119
TABLE 3: Growth rates of selected species from populations incubated
in dialysis tubes and plexiglass - Nuclepore(R) filter cages.
K (doublinss
-1
day )
Species
Tube
Cage
Skeletonema costatum
1. 40
0.95
Nitzschia punsens
1. 91
0.72
Nitzschia closterium
2.72
2.08
Leptocylindrus minimus
0.74
1. 04
Chaetoceros compressus
1.16
0.55
Peridinium quinquicorne
1. 25
1. 60
Prorocentrum minimum
0.00
0.92
The impact of grazing on both diluted and undiluted caged natural
populations must also be considered.
Landry and Hasset (1982) used
the apparent increment in growth rate obtained from a sequential
dilution of natural populations as an estimate of community grazing
pressure by microzooplankt"on.
Their hypothesis assumes that grazers
will be diluted to a greater extent than the phytoplankton population.
This lowers the total grazing impact.
They must also assume that
phytoplankton growth rates are identical at all dilutions as long a
nutrient levels remain constant during the incubation period. Grazing
is calculated as the slope of the regression line for growth vs.
dilution.
The Y-intercept yields an estimate of the apparent Il max •
In one of their experiments maximum growth rates were comparable for
populations incubated in bottles with added nutrients and in dialysis
tubes incubated in situ.
In two other experiments the addition or
deletion
of
nutrients
to
bottle
enclosed populations
yielded
comparable growth rates at high dilution levels whereas depressed
growth rates were found at no or low dilutions without nutrient
additions.
The authors state that this latter response "leads to an
erroneous impression of the magnitude of the micro-zooplankton grazing
impact". I feel that their data demonstrates that nutrient limitation
had a greater impact on the growth of the population than did grazing,
especially since populations at high dilutions had comparable growth
rates with or without nutrient additions. Both of our results suggest
that the dilution method yields elevated growth rates and would be
useful as an indication of nutrient limitation in a natural population
and to obtain an estimate of the carrying capacity for a given set of
environmental variables.
TABLE 3: Growth rates of selected species from populations incubated
in dialysis tubes and plexiglass - Nuclepore(R) filter cages.
K (doublinss
-1
day )
Species
Tube
Cage
Skeletonema costatum
1. 40
0.95
Nitzschia punsens
1. 91
0.72
Nitzschia closterium
2.72
2.08
Leptocylindrus minimus
0.74
1. 04
Chaetoceros compressus
1.16
0.55
Peridinium quinquicorne
1. 25
1. 60
Prorocentrum minimum
0.00
0.92
The impact of grazing on both diluted and undiluted caged natural
populations must also be considered.
Landry and Hasset (1982) used
the apparent increment in growth rate obtained from a sequential
dilution of natural populations as an estimate of community grazing
pressure by microzooplankt"on.
Their hypothesis assumes that grazers
will be diluted to a greater extent than the phytoplankton population.
This lowers the total grazing impact.
They must also assume that
phytoplankton growth rates are identical at all dilutions as long a
nutrient levels remain constant during the incubation period. Grazing
is calculated as the slope of the regression line for growth vs.
dilution.
The Y-intercept yields an estimate of the apparent Il max •
In one of their experiments maximum growth rates were comparable for
populations incubated in bottles with added nutrients and in dialysis
tubes incubated in situ.
In two other experiments the addition or
deletion
of
nutrients
to
bottle
enclosed populations
yielded
comparable growth rates at high dilution levels whereas depressed
growth rates were found at no or low dilutions without nutrient
additions.
The authors state that this latter response "leads to an
erroneous impression of the magnitude of the micro-zooplankton grazing
impact". I feel that their data demonstrates that nutrient limitation
had a greater impact on the growth of the population than did grazing,
especially since populations at high dilutions had comparable growth
rates with or without nutrient additions. Both of our results suggest
that the dilution method yields elevated growth rates and would be
useful as an indication of nutrient limitation in a natural population
and to obtain an estimate of the carrying capacity for a given set of
environmental variables.