Intertidal mussel reefs change the composition and size distribution of diatoms in the biofilm

Migrating diatoms are microscopic ecosystem engineering organisms that have functional consequences on the seascape scale by significantly contributing to the microphytobenthos biofilm. The microphytobenthos biofilm is a thin photosynthesising layer that covers the sediment on intertidal flats. It fuels the food web, increases sediment stability, and enhances the deposition of particles, providing ecosystem services to coastal communities. Here we tested the effect of another ecosystem engineering habitat, intertidal blue mussel reefs, on the composition and properties of migrating diatom communities. Small-scale reefs constructed in the intertidal mimicked and reinforced the natural pattern in diatom community composition and function that we documented in the field. The field experiment adding small reefs to the intertidal ran from 30 April to 10 June 2015 and the field samples were collected around a natural blue mussel bed on the same tidal flat on 7 October 2015 (N 53.489 degrees, E 6.230 degrees). Both the constructed small-scale reefs and the natural reef changed the community composition of diatoms in the biofilm by promoting higher numbers of smaller-sized cells and species. Small diatoms have higher growth and gross photosynthesis rates, indicating that this explains the higher production and chlorophyll-a concentration of the biofilm measured on natural intertidal shellfish reefs. Our results showed that shellfish reefs have a large impact on biofilm functioning. However, biofilms are also fuel for the shellfish, indicating that the two very different ecosystem engineers may facilitate coexistence on tidal flats through a positive feedback loop

Increases of opportunistic species in response to ecosystem change:The case of the Baltic Sea three-spined stickleback

Under rapid environmental change, opportunistic species may exhibit dramatic increases in response to the altered conditions, and can in turn have large impacts on the ecosystem. One such species is the three-spined stickleback (Gasterosteus aculeatus), which has shown substantial increases in several aquatic systems in recent decades. Here, we review the population development of the stickleback in the Baltic Sea, a large brackish water ecosystem subject to rapid environmental change. Current evidence points to predatory release being the central driver of the population increases observed in some areas, while both eutrophication and climate change have likely contributed to creating more favourable conditions for the stickleback. The increasing stickleback densities have had profound effects on coastal ecosystem function by impairing the recruitment of piscivorous fish and enhancing the effects of eutrophication through promoting the production of filamentous algae. The increase poses a challenge for both environmental management and fisheries, where a substantial interest from the pelagic fisheries fleet in exploiting the species calls for urgent attention. While significant knowledge gaps remain, we suggest that the case of the Baltic Sea stickleback increase provides generalisable lessons of value for understanding and managing other coastal ecosystems under rapid change

Mussel beds are biological power stations on intertidal flats

Intertidal flats are highly productive areas that support large numbers of invertebrates, fish, and birds. Benthic diatoms are essential for the function of tidal flats. They fuel the benthic food web by forming a thin photosynthesizing compartment in the top-layer of the sediment that stretches over the vast sediment flats during low tide. However, the abundance and function of the diatom film is not homogenously distributed. Recently, we have realized the importance of bivalve reefs for structuring intertidal ecosystems; by creating structures on the intertidal flats they provide habitat, reduce hydrodynamic stress and modify the surrounding sediment conditions, which promote the abundance of associated organisms. Accordingly, field studies show that high chlorophyll a concentration in the sediment co-vary with the presence of mussel beds. Here we present conclusive evidence by a manipulative experiment that mussels increase the local biomass of benthic microalgae; and relate this to increasing biomass of microalgae as well as productivity of the biofilm across a nearby mussel bed. Our results show that the ecosystem engineering properties of mussel beds transform them into hot spots for primary production on tidal flats, highlighting the importance of biological control of sedimentary systems. (C) 2017 Elsevier Ltd. All rights reserved.</p

Biodiversity change is uncoupled from species richness trends: consequences for conservation and monitoring

Global concern about human impact on biological diversity has triggered an intense research agenda on drivers and consequences of biodiversity change in parallel with international policy seeking to conserve biodiversity and associated ecosystem functions. Quantifying the trends in biodiversity is far from trivial, however, as recently documented by meta-analyses, which report little if any net change in local species richness through time. Here, we summarise several limitations of species richness as a metric of biodiversity change and show that the expectation of directional species richness trends under changing conditions is invalid. Instead, we illustrate how a set of species turnover indices provide more information content regarding temporal trends in biodiversity, as they reflect how dominance and identity shift in communities over time. We apply these metrics to three monitoring datasets representing different ecosystem types. In all datasets, nearly complete species turnover occurred, but this was disconnected from any species richness trends. Instead, turnover was strongly influenced by changes in species presence (identities) and dominance (abundances). We further show that these metrics can detect phases of strong compositional shifts in monitoring data and thus identify a different aspect of biodiversity change decoupled from species richness. Synthesis and applications: Temporal trends in species richness are insufficient to capture key changes in biodiversity in changing environments. In fact, reductions in environmental quality can lead to transient increases in species richness if immigration or extinction has different temporal dynamics. Thus, biodiversity monitoring programmes need to go beyond analyses of trends in richness in favour of more meaningful assessments of biodiversity change

