Wecoma daily report, 10 August 2001

Late on 9 August Wecoma completed SeaSoar/ADCP/HTI/iron sampling on two lines off Cascade as the Thompson continued their times series measurements at station CH-3. At that time winds were just starting to become southerly so upwelling and a 20 cm/s southward jet were still present off Cascade Head. We then began a CTD and zooplankton net section along the Cascade Head line working from offshore (CH-7) to onshore (CH-1). The results of the zooplankton sampling are reported below by Bill Peterson. Discrete samples were taken from the CTD/rosette profiles for analysis of nutrients, iron and chlorophyll. Tim Cowles and Malinda Sutor used the TAPS (Tracor Acoustic Profiling System) mounted on the frame of the MOCNESS zooplankton net to measure the zooplankton acoustically. Combining this data with the results of the net tows and the towed HTI bio-acoustics instrument should give a nice picture of the zooplankton field along the CH line.

Late in the afternoon of 10 August we deployed SeaSoar and started a second small box survey around the Cascade Head line. By this time the southerly surge winds were blowing 10-15 knots to the north. Upper ocean isotherms flattened out toward the coast and currents inshore (< about 100m water depth) reversed and flowed at up to 20 cm/s to the north.

We are now doing a second BigBox survey with SeaSoar, ADCP, HTI and the iron fish. Winds are light and variable. As I type we're passing through the shrimp fishing fleet near the 200m isobath off Lincoln City.

Report from Zooplankton Net Sampling Team

Introduction. The zooplankton net team (Bill Peterson and Jesse Lamb) is sampling the zooplankton in several ways. We filter water from the ship's seawater intake line at fixed time intervals along selected transect lines for samples of the crustacean microzooplankton, diatoms and dinoflagellates, and we sample the zooplankton at fixed stations with a vertical plankton net and the MOCNESS at fixed stations. This was the same protocol as for the first COAST cruise. To date we have sampled every twenty minutes along Small Box Lines A and B (on 9 August), and we sampled with nets along the Cascade Head line at stations CH 1, 2, 3, 4, 5, 6 and 7. A vertical net tow was taken at each CH station and a MOCNESS tow at CH 2, 3, 5 and 7. In addition, we sampled CH 5 during both day and night to obtain a record of changes in zooplankton biomass and species composition due to diel vertical migration.

Results of Vertical Tows. Highest catches of zooplankton were seen at CH stations 2-4 (water depths of 30 to 100 m); the lowest catches were at CH 1 and CH 7. Peaks were seen at CH 2 and 3 (50 and 80 m water depths) with the highest catches at CH 2. The dominant zooplankton species are copepods (Pseudocalanus and Calanus) with goodly numbers of chaetognaths.

Results from MOCNESS. CH 2 catches were dominated by copepods at all depths sampled. In addition, we caught 4 Chrysaora (the large reddish brown jellyfish) in the 25-45 m strata. Most of the copepods were in the upper 30 m. At CH 3, copepods again dominated at all depths but numbers were highest in the upper 50 m. Quite a few chaetognaths were found in the 15-50 m depth range. Three large jellyfish were taken in a sample from the 60 m. At CH 5 (water depth of 130 m), the HTI acoustics showed a dense layer of large zooplankton between 120-130 m, that was situated only a few meters off the bottom. We sampled this layer with the MOCNESS and found it to massive quantities of large copepods (Neocalanus) and euphausiids (a mixture of Euphausia pacifica and Thysanoessa spinifera). The remainder of the nets contained only small copepods.

By night, CH 5 was completely different. Euphausiids were the dominant taxa from 0-35 m (due to diel vertical migration of the dense layer that was living near the sea floor as described above). CH 7 (water depth of 320 m) was only sampled at night, and we found that euphausiids dominated the upper 20 m. In addition, myctophid fish were taken in the upper mixed layer. On L-TOP cruises we always catch these mesopelagic fishes at night but only when we are in waters greater than 250 m depths. An unusual observation at CH 7 was the large number of small juvenile fishes in our nets. We caught several hundred fishes (order 3-5 cm in lengths) of several different species. Some were dover sole, some rockfish but the bulk of them are unknown to me. We do not usually catch many fish in the MOCNESS (other than myctophids and flatfish juveniles) so the catches at CH-7 are noteworthy.

Submitted at 1100 on 10 Aug 2001 by Jack Barth, Chief Scientist, R/V Wecoma.