Long‐term declines in an intertidal foundation species parallel shifts in community composition

The earth is in the midst of a biodiversity crisis, and projections indicate continuing and accelerating rates of global changes. Future alterations in communities and ecosystems may be precipitated by changes in the abundance of strongly interacting species, whose disappearance can lead to profound changes in abundance of other species, including an increase in extinction rate for some. Nearshore coastal communities are often dependent on the habitat and food resources provided by foundational plant (e.g., kelp) and animal (e.g., shellfish) species. We quantified changes in the abundance of the blue mussel (Mytilus edulis), a foundation species known to influence diversity and productivity of intertidal habitats, over the past 40 years in the Gulf of Maine, USA, one of the fastest warming regions in the global ocean. Using consistent survey methods, we compared contemporary population sizes to historical data from sites spanning >400 km. The results of these comparisons showed that blue mussels have declined in the Gulf of Maine by >60% (range: 29–100%) at the site level since the earliest benchmarks in the 1970s. At the same time as mussels declined, community composition shifted: at the four sites with historical community data, the sessile community became increasingly algal dominated. Contemporary (2013–2014) surveys across 20 sites showed that sessile species richness was positively correlated to mussel abundance in mid to high intertidal zones. These results suggest that declines in a critical foundation species may have already impacted the intertidal community. To inform future conservation efforts, we provide a database of historical and contemporary baselines of mussel population abundance and dynamics in the Gulf of Maine. Our results underscore the importance of anticipating not only changes in diversity but also changes in the abundance and identity of component species, as strong interactors like foundation species have the potential to drive cascading community shifts.     

Blue mussels (Mytilus edulis) in the diet of roach (Rutilus rutilus) in outer archipelago areas of the western Gulf of Finland,Baltic Sea

The blue mussel (Mytilus edulis) is one of the key species in the Baltic Sea ecosystem and it is living at the edge of its range in the western Gulf of Finland. Roach (Rutilus rutilus) is a freshwater fish species that has benefited from recent coastal eutrophication and is at present highly abundant in the outer archipelago of the Gulf of Finland. In 2000 and 2001, a total of 516 roach were sampled for diet analysis in three study areas. Shelled molluscs formed over 95% of the diet of roach, blue mussels being the dominant single species. The proportion of this species in the food of roach in the three study areas ranged between 38–61% for smaller roach (<225 mm) and 39–85% for larger (>225 mm) roach, indicating that blue mussel is a highly important food source for roach in outer archipelago areas of the western Gulf of Finland, in contrast to reports from other parts of the northern Baltic Sea. The scarcity of large blue mussels in mussel beds in the easternmost study area was reflected in the lower proportion of blue mussels in the diet of larger roach. However, the growth of roach was not affected by the availability of blue mussels. The twofold differences observed in the annual growth of roach between warm and cold years demonstrated that temperature is an important factor controlling the growth of roach in the western Gulf of Finland.     

Impact assessment of an invasive flatworm, Convoluta convoluta, in the Southern Gulf of Maine

Convoluta convoluta (Abildgaard 1806) is a small (2-3 mm long) acoellous turbellarian flatworm from Europe that has invaded the Gulf of Maine within the last 5 years. Although it has been reported in densities of up to 19 individuals/cm², its ecological impact remains unknown. In its native habitat, it consumes harpacticoid copepods and primary settling mussels <0.5-mm shell length. This study estimated the impact of C. convoluta on juvenile blue mussel populations (Mytilus edulis Linnaeus 1758) around the Isles of Shoals in the southern Gulf of Maine, USA as well as looking at their distribution in their new habitat. We surveyed worm densities at sites of differing wave exposure over three substrates (hard substrates, bladed algae, filamentous algae) to quantify patterns of worm abundance. We found worms on all substrate types with their highest abundances occurring in areas of maximal sunlight exposure and minimal physical disturbance. We showed a definite pattern of consumption of mussels in the lab and found C. convoluta to consume up to 35% of primary settling mussels in the field, but only under certain conditions. Per capita impact on juvenile mussels was found to vary greatly in correlation with mussel recruitment rate and water temperature, but not with the consumption of harpacticoids. Our results also suggested that per capita interaction strength was reduced by intraspecific density-dependent competition and water temperature. The overall impact of C. convoluta on mussel populations in the southern Gulf of Maine is therefore estimated to be minimal.     

