Invertebrate Zooplankton Community Composition

Alexei Pinchuk

Objectives – (1) Describe and quantify the composition and distribution of zooplankton communities across broad temporal and spatial gradients; (2) compare our observations to previous and ongoing sampling programs on the Chukchi Sea Shelf in terms of species composition and their relative abundance/biomass; (3) statistically examine the interactions between our observations, temperature variation, stratification (i.e. timing and intensity/persistence, frequency/intensity of mixing events), water mass distribution, and other factors known to govern Chukchi Sea community structure and function; and (4) provide information on species-specific prey fields available for fish predators.

As mesozooplankton form an essential linkage regulating the distribution and abundance of many upper trophic level organisms including fish, accurate assessment of the zooplankton populations comprising the Chukchi ecosystem are fundamental to the success of the proposed study. The sample gear and tow types were selected to make the data comparable with current BASIS (NOAA Bering Aleutian Salmon International Survey program) sampling protocols. Oblique, paired bongo/pairvet tows (0.505 mm and 0.150 mm mesh) targeting different plankton size categories will be conducted at all stations as part of the comprehensive surface trawl/acoustic survey of the Chukchi Sea. Each net will be equipped with a General Oceanics flowmeter to monitor volume filtered. Physical oceanographic data will be collected simultaneously using a SeaCat profiler mounted in-line with the net array. The sample from one side of the bongo array will be used to enumerate large zooplankton (e.g. oceanic copepods, chaetognaths, hydrozoan jellyfish), and the sample from the other net will be used to enumerate ichthyoplankton (larval fish). Similarly, the sample from one side of the pairvet array will be preserved to enumerate small zooplankton (e.g. neritic copepods, copepodites and larvae) and the sample from the other net will be used for isotope analysis of major zooplankton taxa.

2013 Field Update

At this time, Dr. Alexei Pinchuk reports having received 11 boxes of 500 ml samples and 2 boxes of 250 ml samples. This translates into an approximate total of 292 zooplankton samples.

Pagurid (hermit) crab zoea

Overall, many of the same patterns seen last year in 2012 seem to have been observed in 2013. Gelatinous ctenophores, which bioluminesce a brilliant purple-pink, and Larvaceans, bodies outlined in bright orange (probably Fritillaria), were detected in the Chukchi Sea, especially when close to the ice edge. The transparent mucus material (slime) caught near the ice in 2012 was not observed in 2013 however. Chaetognaths were relatively abundant throughout the CS and NEBS. Copepods, euphausiids, and larval crabs were observed throughout, though levels dropped near the ice edge (exception T. cheiragonas megalopae). In the NEBS in particular, a trend of more abundant large Themisto sp. amphipods and large Calanoid copepods at offshore stations and Chaetognaths, crab larvae, and fish eggs/larvae at inshore stations was seen. Amphipod abundance could be associated with higher abundances of large jellys offshore, as Themisto likely utilizes jelly tentacles as pelagic habitat. The final sampling station, #195, appeared to have very different species assemblages dominated by small benthic Gammarid amphipods and a fiber-like (cotton candy) phytoplankton. In the small mesh nets of the 20-cm Bongo, we observed mostly Pseudocalanus sp. and clumps of algal fragments throughout the sampling region.

Pagurid (hermit) crab glaucothoe

Additionally, biological samples were collected for three separate zooplankton projects; these samples were taken from the FOCI Ichthyoplankton 60-cm Bongo samples prior to being fixed in Formalin solution. In the Chukchi Sea, Jen Marsh, Ph.D. student with Dr. Franz Mueter, requested Neocalanus cristatus and Calanus marshallae/glacialis for stable isotope and trophic food web analyses. Alex Andrews (AFSC) collected Themisto sp. Amphipods and Thysanoessa sp. Euphuasiids per station when available for stable isotope trophic level analysis. And Jared Weems (UAF/AFSC*) collected crab larvae (zoea and megalopae) when available for species identification and trophic level analysis. Chukchi taxa include mostly Telmessus cheiragonas megalopae, Majidae (Hyas sp. and Chionoecetes opilio) zoea, and Paguridae zoea. Telmessus cheiragonas was particularly abundant near the ice edge along the Alaska coast and near Hanna Shoal. Northeastern Bering taxa include mostly Majidae (Hyas sp. and Chionoecetes opilio) megalopae and Paguridae zoea.

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