Alex W. Herman

Design and calibration of a new optical plankton counter capable of sizing small zooplankton

Abstract A new design of optical plankton counter (OPC) capable of sizing zooplankton in the size range of 256μ to 2 cm is presented. The detection sensitivity is the result of new optical design of the previous system ( Herman , 1988, Continental Shelf Research , 8 , 205–221) that originally had a lower detection limit of 550 μm. Both theoretical and experimental calibrations for the OPC are derived and compared. Preserved copepods and eggs introduced in a flow tank demonstrate the response of the OPC and its capability for detecting plankton to a limit of 250 μm. Copepod profiles measured by the OPC mounted on plankton net samplers provided intercomparisons that showed good agreement in identifying copepod layers and identifying some species, for example, copepods as small as Calanus finmarchicus II and up to the adult stages. Profiles containing marine snow and Ceratium illustrate some of the limitations of the OPC in directly measuring and identifying copepods. An additional modification to the OPC may result in a potential lower limit detection of 120 μm, and the practicality of such applications are discussed.

Simultaneous measurement of zooplankton and light attenuance with a new optical plankton counter

Abstract A new optical plankton counter capable of counting and sizing zooplankton and simultaneously measuring light attenuance is described. The submersible model can be towed or lowered on station while a laboratory version is used to process samples from a pump outflow or animals preserved in formalin. The submersible model has been mounted on the Batfish vehicle and towed for extended periods of 15–20 h allowing it to be used as a survey tool. Data showing copepod layers in deep shelf basins are presented and comparisons made between light attenuance measured with the plankton counter and chlorophyll fluorescence measured with an in situ fluorometer also mounted on the Batfish. The submersible counter was also towed while mounted on the BIONESS (a multiple opening and closing net system) and comparisons are made between the measured profiles and size distributions of copepods.

Remote sensing of phytoplankton in the sea: surface-layer chlorophyll as an estimate of water-column chlorophyll and primary production

Abstract The utility is assessed of remotely-sensed chlorophyll data in biological oceanography. Oceanographic data, highly resolved in the vertical, are used as a basis for estimating the (weighted) proportion of the water-column chlorophyll chat is accessible to the remote sensor. Examples are calculated for the continental shelf off Nova Scotia, the Canadian Arctic and the coast of Peru. The analysis is extended to the estimation of phytoplankton production. Remotely-sensed data contain only a small (5 per cent of phytoplankton biomass and 11 per cent of the turnover), but surprisingly stable, fraction of the information for the water column. A modest ground-truthing programme is required to exploit these data to the best advantage.

Sources of zooplankton on the Nova Scotia shelf and their aggregations within deep-shelf basins

Abstract A large-scale survey/study of zooplankton on the Scotian Shelf south of Nova Scotia was performed during 1984–1985. Rapid sampling was accomplished using a Batfish vehicle mounted with an optical plankton counter, a multiple opening/closing net system (BIONESS), and multifrequency acoustics. From the Batfish survey reported here, we present evidence indicating that the dominant supply of zooplankton to the Scotian Shelf is provided by the Nova Scotia Current, a strong localized flow adjacent to the coast. During late winter and spring, the Nova Scotia Current supplies our shelf water with most Calanus species which originate from the Gulf of St Lawrence to the north. Further sampling indicated that the deep Scotian Shelf basins harbor large populations of zooplankton during autumn and winter which consist mainly of stages IV and V of C. finmarchicus, C. hyperboreus, and C. glacialis. This winter storage mechanism is also a dominant source of young copepedite stages in surface waters during the spring. Weak circulation within the shelf basins ensures that copepod populations are not displaced throughout the year.

A continuous pump sampler for profiling copepods and chlorophyll in the upper oceanic layers

Abstract A continuous profiling pumping system has been developed for obtaining biological profiles of copepods, chlorophyll, temperature, and light irradiance in the upper mixed layers to depths of ≈ 100 m. The sampler minimizes the problem of zooplankton avoidance of pump intakes by entrapping animals in a descending plankton net and subsequently removing them by pumping on the cod end. Measured profiles of copepods and chlorophyll indicate a vertical resolution of ≈ 3 to 4 m resulting from mixing in the hose and pump for a sampler descent rate of 0.5 to 1.0 m s −1 . The pumping system delivers ≈ 50 to 60 1 min −1 of seawater on deck. Residence time and mixing in the hose are estimated. Pump and Niskin bottle samples used in primary production measurements are compared and indicate minimal damage to organisms during the production measurements. Applications of the pumping system to production measurements are presented.