Experimental confirmation of multiple community states in a marine ecosystem

Small changes in environmental conditions can unexpectedly tip an ecosystem from one community type to another, and these often irreversible shifts have been observed in semi-arid grasslands, freshwater lakes and ponds, coral reefs, and kelp forests. A commonly accepted explanation is that these ecosystems contain multiple stable points, but experimental tests confirming multiple stable states have proven elusive. Here we present a novel approach and show that mussel beds and rockweed stands are multiple stable states on intertidal shores in the Gulf of Maine, USA. Using broad-scale observational data and long-term data from experimental clearings, we show that the removal of rockweed by winter ice scour can tip persistent rockweed stands to mussel beds. The observational data were analyzed with Anderson’s discriminant analysis of principal coordinates, which provided an objective function to separate mussel beds from rockweed stands. The function was then applied to 55 experimental plots, which had been established in rockweed stands in 1996. Based on 2005 data, all uncleared controls and all but one of the small clearings were classified as rockweed stands; 37% of the large clearings were classified as mussel beds. Our results address the establishment of mussels versus rockweeds and complement rather than refute the current paradigm that mussel beds and rockweed stands, once established, are maintained by site-specific differences in strong consumer control. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Predation on exotic zebra mussels by native fishes: effects on predator and prey

SUMMARY 1. Exotic zebra mussels, Dreissena polymorpha, occur in southern U.S. waterways in high densities, but little is known about the interaction between native fish predators and zebra mussels. Previous studies have suggested that exotic zebra mussels are low profitability prey items and native vertebrate predators are unlikely to reduce zebra mussel densities. We tested these hypotheses by observing prey use of fishes, determining energy content of primary prey species of fishes, and conducting predator exclusion experiments in Lake Dardanelle, Arkansas. 2. Zebra mussels were the primary prey eaten by 52.9% of blue catfish, Ictalurus furcatus; 48.2% of freshwater drum, Aplodinotus grunniens; and 100% of adult redear sunfish, Lepomis microlophus. Blue catfish showed distinct seasonal prey shifts, feeding on zebra mussels in summer and shad, Dorosoma spp., during winter. Energy content (joules g⁻¹) of blue catfish prey (threadfin shad, Dorosoma petenense; gizzard shad, D. cepedianum; zebra mussels; and asiatic clams, Corbicula fluminea) showed a significant species by season interaction, but shad were always significantly greater in energy content than bivalves examined as either ash-free dry mass or whole organism dry mass. Fish predators significantly reduced densities of large zebra mussels (>5 mm length) colonising clay tiles in the summers of 1997 and 1998, but predation effects on small zebra mussels (≤5 mm length) were less clear. 3. Freshwater drum and redear sunfish process bivalve prey by crushing shells and obtain low amounts of higher-energy food (only the flesh), whereas blue catfish lack a shell-crushing apparatus and ingest large amounts of low-energy food per unit time (bivalves with their shells). Blue catfish appeared to select the abundant zebra mussel over the more energetically rich shad during summer, then shifted to shad during winter when shad experienced temperature-dependent stress and mortality. Native fish predators can suppress adult zebra mussel colonisation, but are ultimately unlikely to limit population density because of zebra mussel reproductive potential.     

