In situ Processes Working Group

Chairman: Percy Donaghay

Participants: Joseph Katz, Uwe Kils, Gustav Paffenhofer, Rudi Strickler

Task Statement

The in situ rate processes working group was given the task of identifying optical techniques that would allow direct or indirect estimation of rate processes controlling the dynamics of key zooplankton species. Specific tasks for optics utilization outlined in the Background Document were: (a) obtain estimates of feeding, birth, growth and mortality rates of planktonic animals in situ through continuous observations of individuals or small swarms over seconds to minutes; (b) obtain information on swimming behavior; (c) obtain predation data on zooplankton by using 2 or 3 different scales of observation simultaneously; (d) observe small-scale distribution of zooplankton, especially aggregations of animals smaller than 1 mm; and (e) obtain data on small scale physics and biophysical interactions. Each of these tasks was evaluated both from the perspective of using optics to directly address key scientific issues of U.S. GLOBEC (see Scientific Problem) and as tools to routinely collect data on rates. There was a general consensus that efforts should initially focus on developing optical techniques to directly address key scientific issues. The rationale for this choice is summarized below.

Scientific Problem

A fundamental objective of U.S. GLOBEC is to understand how biophysical interactions at the individual level control zooplankton population dynamics and fish larval recruitment. Small-scale biophysical interactions can potentially affect zooplankton population dynamics through their effects on feeding, swimming and predation vulnerability. The nature of these effects is currently a matter of considerable controversy. In particular, recent experimental and theoretical studies have suggested that increased small-scale mixing may (1) alternatively decrease feeding through reduction of micropatches (e.g., Lasker, 1975; Davis et al., 1991), or increase feeding through the effects of increased small-scale shear on encounter rates (e.g., Rothschild and Osborn, 1988); (2) alter swimming patterns through effects on fine-scale food structure or induction of escape responses (e.g., Donaghay, 1990); and/or (3) modify predation vulnerability through effects on predator encounter rates (e.g., Rothschild and Osborn, 1988), predator detection mechanisms, and zooplankton aggregation (e.g., Kils, 1992). Although there is sufficient theoretical and experimental evidence to indicate that these biophysical interactions may sometimes control individual success and population dynamics, we do not have the technical capability to test these biophysical interaction models in situ, to evaluate the validity of current rate measurements, or to test new techniques that could dramatically improve in situ rate estimates (Price et al., 1988; Marine Zooplankton Colloquium 1, 1989; Schulze et al., 1992). Just as important, these same gaps in technical capability have limited our ability to relate in situ responses to potentially controlling factors and thereby identify "signature" properties that can be used in surveys to link fine to coarse scales and to identify areas where specific processes should dominate.


The primary goal is to develop a combination of techniques that will allow the in situ measurement of zooplankton rate processes (feeding, swimming and predator interactions) along with the potentially controlling fine-scale biological structure, physical structure and biological-physical interactions. Optical techniques will be important in achieving this goal because they provide direct evidence of the interaction of individual organisms with their conspecific neighbors, their prey and potential predators.


In Situ Rate Process Sampling Strategy

Two very different strategies could be used to estimate critical rate processes in zooplankton First, a single individual zooplankter could be followed (tracked) over time (15 minutes or so) while measurements of individual feeding, swimming and predator interactions are recorded. Although this technique provides ideal data for analysis of the mechanism controlling feeding and swimming behavior, the costs of developing equipment for individual tracking in situ appear to be prohibitive at this time. Second, a group of zooplankton can be followed over time while measurements of feeding, swimming and predator interactions are repeatedly recorded for the group. This approach provides statistical estimates of rate processes such as percent of time feeding, percent swimming upwards, average swimming velocity, etc. These statistical estimates are critically needed for parameterizing models of population dynamics and for identifying factors that control these behaviors. In contrast to tracking individuals, the acoustic technology required for tracking groups of individuals is readily available. The following sections therefore presume this latter strategy.

Required Sensor Technologies

Four major types of optical sensors will be required. Each of these types of sensors are summarized briefly below. For a more detailed discussion of these sensors see the short sensor descriptions in the Biomass Working group report and the instrument descriptions in Schulze et al. (1992).


The progammatic goal of in situ rate measurements can only be met by the nonintrusive application of a combination of optical and non-optical sensor technologies. Although most of the required sensor technologies have been developed for use in the laboratory or for coarse-scale sampling in the ocean, their successful application to in situ rate process measurement will require major efforts in the five areas discussed below.