ICES Journal of Marine Science, 53: 945–950. 1996
A method for estimating primary production from chlorophyll
concentrations with results showing trends in the Irish Sea and
the Dutch coastal zone
Peter V. M. Bot and Franciscus Colijn
Bot, P. V. M., and Colijn, F. 1996. A method for estimating primary production from
chlorophyll concentrations with results showing trends in the Irish Sea and the Dutch
coastal zone. – ICES Journal of Marine Science, 53: 945–950.
Analyses of long-term trends in physical, chemical, and biological parameters at
selected locations along the residual current represent one of the objectives of the
North-West European Shelf Programme (NOWESP). Within this framework, trends
in chlorophyll concentrations at two of these localities, the Irish Sea and the Dutch
coastal zone, were analysed. The results show an increase in the Irish Sea from the end
of the 1960s onwards. Chlorophyll concentrations increased from the early 1980s at
two stations in the Dutch coastal zone (Marsdiep and 6 km offshore of Goeree),
followed by a decrease during the end of the 1980s and 1990s to values comparable
with those of the 1970s. In both areas, the increase in chlorophyll is almost exclusively
due to higher summer values. Estimates of gross annual primary production calculated
from the relation between chlorophyll and primary production measurements indicate
a 50 to 100% increase in the Irish Sea over the last three decades. Estimated mean
annual production between 1976 and 1992 at Goeree 6 was 375 gC m "2. Marsdiep
estimates are influenced by an increase of suspended particulate matter (SPM) during
the 1980s followed by a marked reduction during the past 5 years. These changes in
SPM may mask effects of changes in nutrient input.
? 1996 International Council for the Exploration of the Sea
Key words: chlorophyll, Irish Sea, North Sea, primary production.
P. V. M. Bot: RIKZ/National Institute of Coastal and Marine Research, PO Box 20907,
2500 EX The Hague, The Netherlands; F. Colijn: FTZ/Forschungs- und Technologiezentrum Westküste, Hafentörn, D-25761 Büsum, Germany.
Introduction
During the 1975 Århus Symposium (Hempel et al.,
1978) only a few papers were devoted to non-fisheries
issues. An overview on nutrient contents of the North
Sea was given by Postma (1978) showing a general
picture of winter and summer distribution of dissolved
phosphate along a transect running from the Shetlands
to the Dutch coast and into the Channel. He also
focused on the enhanced nearshore nutrient concentrations in the Southern Bight and the relatively high
summer concentrations of dissolved phosphate in the
Wadden Sea, the first indications of eutrophication in
Dutch coastal waters. Information was also presented
on trends in phytoplankton composition and biomass
(Hagmeier, 1978) and nutrients (Lucht and Gillbricht,
1978) near Helgoland since 1962, and in phytoplankton and zooplankton species composition from the
Continuous Plankton Recorder (Reid, 1978; Bainbridge
et al., 1978).
1054–3139/96/060945+06 $18.00/0
No papers were presented dealing with measurements of primary production. Since the introduction of
the 14C method by Steemann Nielsen (1952) numerous
but often spatially restricted and short-term measurements on phytoplankton primary production have
been performed. Even nowadays long-term time
series exist only for a few selected stations, most of
which have unfortunately not been continued until
today.
Within the framework of the North-West European
Shelf Programme (NOWESP, van Leussen et al., 1996),
existing primary production and chlorophyll data are
compiled with the final goal to study inter-annual
variability, spatial variability, and long-term trends.
A serious drawback in the analysis is that no standard methods have been used for measurements of
either primary production or chlorophyll. However, in
view of the limited data all information has to be used
to obtain even a restricted coverage of the large shelf
area.
? 1996 International Council for the Exploration of the Sea
946
P. V. M. Bot and F. Colijn
Even the use of the 14C method does not guarantee
comparability of data sets (Richardson, 1991;
Richardson and Heilmann, 1995), because different
handling procedures cause variability in the measurements, while methods calculating annual values are
sensitive to errors. In a recent review, van Beusekom and
Diel-Christiansen (1994) have compiled primary production data for several ICES rectangles in the North Sea.
