Ecosystem engineering alters density-dependent feedbacks in an aquatic insect population

Ecosystem engineers have large impacts on the communities in which they live, and these impacts may feed back to populations of engineers themselves. In this study, we assessed the effect of ecosystem engineering on density-dependent feedbacks for midges in Lake Mývatn, Iceland. The midge larvae reside in the sediment and build silk tubes that provide a substrate for algal growth, thereby elevating benthic primary production. Benthic algae are in turn the primary food source for the midge larvae, setting the stage for the effects of engineering to feed back to the midges themselves. Using a field mesocosm experiment manipulating larval midge densities, we found a generally positive but nonlinear relationship between density and benthic production. Furthermore, adult emergence increased with the primary production per midge larva. By combining these two relationships in a simple model, we found that the positive effect of midges on benthic production weakened the negative density dependence at low to intermediate larval densities. However, this benefit disappeared at high densities when midge consumption of primary producers exceeded their positive effects on primary production through ecosystem engineering. Our results illustrate how ecosystem engineering can alter density-dependent feedbacks for engineer populations.

The rise of the three-spined stickleback – eco-evolutionary consequences of a mesopredator release

Declines of large predatory fish due to overexploitation are restructuring food webs across the globe. It is now becoming evident that restoring these altered food webs requires addressing not only ecological processes, but evolutionary ones as well, because human-induced rapid evolution may in turn affect ecological dynamics. In the central Baltic Sea, abundances of the mesopredatory fish, the three-spined stickleback (Gasterosteus aculeatus), have increased dramatically during the past decades. Time-series data covering 22 years show that this increase coincides with a decline in the number of juvenile perch (Perca fluviatilis), the most abundant predator of stickleback along the coast. We studied the interaction between evolutionary and ecological effects of this mesopredator take-over, by surveying the armour plate morphology of stickleback and the structure of the associated food web. First, we investigated the distribution of different stickleback phenotypes depending on predator abundances and benthic production; and described the stomach content of the stickleback phenotypes using metabarcoding. Second, we explored differences in the relation between different trophic levels and benthic production, between bays where the relative abundance of fish was dominated by stickleback or not; and compared this to previous cage-experiments to support causality of detected correlations. We found two distinct lateral armour plate phenotypes of stickleback, incompletely and completely plated. The proportion of incompletely plated individuals increased with increasing benthic production and decreasing abundances of adult perch. Stomach content analyses showed that the completely plated individuals had a stronger preference for invertebrate herbivores (amphipods) than the incompletely plated ones. In addition, predator dominance interacted with ecosystem production to determine food web structure and the propagation of a trophic cascade: with increasing production, biomass accumulated on the first (macroalgae) and third (stickleback) trophic levels in stickleback-dominated bays, but on the second trophic level (invertebrate herbivores) in perch-dominated bays. Since armour plates are defence structures favoured by natural selection in the presence of fish predators, the phenotype distribution suggest that a novel low-predation regime favours sticklebacks with less armour. Our results indicate that an interaction between evolutionary and ecological effects of the stickleback take-over has the potential to affect food web dynamics.

Cladoceran (Crustacea) Niches, Sex, and Sun Bathing—A Long-Term Record of Tundra Lake (Lapland) Functioning and Paleo-Optics

Under fundamental ecosystem changes in high latitude lakes, a functional paleolimnological approach may increase holistic understanding of lake responses and resilience to climate warming. A ~2000-year sediment record from Lake Loažžejávri in the tundra of northern Finnish Lapland was examined for fossil Cladocera assemblages to examine long-term environmental controls on aquatic communities. In addition, cladoceran functional attributes, including functional diversity (FD), UV absorbance (ABSUV) of Alona carapaces, and sexual reproduction (ephippia) in Bosmina and Chydoridae were analyzed. Cladoceran communities responded to a major change in benthic habitat quality, reflected as elevated (increasingly benthic) sediment organic matter δ13C signal since the 17th century. FD fluctuations showed association with climate oscillation, FD being generally higher during warm climate periods. These ecological changes were likely attributable to diversification of littoral-benthic consumer habitat space. ABSUV, irrespective of increases during the Little Ice Age (LIA) due to higher UV transparency of lake water, was lower under increasing autochthony (benthic production) suggesting establishment of physical UV refugia by the benthic vegetative substrata. Bosmina ephippia exhibited a decreasing trend associated with increasing benthic production, indicating favorable environmental regime, and, together with chydorid ephippia, transient increases during the climate cooling of the LIA driven by shorter open-water season.

