Moored Optical Plankton Counter

Moored Optical Plankton Counter -- Long-Term Monitoring of Zooplankton and Temperature in Scotian Shelf Waters

by Alex W. Herman

The Scotian Shelf contains several deep basins (Emerald and La Have) which aggregate and harbor zooplankton during fall and winter months. As a result of a sampling program starting in 1984 using Batfish/BIONESS/acoustics (Herman et al. 1991; Sameoto and Herman 1990), we have found that these copepod populations (mainly Calanus) exhibit certain annual characteristics. That is, they represent a large component (~50%) of the shelf Calanus populations from the previous growth season. They also contain the dominant populations that will seed the surface waters at the end of winter, thereby commencing the reproduction and production cycle on the Scotian Shelf. These surface populations then dominate the central Shelf waters and banks. The basins contain warm slope water (9-12°C) and are marked by weak circulation near the bottom thus enabling the copepod populations to reside there. The Emerald and La Have Basins, therefore, are focal/observational points for determining the state of the copepod populations on the Scotian Shelf.

Mounting observational evidence indicates that temperature is a key factor in controlling zooplankton production and that we should be monitoring the Emerald/La Have Basins over decadal periods. If long-term global climate change occurs, will we see its effects in Nova Scotian shelf water temperatures and zooplankton populations? Two independent observations indicate that temperature effects produce an impact on zooplankton populations. First, during the late 1960's, shelf water temperature decreased as a result of an influx of the Labrador Current. Temperature of the slope water residing in the Emerald Basin decreased by 5°C. Intensive zooplankton sampling indicated that species composition and migrational behaviour changed during this period. Second, temperature effects on basin zooplankton populations are currently being observed on decadal time scales. Since 1984 our studies of the Scotian Shelf and Emerald/La Have Basins indicate that basin temperatures have decreased from 11.5°C in 1984 to 9.2°C in 1991. These changes were accompanied by a simultaneous decrease in basin zooplankton populations by a factor of 4-5.

The Moored Optical Plankton Counter

A pilot mooring program was initiated in September 1991 to monitor zooplankton populations and temperature continuously in Emerald Basin. The central sensor of the mooring is a redesigned optical plankton counter (OPC; Herman 1992; Herman 1988) which is mounted inside a foil (Fig. 1). The foil aligns itself in the tidal flow and samples zooplankton via a protruding tunnel. A data logger/power supply mounted below the OPC controls its operations. An on-board computer contains a "look-up" table of peak tidal flows (predicted flows based on experimental data) and "wakes up" twice a day sampling zooplankton for a period of ~1 hr. Temperature and current are monitored via two recording current meters mounted above and below the OPC foil.

Figure 2 shows data collected during a test deployment in Emerald Basin in September 1991. The moored OPC sampled 30 minutes every hour for 24 hours. Current meters situated 5 m below and 15 m above the foil yielded flow and direction, temperature and salinity information. The data show that zooplankton counts for Calanus copepodite stages IV & V increased with current speed. Reliable zooplankton count data can be obtained only at speeds of >7 cm s-1 which represents a detection limit for the OPC. Hence, the OPC must sample during periods of peak tidal flow (which in Emerald Basin ranges between 10-20 cm s-1). Measurement of flow speed is required to estimate volume of seawater processed through the tunnel.

Subsequent to this trial deployment, the moored OPC has been deployed and has functioned reliably in Emerald Basin since September 1991. It is intended to project the program as far into the future as resources will permit.

Sampling Strategy

It is clear that a single depth mooring is insufficient to provide a full and complete picture of zooplankton populations. On the positive side, a single mooring situated deep in the Emerald Basin can provide considerable information that would not be otherwise available in, say, surface layers. The Emerald Basin, as is the case with most shelf basins, represents a stable and quiescent environment for zooplankton. Intrusions into basins from external sources occur slowly and rarely displace zooplankton. Intrusions consist of denser water which cause upward displacement of existing basin water at an extremely slow rate (~0.1 mm sec-1). Emerald Basin copepod stocks are dominated by Calanus finmarchicus stages IV & V (90% of copepod populations present while within the Scotian Shelf basins; 60% of the entire shelf biomass during winter). Therefore they are easily monitored by the OPC. Their migration into the surface layers in winter can be monitored as can the buildup of basin populations during spring and summer. Generation cycles (growth and development) can also be monitored by optical measurement of size distributions. The basin zooplankton layers are generally homogeneous; the layer is approx. 40 m in vertical thickness and spatial variability (horizontal) of concentration (no. m-3) is only a factor of 2X as determined from Batfish sampling. As a result, basin environments are easily sampled and quite respresentative of shelf zooplankton over the long-term. At present our sampling scheme has been extended to 2 depths: one at the center of the copepod layer at 240 m depth; and one just above the layer at 210 m depth. The OPC at the latter depth provides information on upward and downward flux out of and into the deep layer.

Optical Fouling

At depths below 200 m in the Emerald Basin, fouling of the optical windows is not as serious as in the upper layers. During the fall and winter of 1991-92, the moored OPC provided continuous data of high quality and had no fouling problems. However, the spring bloom in April and subsequent "fallout or raining" of material at depth caused immediate and intense fouling of the OPC windows rendering the data unusable. Growth in the upper layers during spring and summer and subsequent fallout limited our sampling periods. Our highest quality data has been obtained for the overwintering, nonproductive period. Various solutions to the fouling problem are being tested for the summer sampling periods.

Preliminary Data

The overwinter period from December 1991 to April 1992 was sampled successfully with a moored OPC deployed at 240 m depth in the Emerald Basin. Our current thought regarding the surface migration of basin populations has been that these animals migrate to the surface nearly instantaneously by responding to some cue. The winter period has always been undersampled by our ships-of-opportunity net-sampling cruises and the migration period has never been adequately resolved. Figure 3 shows a record of raw counts per second (not normalized to volume sampled) of the size group corresponding to Calanus finmarchicus Stages IV-V. The record shows that the counts decrease rapidly, particularly during the months of Febuary-March, dwindling to nil in April. This decrease indicates that the return migration to the surface is a more gradual process than previously believed. Moreover, return of the deep-dwelling populations to the surface is completed just prior to the spring surface bloom.

Future Direction

Due to its large size, it is difficult to deploy enough moored OPC samplers to achieve good spatial resolution of zooplankton abundance in Scotian Shelf Waters. We are currently developing a laser-based OPC that is more compact, requires only a single pressure case, and is capable of measuring zooplankton abundances and sizes in static conditions without flow. The completed development of a laser-based OPC would enable deployment of a larger number of sensors per mooring, which would significantly improve vertical resolution of zooplankton abundance estimates. (Alex Herman is an oceanographer at the Bedford Institute of Oceanography.)

References

Herman, A. W. 1988. Simultaneous measurement of zooplankton and light attenuance with a new optical plankton counter. Cont. Shelf Res. 8: 205-221.

Herman, A. W., D. D. Sameoto, Chen Shunnian, M. R. Mitchell, B. Petrie, and N. Cochrane. 1991. Sources of zooplankton on the Nova Scotia shelf and their aggregations within deep shelf basins. Cont. Shelf Res. 11: 211-238.

Herman, A. W. 1992. Design and calibration of a new optical plankton counter capable of sizing small zooplankton. Deep-Sea Res. 39: 395-415.

Sameoto, D. D., and A. W. Herman. 1990. Life cycle and production of Calanus finmarchicus in deep basins on the Nova Scotia shelf and seasonal changes in Calanus spp. Marine Ecology Progress Series 66: 225-237.