Intrinsic Variability in Shell and Soft Tissue Growth of the Freshwater Mussel Lampsilis siliquoidea

Freshwater mussels are ecologically and economically important members of many aquatic ecosystems, but are globally among the most imperiled taxa. Propagation techniques for mussels have been developed and used to boost declining and restore extirpated populations. Here we use a cohort of propagated mussels to estimate the intrinsic variability in size and growth rate of Lampsilis siliquoidea (a commonly propagated species). Understanding the magnitude and pattern of variation in data is critical to determining whether effects observed in nature or experimental treatments are likely to be important. The coefficient of variation (CV) of L. siliquoidea soft tissues (6.0%) was less than the CV of linear shell dimensions (25.1–66.9%). Size-weight relationships were best when mussel width (the maximum left-right dimension with both valves appressed) was used as a predictor, but 95% credible intervals on these predictions for soft tissues were ∼145 mg wide (about 50% of the mean soft tissue mass). Mussels in this study were treated identically, raised from a single cohort and yet variation in soft tissue mass at a particular size class (as determined by shell dimensions) was still high. High variability in mussel size is often acknowledged, but seldom discussed in the context of mussel conservation. High variability will influence the survival of stocked juvenile cohorts, may affect the ability to experimentally detect sublethal stressors and may lead to incongruities between the effects that mussels have on structure (via hard shells) and biogeochemical cycles (via soft tissue metabolism). Given their imperiled status and longevity, there is often reluctance to destructively sample unionid mussel soft tissues even in metabolic studies (e.g., studies of nutrient cycling). High intrinsic variability suggests that using shell dimensions (particularly shell length) as a response variable in studies of sublethal stressors or metabolic processes will make confident identifications of smaller effect sizes difficult.     

Larvae from deep-sea methane seeps disperse in surface waters

Many species endemic to deep-sea methane seeps have broad geographical distributions, suggesting that they produce larvae with at least episodic long-distance dispersal. Cold-seep communities on both sides of the Atlantic share species or species complexes, yet larval dispersal across the Atlantic is expected to take prohibitively long at adult depths. Here, we provide direct evidence that the long-lived larvae of two cold-seep molluscs migrate hundreds of metres above the ocean floor, allowing them to take advantage of faster surface currents that may facilitate long-distance dispersal. We collected larvae of the ubiquitous seep mussel “Bathymodiolus” childressi and an associated gastropod, Bathynerita naticoidea, using remote-control plankton nets towed in the euphotic zone of the Gulf of Mexico. The timing of collections suggested that the larvae might disperse in the water column for more than a year, where they feed and grow to more than triple their original sizes. Ontogenetic vertical migration during a long larval life suggests teleplanic dispersal, a plausible explanation for the amphi-Atlantic distribution of “B.” mauritanicus and the broad western Atlantic distribution of B. naticoidea. These are the first empirical data to demonstrate a biological mechanism that might explain the genetic similarities between eastern and western Atlantic seep fauna.     

Linking genotype to phenotype in a changing ocean: inferring the genomic architecture of a blue mussel stress response with genome‐wide association

A key component to understanding the evolutionary response to a changing climate is linking underlying genetic variation to phenotypic variation in stress response. Here, we use a genome‐wide association approach (GWAS) to understand the genetic architecture of calcification rates under simulated climate stress. We take advantage of the genomic gradient across the blue mussel hybrid zone (Mytilus edulis and Mytilus trossulus) in the Gulf of Maine (GOM) to link genetic variation with variance in calcification rates in response to simulated climate change. Falling calcium carbonate saturation states are predicted to negatively impact many marine organisms that build calcium carbonate shells – like blue mussels. We sampled wild mussels and measured net calcification phenotypes after exposing mussels to a ‘climate change’ common garden, where we raised temperature by 3°C, decreased pH by 0.2 units and limited food supply by filtering out planktonic particles >5 μm, compared to ambient GOM conditions in the summer. This climate change exposure greatly increased phenotypic variation in net calcification rates compared to ambient conditions. We then used regression models to link the phenotypic variation with over 170 000 single nucleotide polymorphism loci (SNPs) generated by genotype by sequencing to identify genomic locations associated with calcification phenotype, and estimate heritability and architecture of the trait. We identified at least one of potentially 2–10 genomic regions responsible for 30% of the phenotypic variation in calcification rates that are potential targets of natural selection by climate change. Our simulations suggest a power of 13.7% with our study's average effective sample size of 118 individuals and rare alleles, but a power of >90% when effective sample size is 900.     

