Diana J. Hamilton

Predation of Zebra Mussels by Diving Ducks: An Exclosure Study

Zebra mussels (Dreissena polymorpha) are a novel and abundant potential food source for several species of diving ducks in the Great Lakes region. Using predator exclusion cages, behavioral observations, and analyses of duck gizzard contents, we ex- amined the predator-prey relationship between zebra mussels and their duck predators during the fall migratory period in Lake Erie at Point Pelee, Ontario. Diving ducks feeding on zebra mussels in the area reduced mussel biomass by 57% during the period of heaviest feeding, but had no measurable impact on mussel numbers. Birds were size-selective predators, preferentially taking medium and large mussels over the more common small ones, thereby altering the size structure of the mussel population. Ducks were abundant at Point Pelee only during late fall, and differences between cages and control areas had disappeared by the following spring. Overall, ducks had little lasting impact on mussel populations, but mussel abundance may have determined duck concentration in the area. Duck staging populations were higher in 1990, when mussels were more abundant, than in 1991, when mussels were much scarcer. Effective predator control of zebra mussels in the portion of the Great Lakes region that freezes over winter is unlikely because diving ducks are unable to overwinter in the area. However, mussels have become an important food source for diving ducks, and temporary reductions of mussel density through predation are likely in areas where ducks feed during migratory periods. Similarly, in regions where ducks are resident through winter, greater and longer lasting effects may be observed.

Variability in Foraging Behavior and Implications for Diet Breadth among Semipalmated Sandpipers Staging in the Upper Bay of Fundy

Abstract. n During its fall migration stopover on mudflats in the upper Bay of Fundy, Canada, the Semipalmated Sandpiper (Calidris pusilla) is thought to feed primarily on the amphipod Corophium volutator (mudshrimp). Semipalmated Sandpipers typically use a peck-probe foraging strategy and, until recently, there had been little evidence of variability or opportunism in their foraging habits during this stopover. From 2006 to 2008 we recorded data on the sandpipers foraging behavior and food availability at three commonly used mudflats. Behavior and food availability varied considerably at one site in one year. In 2006 at Grande Anse, where mudshrimp densities were exceptionally low and ostracod densities very high, the peck-probe strategy was almost completely abandoned for “skimming,” a foraging behavior novel in this species. Because of similarities between skimming and grazing, used by Western Sandpipers (Calidris mauri) to consume biofilm, we hypothesized that birds switched to skimming to feed on biofilm. However, chlorophyll a concentration in the top 2–3 mm of sediment, an index of biofilm abundance, was not a good predictor of proportion of time spent skimming. Instead, skimming had a strong, positive relationship with ostracod density, suggesting that the sandpipers skimmed opportunistically to feed on ostracods rather than to target biofilm. Thus Semipalmated Sandpipers are capable of adapting to changes at traditional staging areas by using novel foraging mechanisms, apparently to forage opportunistically on alternative prey. If staging habitats continue to change, alternative foods and foraging modes may become increasingly important to the success of this species migration.

Spatial and Temporal Variation in the Population Dynamics of the Intertidal Amphipod Corophium Volutator in the Upper Bay of Fundy, Canada

Abstract The scale of variation in population densities and other demographic variables is an important consideration in the design and interpretation of experiments and sampling programs. Here, we studied spatial and temporal variation in populations of Corophium volutator, an intertidal amphipod that is the most abundant macro-invertebrate on mudflats in the upper Bay of Fundy, Canada. Variables that were quantified included density (peaked in mid-summer at 10,000s per m2), proportion juvenile (an index of population structure), sex ratio (female-biased throughout the year), proportion of females that were ovigerous (reproduction occurs between May and August), and fecundity (number of embryos per female). We studied populations at 4 different sites, i.e., separate mudflats, for 1–2 years, and estimated variance components at 3 spatial scales: metres (samples), 100s of metres (transects) and many kilometres (mudflats). Population density exhibited low variation between years (< 10% of the random variation), but showed high variation at our smallest and largest spatial scales (45% between samples, 2% between transects, and 40% between mudflats), i.e., the distribution of C. volutator within a mudflat was aggregated at the scale of metres, but not at that of 100s of metres. Thus, relatively few transects, but many samples per transect, are required for good representation of density on a mudflat. Fecundity was similar between mudflats, but proportion juvenile, sex ratio, and proportion of ovigerous females were strongly affected by site or the interaction between month and site (> 60% of the random variation in each case). The high variation observed between mudflats for most demographic variables demonstrates that several control sites are necessary for measuring natural variation, a critical consideration in studies of environmental impacts.