For the Northern rectangles no complete annual production cycles are available, but the few temporally
restricted measurements resulted in estimates of about
125 gC m "2 yr "1. For the English East coast, they
derived an estimate of about 80 gC m "2 yr "1. A
much larger data set for the area along the Belgian
and Dutch coast resulted in values of 250–440 in the
nearshore and of about 200 gC m "2 yr "1 in offshore
areas. The authors observed an increasing trend that
was mainly due to higher maximum daily rates during
summer in recent years. The single long-term time series
in this area collected by Cadée and Hegeman (1993)
indicates that primary production increased from 150 to
200–400 gC m "2 yr "1 from the mid-1970s to the mid1980s and early 1990s. Direct measurements of primary
production in the German Bight have not been encountered in the literature, but estimates based on several
other sources (Colijn et al., 1990; Joint and Pomroy,
1992) suggest values of 250 gC m "2 yr "1 and probably
even higher in nearshore areas (Colijn and Ludden,
1983). Recent data for the central North Sea indicate
values around 100 gC m "2 yr "1 (Peeters et al., 1991;
Joint and Pomroy, 1992). Finally, again more data are
available for the Skagerrak and Kattegat area due to
greater observer effort in this region (cf. Colijn, 1992).
Depending on location, estimates range from 100 to
300 gC m "2 yr "1 (Richardson, 1989). Although trends
in primary production have been described for specific
areas (Dutch coast, Wadden Sea, Kattegat/Skagerrak),
contradictory results have been obtained within larger
geographical regions. For instance, a 10-year series for
the Gullmarsfjord (Lindahl, 1995) does not correspond
to the general trend observed in the Kattegat, but only
shows substantial inter-annual variability.
In this paper, a standard algorithm is described for
calculating gross primary production based on chlorophyll measurements, light attenuation, solar irradiance,
and P vs I characteristics. This method enables us to
estimate primary production for areas for which only
chlorophyll data are available. Results are presented for
a station in the Irish Sea and two stations along the
Dutch coast.
Materials and methods
The time series used are stored in the research database
of NOWESP in Hamburg. They include chlorophyll,
suspended matter (SPM), and salinity data collected
Figure 1. Relationship between chlorophyll concentration and
primary production. All data are loge-transformed.
during a bi-weekly Dutch monitoring programme in the
period 1976–1993 from a station in the central part of
the Marsdiep (52)58*N, 04)45*E) and from Goeree 6, a
station 6 km off the Dutch south-west coast (51)52*N,
03)52*E), and chlorophyll data originating from a
station approximately 5 km west of Port Erin on the Isle
of Man (Irish Sea), known as the ‘‘Cypris’’ station
(54)05*N, 04)50*W). The latter time series spans the
period 1966 to 1994, with a sampling frequency of one to
four measurements per month.
Chlorophyll measurements in the Irish Sea were
performed according to Lorenzen (1976). Lorenzen was
also followed up to 1986 at the two Dutch coastal
stations, but the HPLC method was used afterwards
(Gieskes and Kraay, 1984). To estimate the effect of
changes in chlorophyll concentration on primary production, estimates of gross primary production were
made on the basis of the P vs I relationship measured in
the North Sea (Peeters et al., 1993). Linear regression of
loge-transformed chlorophyll and Pmax values derived
from both coastal and open-sea measurements resulted
in a r2 of 0.91 (intercept 1.04, slope 1.27; Fig. 1). In
other words, chlorophyll concentration in this data set
explains 91% of the variance in Pmax. Since primary
production is dependent on chlorophyll and on light
conditions, calculations were based on depth-integrated
chlorophyll (to a depth at which ambient irradiance is
1% of the surface value and the averaged daily irradiance in the water column (Colijn and Ludden, 1983).
The light dependency of primary production in this
relationship was calculated from the quotient of the
average irradiance in the water column and the average
Iopt values (120 W m "2) of the P vs I relationship.
Surface irradiance (PAR) was estimated from the solar
declination and geographic latitude with a correction of
30% for cloud coverage and light reflection.
