Jennifer A. Dijkstra

Experimental and Natural Warming Elevates Mercury Concentrations in Estuarine Fish

Marine food webs are the most important link between the global contaminant, methylmercury (MeHg), and human exposure through consumption of seafood. Warming temperatures may increase human exposure to MeHg, a potent neurotoxin, by increasing MeHg production as well as bioaccumulation and trophic transfer through marine food webs. Studies of the effects of temperature on MeHg bioaccumulation are rare and no study has specifically related temperature to MeHg fate by linking laboratory experiments with natural field manipulations in coastal ecosystems. We performed laboratory and field experiments on MeHg accumulation under varying temperature regimes using the killifish, Fundulus heteroclitus. Temperature treatments were established in salt pools on a coastal salt marsh using a natural temperature gradient where killifish fed on natural food sources. Temperatures were manipulated across a wider range in laboratory experiments with killifish exposed to MeHg enriched food. In both laboratory microcosms and field mesocosms, MeHg concentrations in killifish significantly increased at elevated temperatures. Moreover, in field experiments, other ancillary variables (salinity, MeHg in sediment, etc.) did not relate to MeHg bioaccumulation. Modeling of laboratory experimental results suggested increases in metabolic rate as a driving factor. The elevated temperatures we tested are consistent with predicted trends in climate warming, and indicate that in the absence of confounding factors, warmer sea surface temperatures could result in greater in bioaccumulation of MeHg in fish, and consequently, increased human exposure.

Invasive seaweeds transform habitat structure and increase biodiversity of associated species

Summary The visual landscape of marine and terrestrial systems is changing as a result of anthropogenic factors. Often these shifts involve introduced species that are morphologically dissimilar to native species, creating a unique biogenic structure and habitat for associated species within the landscape. While community-level changes as a result of introduced species have been documented in both terrestrial and marine systems, it is still unclear how long-term shifts in species composition will affect habitat complexity or its potential to influence the biodiversity of species that occur at the base of the food web. We analysed quadrat photos collected at several subtidal sites in the Gulf of Maine over a 30+ year period, and collected individual seaweed species to determine their complexity and the biodiversity of meso-invertebrates associated with each species. By coupling the relationship of 30+ years of shifts in seaweed assemblages, morphological structure of the seaweed assemblage, and their meso-invertebrates, we determined introduced seaweeds have increased by up to 90%, corresponding to a rise in two-dimensional (2D) structure, and a decline in canopy height of subtidal rocky habitats. The highly complex two-dimensional habitat provided by introduced filamentous red seaweeds supports two to three times more meso-invertebrate individuals and species that form the base of the food web than simpler forms of morphological habitat. Synthesis. The present study demonstrates a long-term shift in foundation species towards a dominance of invasive seaweeds that directly reduce canopy height and increase the 2D biogenic structure of the habitat. These introduced seaweeds harbour greater biodiversity of species found at the base of the food web than seaweeds with simpler forms such as the native kelp species. Such shifts in habitat structure will propagate to food webs by influencing the structure of lower trophic-level meso-invertebrates and indirectly upper trophic-level species that feed on these invertebrates and use the seaweed structure as refuge.

Introduced species provide a novel temporal resource that facilitates native predator population growth

Non-native species are recognized as important components of change to food web structure. Non-native prey may increase native predator populations by providing an additional food source and simultaneously decrease native prey populations by outcompeting them for a limited resource. This pattern of apparent competition may be important for plants and sessile marine invertebrate suspension feeders as they often compete for space and their immobile state make them readily accessible to predators. Reported studies on apparent competition have rarely been examined in biological invasions and no study has linked seasonal patterns of native and non-native prey abundance to increasing native predator populations. Here, we evaluate the effects of non-native colonial ascidians (Diplosoma listerianum and Didemnum vexillum) on population growth of a native predator (bloodstar, Henricia sanguinolenta) and native sponges through long-term surveys of abundance, prey choice and growth experiments. We show non-native species facilitate native predator population growth by providing a novel temporal resource that prevents loss of predator biomass when its native prey species are rare. We expect that by incorporating native and non-native prey seasonal abundance patterns, ecologists will gain a more comprehensive understanding of the mechanisms underlying the effects of non-native prey species on native predator and prey population dynamics.

