite occurred in sampling sites mainly sourced by granitoids. An opposite trend was observed where areas with Meso-Cenozoic deposits, less affected by the wildfires than those with basement units, occupy significant proportions of the drainage basins. This trend was ascribed to the wearing out of the weathering profiles covering the basement. Regolith denudation was also testified by a progressive decrease in I5/I10. Shifts in clay mineralogy linked with progressive surface denudation were found to be limited in areas with shallow soils, but may complicate any paleoenvironmental reconstructions based on sediment composition when more evolved weathering profiles have been eroded.

Biological Control of Salvinia molesta (D.S. Mitchell) Drives Aquatic Ecosystem Recovery

Salvinia molesta D.S. Mitchell (Salviniaceae) is a damaging free-floating invasive alien macrophyte native to South America. The biological control programme against S. molesta by the weevil Cyrtobagous salviniae Calder and Sands (Erirhinidae) has been successful in controlling S. molesta infestations in the introduced range, however, there is some debate as to how biological control success is measured. This study measured the response of epilithic algae and aquatic macroinvertebrate communities in a S. molesta-dominated state and subsequently where the weed had been cleared by biological control, as a proxy for ecosystem recovery in a before–after control–impact mesocosm experiment. The restored treatment (S. molesta and C. salviniae) demonstrated epilithic algae and aquatic macroinvertebrate recovery during the “after” biological control phase, defined as similar to the control treatment. Comparatively, the impacted treatment (100% S. molesta) showed a drastic decline in biodiversity and shifts in community assemblages. We conclude that the biological control effort by C. salviniae facilitated biodiversity recovery of the impacted treatment. Furthermore, epilithic algae and aquatic macroinvertebrate communities were reliable biological indicators for measuring ecological impacts of invasion and ecosystem recovery following biological control, and thus represent potential tools for evaluating biological control success and ecological restoration.

Differing limnological responses to late Holocene climate variability in the Cordillera Vilcanota, Peruvian Andes

In recent decades, climate change in Peru’s Cordillera Vilcanota has resulted in massive reductions to its cryosphere and the upslope migration of species and agriculture. Little, however, is known about the response of the region’s many lakes that support local biodiversity and supply water to downstream populations. We analyzed fossil diatom assemblages in dated sediment cores from three lakes with differing morphometry and catchment characteristics to document the limnological response to climate variability over the late Holocene. The study lakes contained similar dominant diatom taxa but recorded markedly different assemblage shifts over time. The two deeper lakes, Laguna Sibinacocha (zmax = 92 m) and Chaca Cocha (zmax = 18 m), contained diatom assemblages that oscillated in dominance between benthic fragilarioids (Staurosirella pinnata, Pseudostaurosira brevistriata) and the planktonic Discostella stelligera. The Chaca Cocha diatom record closely matched the mean state changes in climate inferred from independent proxy records, whereas the record from the glacially influenced Laguna Sibinacocha did not match independent records. In contrast, the shallow study site, Lado del Quelccaya (zmax = 5 m), recorded a complacent diatom profile dominated by benthic fragilarioids with negligible planktonic taxa. The disparate diatom trajectories among the study lakes reflect variations in morphometry and glacial influence and the assemblage shifts appear best explained by climate-driven alterations to thermal stratification. These data offer insight into the primary mechanisms driving limnological change in this region and how different lake types throughout the Andes may respond to future warming.

Saprotrophic and Ectomycorrhizal Fungi Contribute Differentially to Organic P Mobilization in Beech-Dominated Forest Ecosystems

