Population dynamics and seasonal lipid cycles in hyperbenthic crustaceans from a cold-ocean environment (Conception Bay, Newfoundland): Mysis mixta (Mysidacea) and Acanthostepheia malmgreni (Amphipoda)

Richoux, Nicole Bertine (2004) Population dynamics and seasonal lipid cycles in hyperbenthic crustaceans from a cold-ocean environment (Conception Bay, Newfoundland): Mysis mixta (Mysidacea) and Acanthostepheia malmgreni (Amphipoda). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Life cycles and seasonal changes in density, biomass, growth, secondary production and lipids in Mysis mixta and Acanthostepheia malmgreni from a 240-m deep site in Conception Bay, Newfoundland, were studied from October 1998 to November 2000. The primary aim was to provide life-history and ecological information on populations inhabiting the hyperbenthos of Conception Bay, and to relate the seasonal lipid and fatty acid dynamics to each species' life cycle and to the occurrence and quality of the annual phytoplankton bloom. -- Life spans and reproductive cycles were remarkably similar in the two species, with the release of free-living juveniles from mature females occurring in April and May when bloom material was settling to the hyperbenthos. Females reproduced once and then died at age-2.5 years following a 5-month brooding period of larval stages. The biennial life cycles of both populations resulted in the presence of 2 cohorts of each species co-existing at most times, although a marked annual alternation in cohort dominance occurred in the amphipod population. Annual production:biomass (P/B) ratios in the mysid population were similar in both years, indicating consistent recruitment from year to year. In contrast, the alternation in cohort strength in A. malmgreni had significant repercussions in the success of this population, reflected in a lower P/B ratio in 1999 than in 2000. -- Divergent patterns in lipids, fatty acids, and specific fatty acid marker ratios reflected differing life styles, diets and critical periods of energy accumulation and utilisation in M. mixta and A. malmgreni . Influenced by the seasonally productive and perpetually cold environment created by the Labrador Current, M. mixta and A. malmgreni both accumulated high levels of lipid reserves similar to those attained by other cold-water zooplankton. Maximum lipid stores were observed in females, although the maximum lipid content in M. mixta was over twice that in A. malmgreni. Lipid levels increased with maturity, and developing mysids exhibited a particularly rapid accumulation of triacylglycerols and diatom-associated fatty acids at the start of the spring bloom. Unlike the mysids, juvenile amphipods did not begin to develop sexual characteristics or accumulate significant amounts of triacylglycerol or phytoplanktonic fatty acids until after the spring bloom material had already reached the hyperbenthos in May. This study reveals a dependence of M. mixta and A. malmgreni on seasonal lipid accumulation for reproduction, and the existence of trophic links between their populations and the pelagic production cycle (i.e. benthic-pelagic coupling). Increased mobility and a broad range of prey types provided M. mixta with sufficient opportunities to accumulate essential fatty acids rapidly. However, A. malmgreni was restricted to a lower quality diet than was M. mixta, and the trophic connection between A. malmgreni and production in the euphotic zone was less pronounced.

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