Continuous and rapid profiling of zooplankton with an electronic counter mounted on a ‘Batfish’ vehicle

Abstract An in situ electronic zooplankton counter has been developed and tested as a biological tool for counting, sizing, and resolving spatial patterns of zooplankton. The instrument was transported on a towed undulating vehicle, with the simultaneous measurement of chlorophyll a , water temperature, conductivity, and depth. Instrument characteristics, sampler system data, calibration of the zooplankton counter, and tests with zooplankton are discussed. Zooplankton, chlorophyll, and physical data sampled in the coastal waters of Peru are presented and some features of the zooplankton data from both Peru and the Scotian Shelf are discussed.

Rapid underway profiling of chlorophyll with an in situ fluorometer mounted on a ‘Batfish’ vehicle

Abstract A modified version of the Variosens fluorometer employed in the in situ measurement of chlorophyll a fluorescence has been examined as a potential research and survey tool. The instrument was transported on a towed undulating vehicle, with simultaneous measurement of water temperature, conductivity and depth. Frequency response studies, instrument calibration procedures, measurement errors and operating procedures at sea are discussed. The capability of the system is illustrated by examples of chlorophyll and physical data contoured in the vertical plane and sampled from the coastal shelf region adjacent to Nova Scotia.

Numerical modelling of diel carbon production and zooplankton grazing on the Scotian shelf based on observational data

Abstract Using high resolution vertical distributions of chlorophyll and zooplankton, and field observations of photosynthetic parameters, it is shown that on the Scotian Shelf the peak in the vertical profile of primary production generally lies shallower than the chlorophyll maximum, but coincides with the peak in the vertical profile of copepods. A simple numerical model shows that the 24th carbon budget can be balanced using the best available estimates of the rate constants for phytoplankton growth, zooplankton grazing and vertical migration. This calculation is very sensitive to the size of the weight-specific ration and favors values of ∼40% d−1 for it.

Productivity of sea ice algae: In situ vs. incubator methods

Abstract Algae of annual sea ice in the high Arctic (Resolute Passage, N.W.T.) gave little evidence of serious photosynthetic impairment during the seasonal decline of the bottom ice bloom when measured in a light-gradient incubator using a popular technique. Measurements made in situ with a novel sampling and incubating device, however, yielded much lower production rates than expected from the incubator measurements. Maximum assimilation numbers averaged 0.223 mgC·mgChl a −1 ·h −1 in incubator experiments but only 0.018 in situ, while photosynthetic efficiency averaged 0.0137 and 0.0069 [mgC·mgChl a −1 ·h −1 ]·[ μ mol photons·m −2 ·s −1 ] −1 in the incubator and in sutu, respectively. Problems of tracer diffusion, temperature differentials and extracellullar release of dissolved organic carbon can help explain the discrepancy, but differences in the spectral availability of light (PUR) between incubator and estimated in situ conditions would tend to worsen the disagreement. Bottom ice algae appear to remain viable and potentially productive during the decline of the bloom, but may achieve relatively little production in situ, depending on which technique for measuring production is considered most accurate.

Counting and identifying copepod species with an in situ electronic zooplankton counter

Abstract The potential for identifying and separating copepod species and stages with an in situ electronic zooplankton counter, which was developed and tested as a biological tool, has been examined. The instrument was mounted on the Batfish, a depth-controllable towed body equipped to measure chlorophyll a , temperature, salinity, and depth. Zooplankton length and diameter distributions measured by the zooplankton counter were compared with measurements made using a microscope. Detailed analysis of the data shows that the electronic counter has sufficient resolution to separate and identify dominant species of copepods consisting of Clausocalanus arcuicornis, Metridia lucens, Calanus finmarchicus , and Euchirella rostrata sampled from the Scotian Shelf, and Centropages brachiatus, Calanus chiliensis , and Eucalanus inermis sampled from the Peruvian Shelf. The examples of Batfish data included show the separation of dominant species and copepod abundance estimates from the Peruvian Shelf. The relative merits of the electronic measurements and microscope analysis are discussed.