Session Summary

The "Hot Topics" Session was meant to serve as a forum for investigators to briefly (5 minutes) present novel and intriguing research results that either did not fit readily under the theme areas established for the other three sessions or for which investigators did not have adequate time to present during the other sessions. As such, some of these presentations addressed topics only indirectly related to the GLOBEC target species (sand storms on Georges Bank by P. Wiebe; the Chaetoceros patch by D. Gifford et al.; planktonic hydroid growth by Bollens et al.), whereas others addressed more specific aspects of the population biology of the target species (Calanus mating behavior by C. Miller; inter-decadal zooplankton abundances by Landsteiner et al.; Calanus egg viability by J. Runge). But throughout all of this session's diverse presentations runs the common thread of trying to elucidate the intricate web of biological-physical interactions that comprise the Georges Bank ecosystem.

Direct observations and video recordings of mate finding behavior in Calanus marshallae revealed general patterns of female and male activity which lead to encounters and transfer of spermatophores. These patterns were outlined in the talk and will be published in the near future.

Sand Storms on Georges Bank

Peter H. Wiebe

Woods Hole Oceanographic Institution

During the July 1995 broad-scale cruise on Georges Bank (Albatross IV 9508 Figure 1), the towed high frequency (420 kHz) acoustics profiling system encountered extremely high acoustical volume backscattering in the shallower regions of the Bank. One section from the middle of the southern flank (Station 9) to the crest of the Bank (Station 11) and then along the crest to the east (Station 12), illustrated the phenomenon particularly well (Figure 2). In waters that were stratified (depths of ~65 to ~50 m), the acoustical record showed strong surface and subsurface horizontal layers associated with the mixed layer and pcynocline. In a region where the bottom shoaled abruptly from 50 to 40 m, the water column became well mixed and the volume backscattering became dominated by fine-scale vertical lineations from the surface to the bottom. The intensity of the backscattering increased substantially as the depth shoaled to the bank crest. Along the top of the crest, patches of very high volume backscattering were observed which extended from the top to bottom. We suggested previously that such vertical lineation structure is due to the interaction of the particle field with secondary cell circulations associated with strong tidal flow over a rough bottom (Wiebe et al, in press).

On this occasion, net tows (with 150 um mesh nets) and pump sampling (with ~50 um mesh netting) done at stations along the crest contained substantial quantities of sand, even at the surface. An experiment conducted later in the cruise at stations 36, 37, and 38 were specifically designed to examine the relationship between volume backscattering and sand in the water column as determined from pump samples.

Four hypotheses have resulted from this finding:

• It is our impression that much of the well-mixed region on the Bank is susceptible to sand transport to the surface.

• The lunar cycle of tidal forcing and current generation probably causes enhanced turbulent transport of sand into the water column during spring and neap tides.

• It is probable that the sand, once imported to the water column, becomes attached to marine snow particles thus facilitating easier movement towards the surface.

• The presence of substantial amounts of sand in the water column is likely to have considerable ecological impact, particularly on filter feeding planktonic animals (such as salps and pteropods), and may be an important ecological determinant of species composition on the Bank.

Wiebe, P.H., D. Mountain, T.K. Stanton, C. Greene, G. Lough, S. Kaartvedt, J. Manning, J. Dawson, L. Martin, and N. Copley. In Press, Acoustical study of the spatial distribution of plankton on Georges Bank and the relation of volume backscattering strength to the taxonomic composition of the plankton. Deep-Sea Research.

C. socialis colonies were present in densities of ~5 colonies ml-1. Colonies were extremely fragile and were observed only in water siphoned gently through wide-bore tubing from the top of Go-flo bottles. Water drained through spigots at the bottom of Go-flo bottles did not contain intact colonies. The colonies consisted of a few hundreds to a few thousands of cells arranged in chains wrapped into a spheroidal shape. Thin threads converged toward the colony center, where consortia of small heterotrophic flagellates were located. Individual colonies contained on average 300 pg chlorophyll a (range 120-490; n=45 colonies) and 800 pg phaeopigment. Grazing experiments indicated that the colonies were consumed by stage C6 female Calanus finmarchicus.

We suggest that the patch represents a potentially extensive (in space and time) 3-dimensional structure in the water column on the southern flank of Georges Bank. As such, it may provide a concentrated nutritional resource to consumer organisms, or may function as a specific habitat for the nano- and microplankton organisms associated with it.