Light attenuation values in the Marsdiep and at
Goeree 6 were calculated from the relation between
SPM, salinity, and chlorophyll according to Stronkorst
(1988). SPM and salinity data were linearly interpolated,
and chlorophyll data exponentially interpolated, to daily
values for calculating daily light attenuation. Because
Estimating primary production from chlorophyll concentrations
947
Figure 2. Estimates of primary production on the basis of chlorophyll and light (filled rectangles) compared with measured primary
production values interpolated to daily values (open squares) in 1990 at two stations located 20 km (a) and 50 km (b) NW
of Terschelling in the Southern North Sea. Estimated and measured annual primary production were at location A 444 and
441 gC m "2 yr "1 and at location B 144 and 149 gC m "2 yr "1, respectively.
light attenuation data are lacking for the Irish Sea,
constant light transparency during the year has been
assumed with the attenuation value set at 0.3 m "1
(M. Veldhuis, NIOZ pers. comm.). A correction for
self-shading was made by adding 0.015 m "1 per mg
chlorophyll m "3.
For calculating annual primary production, chlorophyll measurements were exponentially interpolated to
daily values, converted to daily production values and
integrated over the year. Since the estimates are based
on the assumption of a constant Pmax/chlorophyll relation, changes in photosynthetic efficiency due to adaptation of phytoplankton to changing light conditions in
the water column are not taken into account.
Examples of estimates based on chlorophyll concentration and light in relation to interpolated primary
production measurements during 1990 at two stations
off the Dutch coast are given in Figure 2.
Results
Compared to the Dutch coastal stations, chlorophyll
concentrations in the Irish Sea are low. With some
exceptions, mean values during the spring bloom (about
3 mg m "3; Fig. 3a) are comparable to values measured
in the central part of the southern North Sea (Joint and
Pomroy, 1993) and about a factor 10 lower than found
at the two stations in the Dutch coastal zone (Fig. 3b,
c). From the end of the 1970s to the early 1990s, summer
chlorophyll concentrations in the Irish Sea increased
by nearly 100%. The period between 1978 and 1981
was characterized by particularly high values
(Fig. 4a). Chlorophyll concentrations at the two Dutch
stations show increasing values from the early 1980s to
the mid- and late 1980s, followed by a decline in the
1990s (Fig. 4b, c). As in the Irish Sea, the changes are
particularly related to the summer season.
The trend in estimated gross annual primary production in the Irish Sea (Fig. 5a) is largely similar to that in
chlorophyll, as might be expected given the method of
calculation. Annual primary production increased from
about 100 gC m "2 yr "1 in the 1960s and early 1970s
to 150–200 gC m "2 yr "1 in the 1980s and 1990s.
However, the inter-annual variation is considerable.
Estimates of primary production for station Goeree
6 indicate very high values in 1983 and 1984 (Fig. 5b),
which are due to a combination of high chlorophyll concentrations and relatively low SPM values
during the summer season. In contrast, low chlorophyll concentrations and relatively high levels of
SPM during the summer season in 1988 result in a
strong reduction in primary production. Estimates
for the Marsdiep (Fig. 5c) range between 100 and
200 gC m "2 yr "1 up to 1988. However, annual production values increase to almost 400 gC m "2 yr "1
during the later years.
948
P. V. M. Bot and F. Colijn
Figure 3. Seasonal variations in surface chlorophyll concentration (based on monthly means) at the ‘‘Cypris’’ station near
the Isle of Man in the Irish Sea (a), at Goeree 6 (b), and in the
Marsdiep (c).
Discussion
Apart from chlorophyll concentrations observed in the
Irish Sea being relatively low, all three time series
indicate an increase in chlorophyll in the late 1970s and
early 1980s, particularly during the summer season.
However, the reduction observed in the Dutch coastal
zone during the end of the 1980s is not apparent in the
Irish Sea series. Changes in chlorophyll concentration
are often related to changes in nutrient availability
during the growing season (Boddeke and Hagel 1991;
Brockmann et al., 1990). Indeed, the perennial changes
in observed nutrient concentrations at the three localities
(Klein and van Buuren, 1992; Laane et al., 1996) occur
in concert with the changes in chlorophyll concentrations. However, in view of the considerable interannual variation in all three data sets, other factors
obviously play an important role. The increase and
decrease in the Marsdiep during the 1980s, for example,
coincide with an increase and decrease of SPM in this
period (Figs 4c and 6). Hence, further studies into
Figure 4. Moving averages over 3 years of mean monthly
surface chlorophyll concentrations during May, June, and July
in the Irish Sea (a), at Goeree 6 (b), and in the Marsdiep (c).
possible links between chlorophyll variability and
physical as well as chemical and biological parameters
are required.