Isolated and combined impacts of blue mussels (Mytilus edulis) and barnacles (Balanus improvisus) on structure and diversity of a fouling community

Effects of two presumably dominant competitors, the blue mussel Mytilus edulis and the barnacle Balanus improvisus on recruitment, population dynamics and community structure on hard substrata were experimentally investigated in the subtidal Kiel Fjord, Western Baltic. The hypothesis that blue mussels and/or barnacles are local dominants and strongly influence succession and community structure was tested by monitoring succession in the presence and absence of simulated predation on either or both species. Manipulations included blue mussel removal, barnacle removal, combined blue mussel and barnacle removal, as well as a control treatment for natural (non-manipulated) succession. In the second part of the experiment, recovery from the treatments was monitored over 1 year.During the manipulative phase of the experiment, blue mussels had a negative effect on recruitment of species, whereas barnacles had no significant effect. Even so, a negative synergistic effect of blue mussels and barnacles was detected. Calculation of species richness and diversity H′ (Shannon Index) showed a negative synergistic effect of blue mussels and barnacles on community structure. Additionally, diversity H′ was negatively affected by the dominant competitor M. edulis. These effects were also detectable in the ANOSIM-Analysis. The non-manipulative phase of the experiment brought about a drastic loss of diversity and species richness. Blue mussels dominated all four communities. Barnacles were the only other species still being able to coexist with mussels. Effects of simulated predation disappeared fast.Thus, in the absence of predation on blue mussels, M. edulis within a few months dominates available space, and diversity of the benthic community is low. In contrast, when mussel dominance is controlled by specific predators, more species may persist and diversity remains high.     

Characteristics and Drivers of High-Altitude Ladybird Flight: Insights from Vertical-Looking Entomological Radar

Understanding the characteristics and drivers of dispersal is crucial for predicting population dynamics, particularly in range-shifting species. Studying long-distance dispersal in insects is challenging, but recent advances in entomological radar offer unique insights. We analysed 10 years of radar data collected at Rothamsted Research, U.K., to investigate characteristics (altitude, speed, seasonal and annual trends) and drivers (aphid abundance, air temperature, wind speed and rainfall) of high-altitude flight of the two most abundant U.K. ladybird species (native Coccinella septempunctata and invasive Harmonia axyridis). These species cannot be distinguished in the radar data since their reflectivity signals overlap, and they were therefore analysed together. However, their signals do not overlap with other, abundant insects so we are confident they constitute the overwhelming majority of the analysed data. The target species were detected up to ∼1100 m above ground level, where displacement speeds of up to ∼60 km/h were recorded, however most ladybirds were found between ∼150 and 500 m, and had a mean displacement of 30 km/h. Average flight time was estimated, using tethered flight experiments, to be 36.5 minutes, but flights of up to two hours were observed. Ladybirds are therefore potentially able to travel 18 km in a “typical” high-altitude flight, but up to 120 km if flying at higher altitudes, indicating a high capacity for long-distance dispersal. There were strong seasonal trends in ladybird abundance, with peaks corresponding to the highest temperatures of mid-summer, and warm air temperature was the key driver of ladybird flight. Climatic warming may therefore increase the potential for long-distance dispersal in these species. Low aphid abundance was a second significant factor, highlighting the important role of aphid population dynamics in ladybird dispersal. This research illustrates the utility of radar for studying high-altitude insect flight and has important implications for predicting long-distance dispersal.     

Evaluation of the Danish mussel fishery: suggestions for an ecosystem management approach