The influence of substrate material on ascidian larval settlement

Submerged man-made structures present novel habitat for marine organisms and often host communities that differ from those on natural substrates. Although many factors are known to contribute to these differences, few studies have directly examined the influence of substrate material on organism settlement. We quantified larval substrate preferences of two species of ascidians, Ciona intestinalis (cryptogenic, formerly C. intestinalis type B) and Botrylloides violaceus (non-native), on commonly occurring natural (granite) and man-made (concrete, high-density polyethylene, PVC) marine materials in laboratory trials. Larvae exhibited species-specific settlement preferences, but generally settled more often than expected by chance on concrete and HDPE. Variation in settlement between materials may reflect preferences for rougher substrates, or may result from the influence of leached chemicals on ascidian settlement. These findings indicate that an experimental plate material can influence larval behavior and may help us understand how substrate features may contribute to differences in settlement in the field.

Dedication: Dr. Michele L. Dionne (1954–2012)

In July 2012, the estuarine and coastal research and conservation community lost one of its greatest champions with the passing of Dr. Michele L. Dionne. Throughout her career, Michele’s contribution to coastal conservation extended well beyond her insights into research. She was tireless in her campaign to advance our understanding of salt marsh ecology, conservation, and restoration. Her love of coastal systemsmade her a visionary and a fighter. She was unselfish. When she did something, it was for her community, her students, or the coastal ecosystems she was charged to protect and conserve. She had the ability to think outside of the box, to see the forest through the trees or the nekton through the mummichogs. Michele began her scientific journey by receiving a Bachelor of Arts in Biology from Bates College in 1977. She then went on to get her Master of Science in Zoology from the University of North Carolina at Chapel Hill in 1982 where she studied the importance of cannibalism in fish populations. She continued her graduate studies by receiving a PhD in Biology from Dartmouth College in 1991 where she investigated habitat complexity and fish behavior. After graduating from Dartmouth College, Michele went on to become the first scientist and Director of Research at the newly created Wells National Estuarine Research Reserve (WNERR). During her tenure as the Director of Research at WNERR, she transformed the research capabilities of the reserve by generating millions of external research funds that went into building the Coastal Ecology Center that serves to house and provide modern laboratory facilities for internal research and education, external research scientists, and graduate students. This facility has and continues to nurture cadres of future ecologists and conservationists, as well as provides the infrastructure needed for well-established scientists to continue doing research in marshes and other coastal ecosystems throughout NewEngland. Her efforts also went into garnering a trust that provides funds for a post-doctoral fellow to perform high-level research in conservation ecology. Michele, her students, and post-doctoral fellows performed a variety of research activities that included studying the utilization of marsh ecosystems by fish (Dionne et al. 1999; MacKenzie and Dionne 2008), coastal restoration (Burdick and Dionne 1994; Burdick et al. 1997; Koniski et al. 2006), assessing human and climate threats on coastal systems (Chen et al. 2009; Crain et al. 2009; Dijkstra et al. 2013; Eberhardt et al. 2011), and investigating species-specific interactions in marsh ecosystems (Dijkstra et al. 2012; Tyrrell et al. 2012). She was a proponent of long-term monitoring that could provide effective baseline data sets to determine the conditions of these ecosystems (Neckles and Dionne 2000). Once asked if there was any part of research she didn’t find interesting. She replied with an emphatic, BNo! I enjoy all aspects of research!^. While Michele was successful in publishing papers and book chapters on marsh ecology and conservation, it was her unwavering support of undergraduate students, graduate students, and post-doctoral students that best describes the legacy she has left behind. She provided opportunities for so many students to discover and to grow; many of these students are now ecologists and conservationists at institutions across North America. She can still be seen today in the many presentations her students or colleagues give at the Coastal and Communicated by Iris C. Anderson