David C. Lasenby

Zinc and cadmium accumulation by the opossum shrimp Mysis relicta

Cadmium and zinc consumption and accumulation rates were determined for immature and adult Mysis relicta feeding on sediments and zooplankton. Although the total body burden of cadmium and zinc increased with increasing mysid size, the concentrations of cadmium and zinc decreased with increasing mysid size. Little or none of the metal taken up by the mysids was assimilated, most of the cadmium and zinc being egested via faecal pellets. Molts were 5 to 8 times higher in cadmium and 4 to 13 times higher in zinc concentration than mysid whole body concentrations. From 21 to 35% of cadmium and 15 to 61% zinc taken up by M. relicta was eliminated via molting. Very little (<1%) metal accumulated as growth. Consumption rates of metal estimated from the amount of metal accumulated in tissue and lost via molting and egestion were higher than direct measurements of consumption estimated from feeding experiments. Most of the metal consumed is recycled to the coprophagous and decomposer community.

Life cycles of the freshwater mysid shrimp Mysis relicta reared at two temperatures

Abstract To assess the role that temperature may play in determining the life cycle of Mysis relicta, we cultured newly hatched juveniles at 4 C and 8 C in the presence of a surfeit of food. Mysids first reached sexual maturity after 8 months at 8 C after molting 12–13 times. Mysids cultured at 4 C molted four to five fewer times in the same 8 months, yet grew the same amount per molt as the mysids cultured at 8 C. We predict that mysids that live in water of at least 8 C during the summer of their first year will complete their life cycle in 1 year, whereas mysids growing in water colder than 4 C during their first summer will require 2 years to complete their life cycle. Received July 12, 1982 Accepted April 9, 1983

SEASONAL TRENDS IN THE HEAD CAPSULE LENGTH AND BODY LENGTH/WEIGHT RELATIONSHIPS OF TWO AMPHIPOD SPECIES

[Amphipod body lengths and weights are important variables in life history, biomass and production determinations. Length and weight are often estimated from head capsule length-body length or weight relationships based on a single field collection. Possible seasonal variation in these relationships is rarely considered. We used seasonal data for two amphipod species (Gammarus lacustris and Diporeia hoyi) to test the hypothesis that head capsule length-body length/weight relationships remain seasonally constant. Slopes of monthly relationships were significantly different (ANCOVA). However, the differences in slope and intercept were trivial, and all monthly regressions yielded similar predictive results. Regressions based on pooled seasonal samples underestimated measured biomass by 5%, while predicted lengths varied between 0 to ± 2 mm from measured lengths. We conclude that head capsule length-body length/weight relationships for these amphipods are seasonally robust, and may be estimated from single collections provided all size classes are obtained. If all size classes can not be obtained from one sample, relationships should be based on pooled samples., Amphipod body lengths and weights are important variables in life history, biomass and production determinations. Length and weight are often estimated from head capsule length-body length or weight relationships based on a single field collection. Possible seasonal variation in these relationships is rarely considered. We used seasonal data for two amphipod species (Gammarus lacustris and Diporeia hoyi) to test the hypothesis that head capsule length-body length/weight relationships remain seasonally constant. Slopes of monthly relationships were significantly different (ANCOVA). However, the differences in slope and intercept were trivial, and all monthly regressions yielded similar predictive results. Regressions based on pooled seasonal samples underestimated measured biomass by 5%, while predicted lengths varied between 0 to ± 2 mm from measured lengths. We conclude that head capsule length-body length/weight relationships for these amphipods are seasonally robust, and may be estimated from single collections provided all size classes are obtained. If all size classes can not be obtained from one sample, relationships should be based on pooled samples.]

An exploration of the role of mysids in benthic‐pelagic coupling and biomagnification using a dynamic bioaccumulation model

A mass-balance model for the uptake of organic contaminants in the opposum shrimp (Mysis relicta) is developed. The model describes the concentration in the mysid as a function of time as it grows and its lipid content changes over a two-year life span. The model can describe uptake in varying proportions from pelagic and benthic sources. Four mysid scenarios are considered: entirely pelagic, entirely benthic, half-pelagic and half-benthic, and following the observed seasonal behavior patterns. The seasonal-scenario mysid yields results consistent with levels of total polychlorinated biphenyls reported for Lake Ontario (Canada/USA). The relative sediment and water fugacities are shown to play a critical role in determining the contamination level in mysids. Inclusion of mysids in a simple food-web model demonstrates higher concentrations in upper-trophic-level organisms by two effects: introduction of another trophic level in the food web, and increased benthic-pelagic coupling.