Parallel ecological networks in ecosystems

In ecosystems, species interact with other species directly and through abiotic factors in multiple ways, often forming complex networks of various types of ecological interaction. Out of this suite of interactions, predator–prey interactions have received most attention. The resulting food webs, however, will always operate simultaneously with networks based on other types of ecological interaction, such as through the activities of ecosystem engineers or mutualistic interactions. Little is known about how to classify, organize and quantify these other ecological networks and their mutual interplay. The aim of this paper is to provide new and testable ideas on how to understand and model ecosystems in which many different types of ecological interaction operate simultaneously. We approach this problem by first identifying six main types of interaction that operate within ecosystems, of which food web interactions are one. Then, we propose that food webs are structured among two main axes of organization: a vertical (classic) axis representing trophic position and a new horizontal ‘ecological stoichiometry’ axis representing decreasing palatability of plant parts and detritus for herbivores and detrivores and slower turnover times. The usefulness of these new ideas is then explored with three very different ecosystems as test cases: temperate intertidal mudflats; temperate short grass prairie; and tropical savannah

Habitat-Mediated Facilitation and Counteracting Ecosystem Engineering Interactively Influence Ecosystem Responses to Disturbance

Recovery of an ecosystem following disturbance can be severely hampered or even shift altogether when a point disturbance exceeds a certain spatial threshold. Such scale-dependent dynamics may be caused by preemptive competition, but may also result from diminished self-facilitation due to weakened ecosystem engineering. Moreover, disturbance can facilitate colonization by engineering species that alter abiotic conditions in ways that exacerbate stress on the original species. Consequently, establishment of such counteracting engineers might reduce the spatial threshold for the disturbance, by effectively slowing recovery and increasing the risk for ecosystem shifts to alternative states. We tested these predictions in an intertidal mudflat characterized by a two-state mosaic of hummocks (humps exposed during low tide) dominated by the sediment-stabilizing seagrass Zostera noltii) and hollows (low-tide waterlogged depressions dominated by the bioturbating lugworm Arenicola marina). In contrast to expectations, seagrass recolonized both natural and experimental clearings via lateral expansion and seemed unaffected by both clearing size and lugworm addition. Near the end of the growth season, however, an additional disturbance (most likely waterfowl grazing and/or strong hydrodynamics) selectively impacted recolonizing seagrass in the largest (1 m2) clearings (regardless of lugworm addition), and in those medium (0.25 m2) clearings where lugworms had been added nearly five months earlier. Further analyses showed that the risk for the disturbance increased with hollow size, with a threshold of 0.24 m2. Hollows of that size were caused by seagrass removal alone in the largest clearings, and by a weaker seagrass removal effect exacerbated by lugworm bioturbation in the medium clearings. Consequently, a sufficiently large disturbance increased the vulnerability of recolonizing seagrass to additional disturbance by weakening seagrass engineering effects (sediment stabilization). Meanwhile, the counteracting ecosystem engineering (lugworm bioturbation) reduced that threshold size. Therefore, scale-dependent interactions between habitat-mediated facilitation, competition and disturbance seem to maintain the spatial two-state mosaic in this ecosystem

Multiple fish predators:Effects of identity, density, and nutrients on lower trophic levels

The ongoing loss of predators is changing the composition of food webs, with largely unknown consequences. In particular, the effects of multiple fish predators on food webs are difficult to predict due to the prevalence of omnivory and intra guild predation. At the same time, many ecosystems experience high nutrient loads that fuel food webs from the bottom up. To test the combined effects of both multiple predators and nutrient enrichment on food web composition, we created monocultures of perch, roach, and stickleback as well as mixed assemblages of all 3 species at 3 different density levels using an additive design, and induced nutrient en richment in half of the cages. The biomass of invertebrate herbivores and algae was measured. Stomach analyses of predators were used to detect prey switching. Herbivore biomass depended mainly on predator identity, while top-down effects on algae were mediated by predator density and nutrient en richment. Specifically, perch strongly reduced amphipods and isopods, and roach mainly reduced gastropods but also isopods, while stickleback had weak overall effects on herbivores. These species-specific effects were attenuated in the mixed fish assemblages, probably due to prey switching. Algal growth strongly increased under high fish density and nutrient enrichment, but was not affected by predator-induced changes in the herbivore composition. This study shows that identity effects from predators in isolation are attenuated in multiple predator assemblages, probably due to increasing interference among the predators. Algal biomass was enhanced by nutrient enrichment and high predator density, which affected algae probably through non-lethal effects, such as reduced activity of the herbivores