cohort was said to be in a given stage was determined as the balance point of the
cohort block in each stage time-series, that is at the abundance-weighted mean date.
Plotted together, these produced growth curves (Fig. 7.12), the slopes of which are the
weight-specific growth rates. Given sufficient nutrition (which seems to have been
available in Jakle’s Lagoon), these animals grow exponentially all through
development, so the same rate can be used for all stages.
Fig. 7.12 Growth progression (as carbon content vs. time) for A. hudsonica cohorts in
Jakle’s Lagoon. Slopes of these semi-log plots are the cohorts’ weight-specific growth
rates. Nauplii – triangles; copepodites – circles.
(^) (After Landry 1978.)
Applying the production equation, the field estimate of biomass in each stage on
each date was multiplied by the cohort growth rate and summed. Estimates of egg
production by females, often an important part of secondary production by a
planktonic species, were added. This gives a time-series of production by the copepod
stock (Fig. 7.13) in units of carbon incorporated in organic matter and assimilated into
new copepod tissue and eggs. Naupliar growth contributed 15%, copepodite growth
47%, and egg production 38%. Production had clear seasonality, deriving mostly from
variation in stock size. Total annual production was ∼8 g C m−2. Acartia hudsonica is
the only abundant mesozooplankter in Jakle’s Lagoon, so its production must
represent most of the “secondary” production. At ∼70 mg C m−2 d−1 during summer,
that production was probably a significant fraction of the primary production. That
wasn’t measured but was likely on the order of 200 mg C m−2 d−1. Clearly, much of
the primary production is consumed by something else. In a shallow lagoon, the
benthos would be important, partly eating settled phytoplankton, partly just filtering
the water column above.
Fig. 7.13 Daily (above) and cumulative production of A. hudsonica in Jakle’s Lagoon.
Oscillations in daily production mostly reflect variations in abundance of biomass,
since growth rates of all cohorts were quite close (Fig. 7.14).
(After Landry 1976.)