The Behavioural Response of Mayfly Nymphs (Stenonema sp.) to Chemical Cues from Crayfish (Orconectes rusticus)

This study was conducted to determine whether mayfly nymphs (Stenonemasp.) have the ability to detect and respond to potential chemical cues from crayfish (Orconectes rusticus) by adopting anti-predator behaviours, and to investigate whether any potential responses would differ under exposure to different light conditions. The average number of mayfly nymphs displaying ‘scorpion’ posture, and the average duration of this behaviour were significantly greater during exposure to crayfish conditioned water than during exposure to unconditioned water. The results also suggested that there was a decrease in drift behaviour and horizontal movement in conditioned water trials relative to controls, as well as an increase in refuge use. However, with the exception of refuge use, differing light intensities did not have a significant effect on the observed prey reactions, although responses did appear to be slightly stronger during exposure to lower light levels relative to higher light conditions. This research provides new insight into the role of invertebrate predators and chemical cues and their influence on the behaviour of an important benthic prey species.

Effect of prey density, prey mobility and habitat structure on size selection and consumption of amphipods by a benthic feeding fish

To examine the interactive effects of prey density, prey mobility and habitat structure on prey selection by benthic feeding fish, feeding experiments were conducted with 95-105 mm (total length) pumpkinseed (Lepomis gibbosus), given four sizes ( 9 mm) of active or immobilized amphipods (Gammarus pseudolimnaeus) as prey. The experiments involved the assessment of prey size selectivity, total number of prey consumed and size-specific indicators of foraging efficiency such as capture success and handling time. Pumpkinseeds were size selective only at medium and high prey densities, preferring large Gammarus when selectivity occurred. The proportion of mobile and immobile prey consumed increased with prey size in unstructured habitat, whereas no selectivity occurred with mobile prey in structured habitat. Both habitat and prey density significantly affected prey consumption, with more prey eaten at higher densities and in unstructured habitat. Prey mobility did not affect the number of prey consumed when mobile or immobile prey were offered in separate trials, but the consumption rate of immobile prey increased in structured habitat when they were offered with mobile prey in the same trial. We conclude that prey density and habitat structure affect pumpkinseed size selectivity and consumption rates in a similar manner, whereas the effect of prey mobility on these factors is more dependent on the combination of density and habitat and on whether mobile and immobile prey are present at the same time.

A comparison of lead and zinc sediment profiles from cores taken by diver and a gravity corer

The zinc and lead concentrations were compared in several pairs of sediment cores obtained with a modified K-B corer and by Scuba diver from Coon Lake, Canada. A slow approach to and penetration of the sediments (ca. 0.1 m s-1) was employed for both coring devices. The modified K-B corer gave similar lead and zinc profiles to the diver cores. The divers observed that the K-B corer caused no disturbance of the surface sediment layers as it approached and entered the sediments. These results suggest that the K-B corer is suitable for use in lake studies requiring short, undisturbed cores from fine-grained sediments.

Particle size selection in cadmium uptake by the opossum shrimp, Mysis relicta

Fecal pellets of plankton play a key role in the recycling of trace contaminants within the aquatic environment. This is especially true for those plankton that undergo vertical migration in the water column. The opossum shrimp Mysis relicta is generally thought of as an omnivorous zooplankter occupying the pelagic zone of lakes, making itself available as a food source to a wide variety of fish. Several mechanisms have been suggested to explain the increase in concentration of metals from food source to fecal pellet. This study examines the feeding behavior of M. relicta to test the hypothesis that particle size selection can account for increased metal concentrations found in the fecal pellets of invertebrates.

The role of cannibalism and contaminant source on bioaccumulation in aquatic food webs

Two aspects of bioaccumulation in an aquatic food web are explored. First, the possible implications of cannibalism, including the scavenging of conspecifics, as a factor influencing food web bioaccumulation and biomagnification are explored by examining the behavior of total polychlorinated biphenyls (PCBs) in a simple aquatic food web consisting of plankton, juvenile and adult Mysis relicta, Diporeia, and alewife. From an analysis of trophic transfer efficiencies and food consumption rates, it is concluded that, for M. relicta, a maximum extent of cannibalism in a population is about 10%, although certain individuals may be more cannibalistic. The model suggests that cannibalism and scavenging of dead conspecifics generally result in an increase in concentration by self-biomagnification, but the effect is small and unlikely to exceed 5% on the average. Concentration differences also are likely to result from changes in the relative amounts of the dietary components. Highly cannibalistic individuals may achieve higher levels of bioaccumulation. In extreme cases, the food web model becomes mathematically unstable because of excessive feedback of high concentrations. A major implication is that differences in extent of cannibalism and scavenging probably contribute significantly to natural concentration variation in a population. Second, and more important, is the effect of benthic versus pelagic sources, especially when significant fugacity differences exist between these zones. A simple method is described by which the separate contributions from these sources can be estimated for organisms at higher trophic levels.

The effect of core compression on the measurement of zinc concentrations and anthropogenic burdens in lake sediments

An attempt was made to demonstrate the significance of core compression (shortening) by the use of geochemical profiles. In Plastic Lake, Canada, the hypothesis was tested that the measured anthropogenic zinc burdens would differ in the sediments while the measured sediment enrichment factors would be comparable, in shortened and unshortened cores. However, there was no significant difference in either the anthropogenic zinc burdens or the sediment enrichment factors calculated from the shortened and unshortened cores. The fact that the anthropogenic zinc is found, almost exclusively, in the top 10 cm of the sediment in Plastic Lake may explain these results.