In Limfjorden, Denmark, an extensive mussel fishery exploits the wild stocks of Mytilus edulis with annual landings of 80,000–100,000 t of mussels. During the last 10 years the impact of mussel dredging on the ecosystem has been studied, including the effect of resuspension of sediment and nutrients and the impoverishment of in- and epi-fauna assemblages. Furthermore, dredging changes the physical structure and complexity of the seabed which affects mussel growth and interactions among zoobenthic species. The blue mussel constitutes the dominant fraction of the zoobenthic suspension feeders, and is important for the transport of material and energy from the pelagic to benthic systems and the control of phytoplankton biomass. In order to evaluate the impact on clearance capacity of a reduction in mussel densities due to mussel dredging, mussel filtration activity measured in situ has been related to the mixing of the water column and the amount of near-bed phytoplankton. Fishery practice for mussel dredging in Limfjorden is discussed in relation to its known impact on the ecosystem and the ecological role of the mussels, and modifications towards an ecosystem management approach and a more sustainable fishery are suggested. The suggested modifications include: a fishery practice where the mussel beds are thinned out when the mussels have attained good quality, and a transplantation practice of mussels from areas with a high mortality to areas with a high growth rate. Both practices intensify the production in a certain area, leaving other areas open for alternative production or for permanent closure for the benefit of the benthic flora and fauna. In addition, other shellfish species represent interesting new resources for fishing or aquaculture. Habitat restoration, such as the relaying of mussel shells from the mussel industry, is another important management tool that should be included in an ecosystem management approach of the mussel fishery. Electronic Publication     

An indirect effect of biological invasions: the effect of zebra mussel fouling on parasitisation of unionid mussels by bitterling fish

Invasive species represent a major threat with both direct and indirect effects on natural ecosystems, including effects on established and coevolved relationships. In a series of experiments, we examined how the interaction between two native species, a unionid mussel (Unio pictorum) and the European bitterling (Rhodeus amarus), a fish that parasitises unionids, was affected by the non-native zebra mussel (Dreissena polymorpha). The zebra mussel fouls hard substrates, including shells of living unionids, and its presence is often associated with a decrease in population density of native unionid mussels. Bitterling lay their eggs into live unionids and the embryos develop inside their gills. Using a range of zebra mussel densities, we demonstrated that zebra mussel fouling had a negative effect on the number of bitterling eggs inside the mussel host, with abundances of 5–10 zebra mussels (shell size 15–25 mm) per unionid critical for bitterling ability to utilise the host. In a further experiment, we found that bitterling did not discriminate between unfouled unionids and those fouled with five zebra mussels. Most ovipositions into fouled hosts, however, were unsuccessful as eggs failed to reach the unionid gills. We discuss implications of such unsuccessful ovipositions for bitterling recruitment and population dynamics.     

The site of settlement indicates commensalism between bluemussel and its epibiont

Summary The site of settlement of barnacles (Balanus improvisus) attached on shells of bluemussels (Mytilus edulis) was mapped from a sample of mussels collected in the Baltic Sea. Most barnacles had settled near the siphonal apertures of the mussel. An experiment was made to measure the disadvantages and advantages that living in close association brings to barnacles and mussels. The barnacles on shells of living mussels were shown to grow significantly faster than those on empty mussel shells. Presence of barnacles had no effects on growth of mussels. The two-species association under study was demonstrated to be a case of commensalism.     

Preferences of the Ponto-Caspian amphipod Dikerogammarus haemobaphes for living zebra mussels

A Ponto-Caspian amphipod Dikerogammarus haemobaphes has recently invaded European waters. In the recipient area, it encountered Dreissena polymorpha , a habitat-forming bivalve, co-occurring with the gammarids in their native range. We assumed that interspecific interactions between these two species, which could develop during their long-term co-evolution, may affect the gammarid behaviour in novel areas. We examined the gammarid ability to select a habitat containing living mussels and searched for cues used in that selection. We hypothesized that they may respond to such traits of a living mussel as byssal threads, activity (e.g. valve movements, filtration) and/or shell surface properties. We conducted the pairwise habitat-choice experiments in which we offered various objects to single gammarids in the following combinations: (1) living mussels versus empty shells (the general effect of living Dreissena ); (2) living mussels versus shells with added byssal threads and shells with byssus versus shells without it (the effect of byssus); (3) living mussels versus shells, both coated with nail varnish to neutralize the shell surface (the effect of mussel activity); (4) varnished versus clean living mussels (the effect of shell surface); (5) varnished versus clean stones (the effect of varnish). We checked the gammarid positions in the experimental tanks after 24 h. The gammarids preferred clean living mussels over clean shells, regardless of the presence of byssal threads under the latter. They responded to the shell surface, exhibiting preferences for clean mussels over varnished individuals. They were neither affected by the presence of byssus nor by mussel activity. The ability to detect and actively select zebra mussel habitats may be beneficial for D. haemobaphes and help it establish stable populations in newly invaded areas.     