Inter-Decadal Comparison of Monthly Mean Zooplankton Abundances on Georges Bank

Mary Landsteiner1, Stephen Bollens1, Cabell Davis1, Andy Solow1 and Jack Green2

1Woods Hole Oceanographic Institution
2National Marine Fisheries Service, Narragansett

We examine data collected during 1939-41 (R/V Atlantis) and 1977-87 (MARMAP) for any possible inter-decadal changes in zooplankton abundances. Currently the Atlantis data and the MARMAP data allow the only quantitative inter-decadal comparison of zooplankton abundances on Georges Bank. The Atlantis data, a largely unpublished and recently uncovered data set, consist of vertically-stratified net samples collected at 21 to 52 stations on each of 11 cruises using a Clarke Bumpus plankton sampler (12.7 diameter mouth opening; ~250 mm mesh netting). The MARMAP data consist of vertically-integrated net samples collected at 15 to 50 stations on cruises conducted 6 to 12 times per year using a bongo net (61 cm diameter mouth opening; 333 mm mesh netting). Only adult stages for selected species were compared so as to avoid possible biases due to the different mesh sizes. Differential avoidance of the two samplers may be another possible sampling bias; however, work by Clarke et al. (1943) showed for Sagitta elegans that this is unlikely to be a problem.

We present a preliminary comparison of the monthly mean abundances of the dominant zooplankton species (Calanus finmarchicus, Pseudocalanus minutus, Metridia lucens, and Centropages hamatus) on the Bank (< 200 m) (Figure 1). Although seasonal abundance patterns for C. finmarchicus, P. minutus, and C. hamatus are comparable between the two time periods, absolute abundances of C. finmarchicus, M. lucens and C. hamatus are generally less during 1939-41 than 1977-87. This could be due to any number of biological or physical processes during this period. One intriguing possibility is inter-decadal variability in the abundance of vertebrate predators. For example, abundances of mackerel, a known predator of C. finmarchicus (Madin et al.; Session II), were relatively low during the 1970s (Fogarty et al.; Session II).

Future work with the MARMAP data will examine the effect of storminess on mean abundances and spatial distributions of zooplankton and the results will be compared with our investigations of the Atlantis data (Bollens et al.; Session I).

Reference:

Clarke, G. L., E. L. Pierce, and D. F. Bumpus. 1943. The distribution and reproduction of Sagitta elegans on Georges Bank in relation to hydrographical conditions. Biol. Bull. 85: 201 226.

Figure 1: Monthly mean abundances and variances [vertical bars = 2 SE] of adult zooplankton on Georges Bank (bottom depths < 200 m); longitude east of 69.1W). [circles = Atlantis (1939-41); solid diamonds = MARMAP (1977-87)]

Recent studies by S. Poulet, A. Ianora and co-workers show that ingestion of certain diatom species by female copepods inhibits development of their offspring. Experimental results are most consistent with the hypothesis that these diatoms contain a toxin that either blocks embryogenesis entirely or induces deformities in the early naupliar stages. In laboratory experiments, we have observed that feeding by Calanus finmarchicus females on the common diatom, Thalassiosira nordenskioldii, greatly reduces hatching success of their eggs. A control group of females feeding on the dinoflagellate, Prorocentrum micans, produced eggs that hatched with a 90% success rate. During 1995 process cruises on Georges Bank, eggs released by C. finmarchicus typically hatched with a success rate of 60-90%, although at some stations only 30-35% of the C. finmarchicus eggs were viable (Figure 1). These observations suggest that the influence of maternal diet on the viability of offspring may contribute to patterns of variability in recruitment into the region's copepod populations.

Figure 1. Hatching success of eggs released by C. finmarchicus females captured at Georges Bank stations during 1995 process cruises.

During spring of 1994, we found huge numbers of hydroids (principally Clytia gracilis) suspended in the plankton on Georges Bank. We subsequently showed that the hydroids were capable of consuming cod larvae and young copepods; in the latter case having a potentially devastating effect on their prey (Madin et al., In Press; Session II). Based on these initial findings we mounted a much more extensive field study during spring and summer of 1995 on the distribution and abundance of hydroids in both the water column and on the bottom of Georges Bank (see Sullivan et al.; Session I). Additionally, we undertook a series of laboratory investigations on the feeding, growth and life history characteristics of planktonic hydroids, some preliminary results of which we summarize below.