Invertebrate Zooplankton Community Composition

Alexei Pinchuk

Final Report
Sample Processing in the Lab
Large Zooplankton Preliminary Results
Small Zooplankton Preliminary Results

Final Report

Final Report to CIAP and BOEM:
Pinchuk A. I., and Eisner L. B. 2016. Spatial heterogeneity in zooplankton distribution in the eastern Chukchi Sea and northern Bering Sea as a result of large-scale interactions of water masses. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 42 pp. – DRAFT REPORT

Pinchuk, A.I., Eisner, L. in review. A comparison between late summer 2012 and 2013 water masses, macronutrients, and phytoplankton standing crops in the northern Bering and Chukchi Seas. Deep Sea Res. II: Arctic Eis Special Issue.

Interest in the Arctic shelf ecosystems has increased in recent years as the climate has rapidly warmed and sea ice declined. These changing conditions allowed conduct of large scale surveys aimed at systematic, comparative analyses of interannual variability of the shelf ecosystem. In this study, we compared zooplankton composition and geographical distribution in relation to water properties on the eastern Chukchi and Bering Sea shelves during the summer of 2012 and 2013 as a part of the Arctic Ecosystem Integrated Survey (Arctic Eis) program. In 2013, shifts in water mass distribution manifested in a stronger influence of Chukchi Winter and Ice Melt waters, markedly affecting the distribution of expatriate and resident zooplankton species. This pattern was apparent not only in the spatial coverage, but also in their relative abundance and biomass, thus demonstrating the importance of the Arctic community on the northeastern Chukchi shelf. In contrast, zooplankton biomass of Pacific origin decreased in 2013 in the northern Chukchi shelf suggesting a change of its advection pathways into the Arctic. The observed interannual variability in distribution of water properties on the Chukchi Sea shelf is in agreement with previously described systematic oceanographic patterns derived from long-term observation time series. Our study is the first attempt to relate variability in zooplankton distribution in the eastern Chukchi Sea to these oceanographic patterns in the context of changing climatic conditions and to explore potential consequences for the local biotas of the Arctic shelf ecosystem.

Sample Processing

In the lab, the preserved mesozooplankton samples were processed as follows: each sample was poured into a sorting tray and large organisms, primarily shrimp and jellyfish, were removed and enumerated. The sample were then sequentially split using a Folsom splitter until the smallest subsample contained about 200-300 specimens of the most abundant taxa.

All taxa in the smallest subsamples were identified, staged, enumerated and weighed. Each larger subsample was examined to identify, enumerate and weigh the larger, less abundant taxa. Blotted wet weights of all specimens of each taxa and stage in each sample were taken as outlined in earlier papers (Coyle et al., 2008, 2011) and the coefficient of variation in average wet weight were computed. If the coefficient of variation for any given taxa and stage changes by less than 5% when additional weights were taken from subsequent samples, wet weights were no longer measured for that taxa for that cruise, and the wet weight biomass was estimated by multiplying the specimen count by the mean wet weight.

In practice, only calanoid copepods had consistent wet weights after weighing each taxa and stage in about 10-15 samples. Therefore, wet weights on euphausiids, shrimp and other larger taxa were measured and recorded individually for each sample. Wet weight measurements were done on a Cahn Electrobalance or Mettler top loading balance, depending on the size of the animal.

All animals in the samples were identified to the lowest taxonomic category possible. Copepodid stages were identified and recorded. The above analytical and collection procedures insure that all data are directly comparable with earlier late summer/early fall BASIS data (Coyle et al., 2008, 2011).

Large Zooplankton Preliminary Results

Figure 1. Total zooplankton biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 (L) and 2013 (R) Arctic Eis Surveys.

Click on figures to enlarge.

Figure 2. Total Calanus spp. biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 (L) and 2013 (R) Arctic Eis Surveys. (Calanus marshallae/glacialis)

Figure 3. Total Sagitta elegans biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 (L) and 2013 (R) Arctic Eis Surveys. (Chaetognatha, Arrow Worm)

Figure 4. Total Oikopleura spp. biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 (L) and 2013 (R) Arctic Eis Surveys. (Larvacean)

Figure 5. Total Euphausiid furcilia biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 (L) and 2013 (R) Arctic Eis Surveys. (Krill Larvae)

Figure 6. Total Neocalanus and Eucalanus spp. biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 (L) and 2013 (R) Arctic Eis Surveys. (Very Large Copepods)

Figure 7. Total Calanus hyperboreus biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 (L) and 2013 (R) Arctic Eis Surveys. (Very Large Arctic Copepod)

Analysis of spatial distribution of zooplankton biomass showed that most of the biomass occurred on the Chukchi Sea shelf with maximum values exceeding 1500 mg m-3 found in the northwest in 2012; however, unlike in the previous year, most of the biomass in 2013 occurred on the southern Chukchi Sea shelf exceeding 2500 mg m-3 at a single station (Figure 1). The bulk of the total zooplankton biomass (>50%) consisted of the Arctic copepods Calanus spp. (most likely C. glacialis) in both years.

Analysis of distribution of major water properties revealed presence of three major pelagic habitats: well mixed warm (>7°C) and less saline (<31 psu) nearshore with surface pools of even fresher (<27 psu) water, well stratified offshore with cold (<1.5°C) and saline (>32 psu) bottom layer extended over the western shelf, and very cold (<1°C) and less saline (<30 psu) water observed in the northeastern Chukchi Sea. This was a consistent trend over both years; however with substantial variation in high temperatures and spatial coverage.