Genetic Stratigraphy of Key Demographic Events in Arabia

At the crossroads between Africa and Eurasia, Arabia is necessarily a melting pot, its peoples enriched by successive gene flow over the generations. Estimating the timing and impact of these multiple migrations are important steps in reconstructing the key demographic events in the human history. However, current methods based on genome-wide information identify admixture events inefficiently, tending to estimate only the more recent ages, as here in the case of admixture events across the Red Sea (∼8–37 generations for African input into Arabia, and 30–90 generations for “back-to-Africa” migrations). An mtDNA-based founder analysis, corroborated by detailed analysis of the whole-mtDNA genome, affords an alternative means by which to identify, date and quantify multiple migration events at greater time depths, across the full range of modern human history, albeit for the maternal line of descent only. In Arabia, this approach enables us to infer several major pulses of dispersal between the Near East and Arabia, most likely via the Gulf corridor. Although some relict lineages survive in Arabia from the time of the out-of-Africa dispersal, 60 ka, the major episodes in the peopling of the Peninsula took place from north to south in the Late Glacial and, to a lesser extent, the immediate post-glacial/Neolithic. Exchanges across the Red Sea were mainly due to the Arab slave trade and maritime dominance (from ∼2.5 ka to very recent times), but had already begun by the early Holocene, fuelled by the establishment of maritime networks since ∼8 ka. The main “back-to-Africa” migrations, again undetected by genome-wide dating analyses, occurred in the Late Glacial period for introductions into eastern Africa, whilst the Neolithic was more significant for migrations towards North Africa.     

The contribution of marine snow to the particle food supply of the benthic suspension feeder, Mytilus edulis

In order to examine the importance of the settling of large particles to the food supply and feeding behavior of a benthic culture of the blue mussel, Mytilus edulis, we investigated the tidal dynamics of large (>0.5 mm diameter) marine aggregates, commonly known as marine snow, during three tidal cycles in July 1998 at a shallow, subtidal, low current flow regime site along the coast of Maine (Shorey Cove, Roque Island, Englishman's Bay, Maine).In situ, optically measured marine snow showed a distinct tidal signal displaying an increase in size and abundance through high tide with a peak on the early ebb tide as it settled to the bottom. Marine snow volume ranged an order of magnitude through the tidal cycle, from under 8 to over 80 mm³ l⁻¹. An increase in the in situ marine snow volume corresponded with an increase in benthic mussel feeding activity (from 20% to 60% of maximum exhalant siphon area, which is an estimate of pumping rate) and maximum rates of pseudofeces production by the mussels during periods of low tidal current speeds. In contrast, mussels from the same population feeding on surface waters in shipboard chambers produced no pseudofeces and had high pumping rates (80-100% maximum exhalant siphon area) over the whole tidal period. A second peak in benthic mussel pumping rates also occurred during flood tide.Food quality was lower in the bottom waters due to significantly higher particulate inorganic matter (PIM, >2 mg l⁻¹) when compared with the surface waters. PIM accounted for 95% of the total settled mass flux of 3.4 g m⁻² day⁻¹ measured in sediment traps deployed 1 m off the bottom, with organic carbon representing only 2.5% of the mass flux during the mid-summer conditions. At low-current sites such as Shorey Cove, Roque Island, Maine, the settling of marine snow provides an important additional source of food, albeit of low quality, to benthic populations of blue mussels.     