Trophodynamics of nekton assemblages and relationships with estuarine habitat structure across a subtropical estuary

Understanding trophodynamics of estuarine and marine ecosystems is important in developing food-web models prevalent in ecosystem-based approaches to fisheries management, as well as assessing ecosystem condition and function. Few studies have examined seasonal and spatial trophodynamics of nekton assemblages in shallow subtropical waters on estuary-wide spatial extent. We used fisheries-independent monitoring data to examine the spatial distribution and seasonal trophodynamics of nekton assemblages in shallow waters (≤ 1.5m) of Charlotte Harbor, Florida, USA, as well as the lower Peace and Myakka Rivers. Overall nekton densities were generally high in spring (April - May), especially in the lower rivers, and declined through the summer (June – October), and fall (November – December), with small increases in mean nekton densities in winter (January – March). We found that species composition as well as trophic structure of nekton assemblages changed along a broad spatial gradient from the lower portions of the Peace and Myakka Rivers to lower Charlotte Harbor near the passes to the Gulf of Mexico. Nekton assemblages dominated by planktonic-feeding species were located in lower Peace and Myakka Rivers and extended into upper Charlotte Harbor in apparent response to seasonal fluctuations in freshwater inflow. In contrast, most of the nekton assemblages within Charlotte Harbor proper were dominated by nekton species feeding within the benthos (those feeding on benthic infauna, epifauna, seagrass, and/or detritus) throughout the year. The proportion of benthic feeders was positively correlated with landscape metrics describing the area of continuous seagrass beds mapped from aerial photography, and negatively correlated with distance to the nearest pass to the Gulf of Mexico. These relationships were consistent throughout all four seasons of the year. The proportion of benthic feeders varied seasonally, and was relatively high in the fall and winter compared with the spring and summer. The predominance of benthic feeding species during fall and winter when overall nekton densities were low, suggests that benthic production is an important in supporting populations of forage fishes for higher trophic levels, such as piscivorous fishes and wading birds during this time of year. This is an important time for overwintering wading birds, as well as several economically-important fishery species who are recruiting into shallow water areas of Charlotte Harbor. Planktonic production is often the focus of food-web models with benthic production sometimes not included. We found both planktonic and benthic production to be important in Charlotte Harbor. Therefore, both sources of production need to be included in food-web models for Charlotte Harbor, as well as similar subtropical estuaries with relatively large areas of seagrass beds.

Trophodynamics of nekton assemblages and relationships with estuarine habitat structure across a subtropical estuary

Understanding trophodynamics of estuarine and marine ecosystems is important in developing food-web models prevalent in ecosystem-based approaches to fisheries management, as well as assessing ecosystem condition and function. Few studies have examined seasonal and spatial trophodynamics of nekton assemblages in shallow subtropical waters on estuary-wide spatial extent. We used fisheries-independent monitoring data to examine the spatial distribution and seasonal trophodynamics of nekton assemblages in shallow waters (≤ 1.5m) of Charlotte Harbor, Florida, USA, as well as the lower Peace and Myakka Rivers. Overall nekton densities were generally high in spring (April - May), especially in the lower rivers, and declined through the summer (June – October), and fall (November – December), with small increases in mean nekton densities in winter (January – March). We found that species composition as well as trophic structure of nekton assemblages changed along a broad spatial gradient from the lower portions of the Peace and Myakka Rivers to lower Charlotte Harbor near the passes to the Gulf of Mexico. Nekton assemblages dominated by planktonic-feeding species were located in lower Peace and Myakka Rivers and extended into upper Charlotte Harbor in apparent response to seasonal fluctuations in freshwater inflow. In contrast, most of the nekton assemblages within Charlotte Harbor proper were dominated by nekton species feeding within the benthos (those feeding on benthic infauna, epifauna, seagrass, and/or detritus) throughout the year. The proportion of benthic feeders was positively correlated with landscape metrics describing the area of continuous seagrass beds mapped from aerial photography, and negatively correlated with distance to the nearest pass to the Gulf of Mexico. These relationships were consistent throughout all four seasons of the year. The proportion of benthic feeders varied seasonally, and was relatively high in the fall and winter compared with the spring and summer. The predominance of benthic feeding species during fall and winter when overall nekton densities were low, suggests that benthic production is an important in supporting populations of forage fishes for higher trophic levels, such as piscivorous fishes and wading birds during this time of year. This is an important time for overwintering wading birds, as well as several economically-important fishery species who are recruiting into shallow water areas of Charlotte Harbor. Planktonic production is often the focus of food-web models with benthic production sometimes not included. We found both planktonic and benthic production to be important in Charlotte Harbor. Therefore, both sources of production need to be included in food-web models for Charlotte Harbor, as well as similar subtropical estuaries with relatively large areas of seagrass beds.