Phosphorus (P) is an essential nutrient, but European forest ecosystems are experiencing widespread declines in soil P concentrations. To clarify the roles of ectomycorrhizal (EM) and saprotrophic (SAP) fungi in P cycling in forest soils that differ in inorganic and organic P availability, we conducted an ingrowth tube (IGT) experiment over 18 months of exposure in five beech (Fagus sylvatica L.) forests in Germany. To separate the contributions of both fungal guilds in situ, two different types of IGTs were used: (i) open IGTs with micromesh windows to allow EM fungi to regrow into the tubes, and (ii) closed IGTs (without mesh windows) in which recolonization by EM hyphae was restricted. We then measured phosphatase and oxidase activities in EM (open IGTs) and SAP (closed IGTs) dominated fungal assemblages. Our results show that shifts in fungal assemblages occurred over time, but that the speed and extent of those shifts differed. In three forest sites fungal community composition shifted towards greater dominance of EM fungi in open IGTs after a period of 18 months, but these changes were not apparent in two other sites. Although shifts in fungal community composition were noticeable between treatments, we found no effect on phosphomonoesterase activity. Since phosphomonoesterase is present in both fungal guilds, this indicates functional redundancy. Phosphodiesterase activity was slightly reduced in SAP dominated IGTs, indicating that SAP fungi contribute to phosphodiesterase activity to a lesser extent than do EM fungi. P cycling enzymes did not appear to have been influenced by the total P stocks of the forest sites but may have been affected by additional abiotic factors. Contrary to our expectation, phenoloxidase activity was unaffected by fungal community composition, and only peroxidase activity was higher in SAP dominated IGTs, indicating that the contribution of EM fungi to degradation of complex organic material is driven by the need to obtain nutrients in addition to C. Overall, the results of this study contribute to a better understanding of the roles of EM and SAP fungi in P cycling in forest soils.

Environmental effects on daytime sheltering behaviors of California horn sharks (Heterodontus francisci)

California horn sharks (Heterodontus francisci) are relatively small (122 cm maximum total length; TL), nocturnally active sharks that have been observed refuging in shelters during the day; however, little is known about how they select their daytime resting shelters, and what factors may drive their degree of shelter fidelity. Depending on body size, thermal tolerance, and ability to avoid predators, horn sharks may need to balance tradeoffs when selecting a quality shelter. This study quantified vertical daytime distributions via dive surveys and shelter fidelity via acoustic telemetry during June – August 2016 and June – November 2017 in a temperate rocky reef (33˚26’ N, -118˚29’ W). Immature individuals ( 58 cm TL) were found in deeper (> 10 m), cooler habitats sheltering in larger caves and rock ledges. The sizes and relative abundances of sharks varied between land-associated and offshore-isolated reefs, indicating habitat type may be a likely driver of ontogenetic shifts in shelter behavior. No sharks returned to their shelters in 2016 coinciding with the period of El Nino conditions (increased temperature, shifts in community assemblage), yet shelter fidelity was 47% in 2017 under non-El Nino conditions. By quantifying sheltering behavior in a residential mesopredator, this study provides a framework for understanding how environmental conditions influence horn shark behaviors and their potential impacts on their community.

Fish and fishing in Lake Peipsi (Estonia/Russia) since 1851: Similarities and differences between historical and modern times

The first survey data on the fisheries in Lake Peipsi were collected by natural scientist Karl Ernst von Baer as a result of the world's first special fishery expedition in 1851–1852. In the current study, all available numerical datasets on fishing in Lake Peipsi from 1851 to 2018 have been drawn together in order to analyse the long-term changes in the lake’s fish assemblages. As the study indicates, the overexploitation, catching of juvenile fish and predator–prey imbalance have been common problems associated with fisheries in Lake Peipsi for the last two centuries. Similar to many inland waterbodies worldwide, total catch of fish from Lake Peipsi has decreased about four times since the mid-19th century, and the catch of lake (dwarf) smelt, a previously dominant species, has periodically fluctuated with long-term gradual reduction. The decline of cool-water fish, such as vendace, burbot and Peipsi whitefish during the last two centuries, and the domination of Eurasian perch, common bream and pike-perch in the catches of the last 20 years mark considerable structural shifts in fish assemblages. Of the multiple stressors (e.g., nutrient enrichment, increased water temperature, overfishing) triggering shifts in fish assemblages, the impact of climate warming, especially extreme weather events such as heatwaves, seems to be the strongest during the last three decades. However, none of the fish species have completely disappeared from the lake during the last century although the share of cool-water fish in the total catch has declined about tenfold since the 1930s.