Apparent redox potential discontinuity (aRPD) depth as a relative measure of sediment oxygen content and habitat quality

As hypoxic conditions spread in our oceans, indices that quickly and efficiently assess oxygen content in sediment pore water, and habitat quality are increasingly becoming desirable. Depth to the apparent redox potential discontinuity (aRPD) in sediment is one such index; however, the relationship between aRPD depth and oxygen content is still unclear. We quantified oxygen content in sediment pore water with a dissolved oxygen (DO) probe, utilizing luminescent technology, to determine how DO concentration varies with depth in the sediment, and with position relative to the aRPD (above versus below) on an intertidal mudflat in the Bay of Fundy, Canada. Based on generalized linear regression models, we found that depth in the sediment was a poor predictor of pore water DO concentration, while position in the sediment relative to the aRPD was a good predictor. Further, sediment above the aRPD had higher DO content than sediment below. We maintain that aRPD depth is a good measure of relative oxygen content in sediment pore water, as well as habitat quality. However, we recommend caution as the factors that influence aRPD depth are numerous and variable; therefore, the relationship between aRPD depth and DO content should be experimentally confirmed in any system of interest before aRPD depth is used as an indicator.

Dispersal of marine benthic invertebrates through ice rafting

Knowledge of dispersal vectors used by organisms is essential to the understanding of population and community dynamics. We report on ice rafting, a vector by which intertidal benthic invertebrates can be transported well outside their normal dispersal range during winter in temperate climates. We found multiple invertebrate taxa in sediment-laden ice blocks sampled in the intertidal zone. A large proportion of individuals were alive and active when freed from the ice. Using radio tracking, we found that ice blocks can travel over 20 km within a few days. Given the abundance of highly mobile ice blocks carrying viable invertebrates, we conclude that ice-rafting is likely an important dispersal vector, contributing to spatial community dynamics in intertidal systems. This mechanism helps explain observed genetic structure of populations, but it also raises concerns about potential negative impacts of climate change on connectivity between populations.

Influence of Intra- and Interspecific Interactions on Short-term Movement of the Amphipod Corophium volutator in Varying Environmental Conditions

Movement of animals on intertidal mudflats can have profound consequences for population and community dynamics. Such movement is often influenced by biotic interactions, but the abiotic environment can also be important, affecting the strength of species’ interactions and even changing their outcome. Here, we focused on two biotic interactions and examined their effect on movement of the burrow-dwelling amphipod Corophium volutator: intraspecific interactions (specifically, the effect of conspecific density) and interspecific interactions with a competitor–predator, the mud snail Ilyanassa obsoleta (=Nassarius obsoletus). We assessed the magnitude of the temporal variation and examined the influence of pertinent abiotic variables (lunar cycle, presence of tide pools). Despite substantial temporal variation in movement levels, density of conspecifics did not influence emigration or immigration rates of C. volutator. Mud snails negatively affected immigration of C. volutator, and this was consistent across all lunar phases. However, mud snails influenced mortality and emigration of C. volutator in a contrasting way relative to lunar phases; sometimes amphipods evaded snails and survived, but at other times did not emigrate and were killed. The presence of tide pools did not change the nature or strength of the snail–amphipod interaction. Our results further clarify the complex interactions between C. volutator and I. obsoleta and suggest that resources on the mudflat are sufficient to sustain high densities of amphipods in a density-independent manner. Further, they highlight the importance of considering multiple environmental variables when examining interspecific interactions.

Short-Term Response of a Downstream Marine System to the Partial Opening of a Tidal-River Causeway

The spillway gates of the Petitcodiac Causeway, a hydraulic structure ~35xa0km upstream of the mouth of the Petitcodiac River in New Brunswick, Canada, were permanently opened in April 2010. The short-term effect opening the spillway gates had on downstream intertidal mudflats of the upper Bay of Fundy was investigated. Specifically, a multivariate before-after-control-impact design was used to determine if opening the spillway gates affected the invertebrate community (crustaceans, polychaetes, and molluscs), abiotic sediment conditions (sediment water content, mean particle size, penetrability, and depth of the apparent redox potential discontinuity), or resource availability (sediment chlorophyll a concentration and organic matter content) of five intertidal mudflats (two impacted sites, three reference sites) spanning Chignecto Bay, the northern arm of the upper Bay of Fundy, up to 5xa0months post-opening. No biologically or statistically meaningful differences were detected between impacted and reference sites for any of the measured variables. This suggests that opening the causeway did not have a quantifiable impact on these intertidal mudflats, at least within half a year of the opening. This is likely a result of the macrotidal nature of the Bay of Fundy that overwhelmed any immediate changes to hydrodynamics that occurred after the opening of the causeway gates.