While Calanus spp. (Figure 2) occurred over the entire study area, on average, Calanus spp. biomass was 4-fold more on the Chukchi shelf (347 mg m-3) than that on the northern Bering Sea shelf (84 mg m-3) in 2012, with highest values occurring in the offshore habitat. The northwest Chukchi shelf was not re-sampled in 2013, thus no comparison could be made to supra-abundance of Calanus in that region from 2012. Calanus in 2013 was most abundant in the southern Chukchi. Overall, the Calanus population consisted of nearly equal numbers of all copepodite stages, while adult females and males were nearly absent. The population structure indicates extended spawning over the study area which occurred earlier in spring and summer following sea ice retreat.

The distribution of non-crustacean Sagitta elegans mirrored that of Calanus spp. in 2012 and made up approximately 25% of the overall biomass that year. Interestingly, in 2013, S. elegans was overall less abundant and had a population break across the north-central Chukchi between two spikes in population numbers (Figure 3). Chaetognaths, or arrow worms, such as S. elegans are large, voracious pelagic predators of other zooplankton.

The larvacean Oikopleura spp. (Figure 4) were almost exclusively restricted to the stratified shelf habitat, near the ice edge, and absent near shore in both 2012 and 2013. As an interesting side note, the bottom trawl scientists noted lots of mysterious green ooze covering the epibenthic invertebrates in the northern Chukchi in 2012. This could have been attributed to Oikopleura‘s feeding biology, which includes secreting and re-ingesting mucilaginous ‘houses’ used to collect microscopic algae and invertebrates; they then collect the prey items, discard the used mucus, and create a new mucus ‘house’. The used mucus then sinks to the bottom, providing nutrients to the sediments as well as falling upon sessile or slow moving invertebrates.

Euphausiid furcilid larvae (Figure 5) occurred in large numbers on a few stations in vicinity of the Bering Strait and in the northern Bering Sea, where they apparently originated from. This was largely inconsistent with 2013 observations, as furcilids were mostly found in the south central Chukchi most closely associated with the influx of the Bering Sea watermass. Euphausiids are known to be a major link in the North Pacific food web and no doubt play an important role locally where large aggregations can create optimum fish, whale and seabird feeding grounds.

Similarly, large oceanic Bering Sea copepods Neocalanus cristatus, Neocalanus flemingeri / plumchrus, and Eucalanus bungii were most common to the south and north of the Bering Strait, indicating the influence of the Bering Sea shelf water mass (Figure 6). Interestingly, in 2013, smaller pockets did not extend into the northern Chukchi Sea as seen in 2012. This could be related to the apparent stop in northward flow and westward deflection of the Alaska Coastal Current seen in 2013 (see oceanographic page). We will examine this ‘intrusion’ within the zooplankton community to test if Bering Sea Oceanic water was displaced.

Finally, the large arctic copepods Calanus hyperboreus which inhabit the central Arctic basin were found in the northeast indicating the presence and upwelling of the Arctic water mass onto the shelf, likely through Barrow Canyon (Figure 7). This pattern was seen both years; however 2013 C. hyperboreus were much more abundant and wide-spread in the region suggesting more Arctic and Atlantic water was flowing south onto the shelf.

We anticipate a manuscript on zooplankton community composition and variability in relation to oceanographic conditions and will contribute to one or more synthesis manuscripts.

Crab Meroplankton Preliminary Results

Figure 8. Total crab zoeae biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 (L) and 2013 (R) Arctic Eis Surveys. (Early Stage Crab Larvae)

Chionocetes opilio ZI ZII and Megalopa

Figure 9. Total crab megalopae and glaucothoe biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 (L) and 2013 (R) Arctic Eis Surveys. (Late Stage Crab Larvae)

King Crab Glaucothoe

Crab larvae biomass includes hotspots specific to both zoeae and megalopae/glaucothoe stages (Figures 8 & 9) . These include large catches of: Majidae zoeae in the southcentral NBS, Majidae megalopae in Chirokov Basin north of St. Lawrence Island, and Telmessus cheiragonus megalopae southwest of Pt. Barrow. Further analysis of crab meroplankton is being explored and is currently seeking funding.

Labidochirus splendescens ZIII

Continued processing of crab larvae from the 2012 and 2013 is being conducted by Jared Weems. Interesting results from methodological testing can be seen in poster format:

Weems et al. Poster – AMSS 2015
Weems et al. Poster – AMSS 2016

Telmessus cheirogonus megalopa

Small Zooplankton Preliminary Results

Figure 10. Pseudocalanus spp. and Oithona similis (small neritic copepods) biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 Arctic Eis Survey.

Figure 11. Centropages abdominalis and Acartia hudsonica (small neritic and brackish copepods) biomass (mg/m^3) from the 60cm Bongo net (505um mesh) during the 2012 Arctic Eis Survey.

The small mesh zooplankton net samples were deemed unreliable due to the loss of flow meter volumetric filtration data in 2013. These samples were not processed.

However, in 2012, the nearshore habitat was dominated by small Pseudocalanus spp. copepods, while neritic Acartia hudsonica, Centropages abdominalis, Eurythemora hermanii (Figure 8 and 9) and cladocerans Podon spp. and Evadne spp. mostly occurred in the brackish areas influenced by freshwater runoff.