Last glacial maximum ecology and climate from terrestrial gastropod assemblages in Peoria loess, western Kentucky

ABSTRACTThe Rocks loess section, in unglaciated western Kentucky, provides a high‐resolution environmental record during the last glacial maximum onset. The Peoria Silt (9 m thick) contains 26 terrestrial gastropod species, with up to 15 species within a single 5 cm interval. Thirteen radiocarbon ages, using shells or charcoal, range between 30 and 24.5 cal ka; younger loess has been leached or eroded. Stratigraphic shifts in gastropod assemblages imply significant cooling, particularly ~27 cal ka, as solar insolation was decreasing and the southern Laurentide Ice Sheet rapidly advancing. Midwestern to southern species (e.g. Anguispira kochi, Gastrocopta pentodon, Hawaii miniscula, Helicodiscus parallelus, Vallonia perspectiva) occur only in the lowermost Peoria Silt (~30–27 cal ka). In contrast, cold‐tolerant species (Columella alticola, Vertigo modesta, Vallonia gracilicosta) occur only in full glacial Peoria Silt (27–24.5 cal ka). Inferred mean July temperatures, from mutual climatic range methods, range from ~23 °C at 30 cal ka, cooling to ~18 °C by 26 cal ka; about 3–8 °C cooler than today (~26 °C). Superimposed on this cooling trend are multi‐centennial variations in detrital carbonate, fossil shell concentrations, palaeotemperature estimates, and oxygen isotope values (Vertigo, Discus, Helicodiscus). The finer‐scale variations imply relatively synchronous fluctuations in glacial sediment supply, loess sedimentation, and climate.

Contrasting shifts in coral assemblages with increasing disturbances

Increasing incidence of major disturbances is contributing to extensive and widespread coral loss, thereby undermining the biodiversity, structure and function of reef ecosystems. The composition of coral assemblages is already changing due to selective effects of recurrent disturbances, combined with marked differences in the underlying life-history dynamics of corals, which affects their recovery. This study quantifies the effects of varying disturbance regimes on two groups of corals with divergent life histories: short-lived species with rapid growth (bushy and tabular Acropora) and long-lived species with slow growth (massive and columnar Porites). Inter-decadal shifts in the coral assemblages across four locations suggest that a high frequency of moderate disturbances favours Porites, whereas infrequent, but severe disturbances favour rapidly replenishing Acropora. Using empirical modelling, we expand these observations to show that Acropora continues to dominate so long as the interval between major disturbances is > 2 years. The only disturbance regime we considered that favoured Porites was high frequency (2-year recurrence) of moderate disturbance, whereas high frequency of severe disturbances led to local extirpation of both Acropora and Porites. Our results show that increasing incidence of major disturbances will not necessarily lead to selective loss of species that are most susceptible to disturbance, as long as these species can continue to colonise vacant space and grow quickly in the aftermath of such disturbances. This study highlights the need to consider the sensitivity of taxa to changes in both disturbance frequency and severity when forecasting changes in the composition of coral assemblages under new disturbance regimes.

A regime shift in intertidal assemblages triggered by loss of algal canopies: A multidecadal survey

Canopy-forming macroalgae recently experienced a worldwide decline. This is relevant, because canopies sustain complex food webs in temperate coasts. We assessed the die-back of the canopy-forming alga Fucus serratus in N Spain, at its warm distributional range boundary, and its effects on associated assemblages. We combined long-term descriptive surveys with canopy-removal experiments. Results showed that rapid shifts to turf-forming communities were mostly the direct consequence of the canopy loss, rather than a concurrent process directly triggered by climate change. The switch alters the whole food web, as the prominent role of F.serratus and other cold-temperate intertidal fucoids is not being replaced by functionally equivalent species. Canopy loss caused a rapid biotic homogenization at regional scale which is spreading towards the west, from the edge to the central part of the former distributional range of F.serratus in N Spain. The most obvious effect is the ecological and functional impoverishment of the coastal system.