Human Influence at the Coast: Upland and Shoreline Stressors Affect Coastal Macrofauna and Are Mediated by Salinity

Anthropogenic stressors can affect subtidal communities within the land-water interface. Increasing anthropogenic activities, including upland and shoreline development, threaten ecologically important species in these habitats. In this study, we examined the consequences of anthropogenic stressors on benthic macrofaunal communities in 14 subestuaries of Chesapeake Bay. We investigated how subestuary upland use (forested, agricultural, developed land) and shoreline development (riprap and bulkhead compared to marsh and beach) affected density, biomass, and diversity of benthic infauna. Upland and shoreline development were parameters included in the most plausible models among a candidate set compared using corrected Akaike’s Information Criterion. For benthic macrofauna, density tended to be lower in subestuaries with developed or mixed compared to forested or agricultural upland use. Benthic biomass was significantly lower in subestuaries with developed compared to forested upland use, and biomass declined exponentially with proportion of near-shore developed land. Benthic density did not differ significantly among natural marsh, beach, and riprap habitats, but tended to be lower adjacent to bulkhead shorelines. Including all subestuaries, there were no differences in diversity by shoreline type. In low salinities, benthic Shannon (H′) diversity tended to be higher adjacent to natural marshes compared to the other habitats, and lower adjacent to bulkheads, but the pattern was reversed in high salinities. Sediment characteristics varied by shoreline type and contributed to differences in benthic community structure. Given the changes in the infaunal community with anthropogenic stressors, subestuary upland and shoreline development should be minimized to increase benthic production and subsequent trophic transfer within the food web.

Benthic macrofaunal structure and secondary production in tropical estuaries on the Eastern Marine Ecoregion of Brazil

Estuaries are highly productive and support diverse benthic assemblages, but few estimates of benthic production are available for most ecosystems. In tropical estuaries mangroves and tidal flats are typical habitats with high spatial heterogeneity of benthic macrofaunal assemblages. However, accessing differences and similarities of benthic assemblages within estuarine habitats and between regional ecosystems may provide scientific support to management of those ecosystems. Here we studied three tropical estuaries in the Eastern Marine Ecoregion of Brazil to assess the spatial variability of benthic assemblages from vegetated (mangroves) and unvegetated (tidal flats) habitats. A nested sampling design was used to determine spatial scales of variability in benthic macrofaunal density, biomass and secondary production. Habitat differences in benthic assemblage composition, biomass, density and secondary production were significant, but also varied between estuaries. Macrofaunal secondary production differed between estuaries and between tidal flat and mangrove habitats, and those differences were related to the composition of benthic assemblages. High benthic production were associated with tidal flats in estuaries with presumable less human impacts, although benthic assemblages from mangrove sediments had similar production irrespective of human disturbances. Given variable levels of human impacts and predicted climate change effects on tropical estuarine assemblages in Eastern Brazil, our data support the use of benthic secondary production to address long-term changes and improved management of estuaries in Eastern Brazil.

Benthic macrofaunal structure and secondary production in tropical estuaries on the Eastern Marine Ecoregion of Brazil

Estuaries are highly productive and support diverse benthic assemblages, but few estimates of benthic production are available for most ecosystems. In tropical estuaries mangroves and tidal flats are typical habitats with high spatial heterogeneity of benthic macrofaunal assemblages. However, accessing differences and similarities of benthic assemblages within estuarine habitats and between regional ecosystems may provide scientific support to management of those ecosystems. Here we studied three tropical estuaries in the Eastern Marine Ecoregion of Brazil to assess the spatial variability of benthic assemblages from vegetated (mangroves) and unvegetated (tidal flats) habitats. A nested sampling design was used to determine spatial scales of variability in benthic macrofaunal density, biomass and secondary production. Habitat differences in benthic assemblage composition, biomass, density and secondary production were significant, but also varied between estuaries. Macrofaunal secondary production differed between estuaries and between tidal flat and mangrove habitats, and those differences were related to the composition of benthic assemblages. High benthic production were associated with tidal flats in estuaries with presumable less human impacts, although benthic assemblages from mangrove sediments had similar production irrespective of human disturbances. Given variable levels of human impacts and predicted climate change effects on tropical estuarine assemblages in Eastern Brazil, our data support the use of benthic secondary production to address long-term changes and improved management of estuaries in Eastern Brazil.