Biological Oceanography

(^) where 2°P is the daily, secondary production of the population, i signifies the life-
cycle stage (or any marker with respect to age or size), Gi is the weight-specific
growth rate of the ith stage (weight added per weight per day), and Bi is the mean
biomass of the life-cycle stage in the habitat. A rather different approach to secondary
production is to evaluate growth directly from field observations of the increase in
size of individuals in a cohort (see below), sometimes with help from lab rearing to
determine stage duration. Total biomass of stages is evaluated from the same field
sampling data as numbers × stage biomass. If samples are taken often enough, then
these short-term estimates of 2 °P can be added to give the cumulative production of
the population during some suitable period, such as a year or a growing season.
(^) This is very difficult to do convincingly in the ocean. The water and its contained
plankton are on the move, and the fauna present on any given date usually are
different from those present the day before or the day after. Species and stage
composition are usually (not always) too erratic for reliable estimation of growth.
Individuals captured today may be younger and smaller on average than those
captured yesterday, which probably isn’t a biological change in a consistently sampled
population. It happens because populations shift with the flow. Observations of Bi are
afflicted by large variability, variability so great that estimates must be treated as
having multiplicative confidence intervals (½ to two-fold, or worse).
(^) For confined populations in lakes and estuaries, this method has been used with
some success, as for example in Landry’s (1978) study of Acartia hudsonica
production in Jakle’s Lagoon, a small, enclosed, marine pond of 3.5 m depth adjacent
to Puget Sound. Over 2 years (we will only look at one), Landry sampled this small
copepod in the lagoon; his collections included the whole water column so that
abundance could be estimated on a per unit area basis (Fig. 7.11).
Fig. 7.11 Stacked time-series of stage-abundance estimates for Acartia hudsonica in
Jakle’s Lagoon, Washington State, USA, in 1973. Groups (I through VI) identified as
“cohorts” are alternately shaded or white.
(^) (After Landry 1978.)