Return of the “age of dinoflagellates” in Monterey Bay: Drivers of dinoflagellate dominance examined using automated imaging flow cytometry and long‐term time series analysis

AbstractPhytoplankton biomass in Monterey Bay, California is typically dominated by diatoms, but it shifted to dinoflagellates twice in the past 18 years (2004–2007, 2017–2018), which was associated with increased harmful algal blooms. Located within the central California Current System (CCS), Monterey Bay is strongly influenced by cycles of upwelling‐favorable winds and their relaxation or reversal. Both dinoflagellate‐dominated periods were linked to a negative North Pacific Gyre Oscillation (NPGO) and increased river discharge, but each had a different relationship with upwelling. To examine the connection between large‐scale and local forcings underlying floristic shifts in the phytoplankton assemblage, an Imaging FlowCytobot (IFCB) was deployed within the Monterey Bay upwelling shadow for a full year. A machine learning classifier differentiating IFCB images of the phytoplankton assemblage was developed. Despite anomalously strong upwelling in 2018, dinoflagellates comprised 57% of the annual phytoplankton‐specific biomass. During upwelling, dinoflagellates appear to have accumulated at convergent fronts, while during relaxation these frontal populations were transported to the nearshore where they seeded local blooms. Frequent upwelling‐relaxation cycles and local wind anomalies generated an unusually retentive circulation pattern in the upwelling shadow, producing a warm and stratified bloom incubator. Thus, local features and forcings (upwelling shadow, winds, river discharge) modified the effects of regional‐ and basin‐scale oceanographic variability (regional upwelling, NPGO), altering local phytoplankton patterns. As North Pacific decadal variability and CCS upwelling intensity increase under climate warming, dinoflagellates may become more common in some CCS regions, due to the enhancement or mitigation of large‐scale trends by local forcings.

Nowhere to swim: interspecific responses of prairie stream fishes in isolated pools during severe drought

Extreme drying can dramatically shift abiotic characteristics of streams and species respond differently as conditions change. We modified a conceptual model describing responses of aquatic invertebrate assemblages to stream drying to facilitate predictions of fish assemblage shifts in which thresholds of physiochemical gradients lead to punctuated losses of fish diversity as stream connectivity diminishes. We tested the applicability of this model during a record-breaking drought that reduced a perennial reach of a headwater prairie stream to a series of 12 isolated pools with variable levels of hyporheic connectivity that was associated with pool volume. We tracked fish assemblage structure in pools over seven weeks and measured abiotic conditions. Inter-specific differences in drought resistance traits were tested in the lab (i.e., thermal and hypoxia tolerance) for common species experiencing drought conditions. Species richness declined rapidly with pool volume associated with hyporheic connectivity. Abundance of the most common species prior to drought, Chrosomus erythrogaster (southern redbelly dace) declined 95% and was extirpated from five pools during drought due to its low thermal tolerance. Despite being the least tolerant to hypoxia, Campostoma anomalum (central stoneroller) populations persisted in most pools. The remaining pools became dominated by a non-native extremophile fish, Gambusia affinis (western mosquitofish) and abundances of less resistant native species declined. Our study demonstrates that richness in drying pools is driven by factors associated with pool volume, and assemblage composition shifts from dominance by native species adapted to variable hydrological conditions to non-native species able to resist increasing temperatures and hypoxia. Sudden species loss associated with reduced stream connectivity can shift abundance patterns of prairie stream fish communities with potentially lagged effects on population and community dynamics.

Changes in plant function and root mycobiome caused by flood and drought in a riparian tree.

Under increasingly harsh climatic conditions, conservation of threatened species requires integrative studies to understand stress tolerance. Riparian Ulmus minor Mill. populations have been massively reduced by Dutch Elm disease (DED). However, resistant genotypes were selected to restore lost populations. To understand the acclimation mechanisms to the succession of abiotic stresses, ramets of five DED-tolerant U. minor genotypes were subjected to flood and subsequently to drought. Physiological and biochemical responses were evaluated together with shifts in root-fungal assemblages. During both stresses, plants exhibited a decline in leaf net photosynthesis and an increase in percentage loss of stem hydraulic conductivity and in leaf and root proline content. Stomatal closure was produced by chemical signals during flood and hydraulic signals during drought. Despite broad similarities in plant response to both stresses, root-mycobiome shifts were markedly different. The five genotypes were similarly tolerant to moderate drought, however, flood tolerance varied between genotypes. In general, flood did not enhance drought susceptibility due to fast flood recovery, nevertheless, different responses to drought after flood were observed between genotypes. Associations were found between some fungal taxonomic groups and plant functional traits varying with flood and drought (e.g. proline, chlorophyll and starch content) indicating that the thriving of certain taxa depends on host responses to abiotic stress.