Distribution and Habitat Use of the Invasive Carcinus maenas L. (European Green Crab) and the Native Cancer irroratus (Say) (Rock Crab) in Intertidal Zones in the Upper Bay of Fundy, Canada

Abstract Carcinus maenas (European Green Crab) is an invasive marine portunid crab that has established populations globally outside of its native range and has been implicated in declines of benthic invertebrates in invaded ecosystems. Observations of Green Crab on intertidal mudflats in the upper Bay of Fundy have increased in recent years. We assessed the distribution and relative abundance of crab populations in Chignecto Bay, an arm of the upper Bay of Fundy, by trapping Green Crab and native Cancer irroratus (Say) (Rock Crab) at mudflats and in rocky intertidal zones in 2013 and 2014. Spatial distribution of Green Crabs indicated a preference for rocky intertidal habitats and greater abundance geographically lower in the Bay, which would correspond with an initial introduction at the mouth of the Bay and subsequent inward expansion. Abundance declined drastically from 2013 to 2014, suggesting that Green Crab may not yet be well established in Chignecto Bay. Carapace width indicated that crab age may be less variable further into the Bay, suggesting these sites may only be colonized in years with favorable environmental conditions. The population may be vulnerable under poorer conditions in other years, like 2014, when high overwintering mortality is a possible cause for the observed decline. There was not a corresponding decline in native Rock Crab. While Green Crab abundance is currently relatively low in Chignecto Bay, and their impact on mudflats likely minimal, prolonged favorable environmental conditions could lead to an increased presence.

Behavioral Response of Corophium volutator to Shorebird Predation in the Upper Bay of Fundy, Canada

Predator avoidance is an important component of predator-prey relationships and can affect prey availability for foraging animals. Each summer, the burrow-dwelling amphipod Corophium volutator is heavily preyed upon by Semipalmated Sandpipers (Calidris pusilla) on mudflats in the upper Bay of Fundy, Canada. We conducted three complementary studies to determine if adult C. volutator exhibit predator avoidance behavior in the presence of sandpipers. In a field experiment, we monitored vertical distribution of C. volutator adults in bird exclosures and adjacent control plots before sandpipers arrived and during their stopover. We also made polymer resin casts of C. volutator burrows in the field throughout the summer. Finally, we simulated shorebird pecking in a lab experiment and observed C. volutator behavior in their burrows. C. volutator adults were generally distributed deeper in the sediment later in the summer (after sandpipers arrived). In August, this response was detectably stronger in areas exposed to bird predation than in bird exclosures. During peak predator abundance, many C. volutator adults were beyond the reach of feeding sandpipers (>1.5 cm deep). However, burrow depth did not change significantly throughout the summer. Detailed behavioral observations indicated that C. volutator spent more time at the bottom of their burrow when exposed to a simulated predator compared to controls. This observed redistribution suggests that C. volutator adults move deeper into their burrows as an anti-predator response to the presence of sandpipers. This work has implications for predators that feed on burrow-dwelling invertebrates in soft-sediment ecosystems, as density may not accurately estimate prey availability.

Long-term continental changes in wing length, but not bill length, of a long-distance migratory shorebird

Abstract We compiled a >50‐year record of morphometrics for semipalmated sandpipers (Calidris pusilla), a shorebird species with a Nearctic breeding distribution and intercontinental migration to South America. Our data included >57,000 individuals captured 1972–2015 at five breeding locations and three major stopover sites, plus 139 museum specimens collected in earlier decades. Wing length increased by ca. 1.5 mm (>1%) prior to 1980, followed by a decrease of 3.85 mm (nearly 4%) over the subsequent 35 years. This can account for previously reported changes in metrics at a migratory stopover site from 1985 to 2006. Wing length decreased at a rate of 1,098 darwins, or 0.176 haldanes, within the ranges of other field studies of phenotypic change. Bill length, in contrast, showed no consistent change over the full period of our study. Decreased body size as a universal response of animal populations to climate warming, and several other potential mechanisms, are unable to account for the increasing and decreasing wing length pattern observed. We propose that the post‐WWII near‐extirpation of falcon populations and their post‐1973 recovery driven by the widespread use and subsequent limitation on DDT in North America selected initially for greater flight efficiency and latterly for greater agility. This predation danger hypothesis accounts for many features of the morphometric data and deserves further investigation in this and other species.