Classification of Domain Movements in Proteins Using Dynamic Contact Graphs

A new method for the classification of domain movements in proteins is described and applied to 1822 pairs of structures from the Protein Data Bank that represent a domain movement in two-domain proteins. The method is based on changes in contacts between residues from the two domains in moving from one conformation to the other. We argue that there are five types of elemental contact changes and that these relate to five model domain movements called: “free”, “open-closed”, “anchored”, “sliding-twist”, and “see-saw.” A directed graph is introduced called the “Dynamic Contact Graph” which represents the contact changes in a domain movement. In many cases a graph, or part of a graph, provides a clear visual metaphor for the movement it represents and is a motif that can be easily recognised. The Dynamic Contact Graphs are often comprised of disconnected subgraphs indicating independent regions which may play different roles in the domain movement. The Dynamic Contact Graph for each domain movement is decomposed into elemental Dynamic Contact Graphs, those that represent elemental contact changes, allowing us to count the number of instances of each type of elemental contact change in the domain movement. This naturally leads to sixteen classes into which the 1822 domain movements are classified.     

Contemporary Land Change Alters Fish Communities in a San Francisco Bay Watershed,California, U.S.A.

Urbanization is one of the leading threats to freshwater biodiversity, and urban regions continue to expand globally. Here we examined the relationship between recent urbanization and shifts in stream fish communities. We sampled fishes at 32 sites in the Alameda Creek Watershed, near San Francisco, California, in 1993–1994 and again in 2009, and we quantified univariate and multivariate changes in fish communities between the sampling periods. Sampling sites were classified into those downstream of a rapidly urbanizing area (“urbanized sites”), and those found in less impacted areas (“low-impacted sites”). We calculated the change from non-urban to urban land cover between 1993 and 2009 at two scales for each site (the total watershed and a 3km buffer zone immediately upstream of each site). Neither the mean relative abundance of native fish nor nonnative species richness changed significantly between the survey periods. However, we observed significant changes in fish community composition (as measured by Bray-Curtis dissimilarity) and a decrease in native species richness between the sampling periods at urbanized sites, but not at low-impacted sites. Moreover, the relative abundance of one native cyprinid (Lavinia symmetricus) decreased at the urbanized sites but not at low-impacted sites. Increased urbanization was associated with changes in the fish community, and this relationship was strongest at the smaller (3km buffer) scale. Our results suggest that ongoing land change alters fish communities and that contemporary resurveys are an important tool for examining how freshwater taxa are responding to recent environmental change.     

Biomineralization plasticity and environmental heterogeneity predict geographical resilience patterns of foundation species to future change

Although geographical patterns of species' sensitivity to environmental changes are defined by interacting multiple stressors, little is known about compensatory processes shaping regional differences in organismal vulnerability. Here, we examine large‐scale spatial variations in biomineralization under heterogeneous environmental gradients of temperature, salinity and food availability across a 30° latitudinal range (3,334 km), to test whether plasticity in calcareous shell production and composition, from juveniles to large adults, mediates geographical patterns of resilience to climate change in critical foundation species, the mussels Mytilus edulis and M. trossulus. We find shell calcification decreased towards high latitude, with mussels producing thinner shells with a higher organic content in polar than temperate regions. Salinity was the best predictor of within‐region differences in mussel shell deposition, mineral and organic composition. In polar, subpolar, and Baltic low‐salinity environments, mussels produced thin shells with a thicker external organic layer (periostracum), and an increased proportion of calcite (prismatic layer, as opposed to aragonite) and organic matrix, providing potentially higher resistance against dissolution in more corrosive waters. Conversely, in temperate, higher salinity regimes, thicker, more calcified shells with a higher aragonite (nacreous layer) proportion were deposited, which suggests enhanced protection under increased predation pressure. Interacting effects of salinity and food availability on mussel shell composition predict the deposition of a thicker periostracum and organic‐enriched prismatic layer under forecasted future environmental conditions, suggesting a capacity for increased protection of high‐latitude populations from ocean acidification. These findings support biomineralization plasticity as a potentially advantageous compensatory mechanism conferring Mytilus species a protective capacity for quantitative and qualitative trade‐offs in shell deposition as a response to regional alterations of abiotic and biotic conditions in future environments. Our work illustrates that compensatory mechanisms, driving plastic responses to the spatial structure of multiple stressors, can define geographical patterns of unanticipated species resilience to global environmental change.