The annual primary production at the ‘‘Cypris’’
station has apparently increased by between 50 and
100% over the past three decades. This estimate is in
agreement with primary production measurements of
Boalch (1987) at station E1 in the inlet of the English
Channel, where daily production values during the
spring phytoplankton bloom increased from about
1 gC m "2 d "1 in the 1970s to 1.5–2.0 gC m "2 d "1 in
the 1980s. It is not clear whether the station near the
Isle of Man may be considered representative of the
entire Irish Sea (Slinn, 1974), but, if so, this would
mean a significant increase in primary production in
this area. However, the estimates are based on a constant light transparency during the year and further
Estimating primary production from chlorophyll concentrations
949
Figure 6. Suspended matter concentrations in the Marsdiep.
Figure 5. Estimates of gross annual primary production (filled
squares) on the bases of chlorophyll and light in the Irish Sea
(a), at Goeree 6 (b), and in the Marsdiep (c). Open triangles
represent measurements by Cadée and Hegeman (1993).
information on local light conditions and attenuation
is needed.
At station Goeree 6, no clear perennial trend is
observed. The mean annual primary production estimate over the period 1976–1992 is 375 gC m "2 yr "1.
This value agrees well with recent estimates for nearshore parts of the Dutch coast of 250–440 gC m "2 yr "1
(Peeters et al., 1991).
The estimates for the Marsdiep show a different
picture. The estimated values of 100–200 gC m "2 yr "1
for the 1970s agree well with measurements by Cadée
and Hegeman (1974, 1979) at another station in the
same area. The same is true for the 1990s, with estimated
annual values of 259, 395, and 388 gC m "2 yr "1
compared to measured values of 254, 385, and
370 gC m "2 yr "1 in 1990, 1991, and 1992, respectively
(Cadée and Hegeman, 1993). In the 1980s, however, the
estimated values are two to three times lower than those
measured by Cadée and Hegeman (1991), whereas the
mean annual values for chlorophyll show comparable
patterns. The correlation between the loge-transformed
incubator measurements of these authors and estimated
primary production values based on their chlorophyll
concentrations gave a r2 of 0.82 (intercept 1.59, slope
0.95; three outliers among the 173 measurements having
been excluded), and a r2 of 0.81 (intercept 1.81, slope
0.87) if measurements of the 1980s alone were included.
This indicates that the difference between measured and
estimated production in the 1980s is not due to differences in chlorophyll concentrations or to the method of
calculation, but to different values for light attenuation
and to the way the impact of light attenuation is
calculated.
It is worth discussing the impact of SPM in this
context. SPM concentrations in the Marsdiep increased
considerably in the 1980s and declined to very low levels
in the 1990s (Fig. 6). These changes have a strong effect
on the calculation of the light attenuation values used in
the estimation procedure, and it is quite possible that the
role of SPM is overestimated at high concentrations.
The same trend of reduced light transparency in the
1980s followed by an increase in the 1990s is also
revealed by Secchi disk data from the Marsdiep station
(data not shown). This is in contrast to the Secchi disk
readings used by Cadée and Hegeman (1993) to calculate column production which remained virtually
unchanged from the 1970s until the 1990s. A possible
explanation for this apparent discrepancy is a difference
in sampling strategy. Samples at the Marsdiep station
were collected 2 h after high tide when the contribution
of water coming from the Wadden Sea is relatively high,
whereas the samples of Cadée and Hegeman were collected during high tide from the NIOZ jetty when the
contribution of North Sea water is at a maximum. This
would imply that the high annual production values
observed by Cadée and Hegeman in the 1980s are
characteristic of incoming water from the North Sea.
950
P. V. M. Bot and F. Colijn
Acknowledgements
The authors thank Dr D. J. Slinn and Dr J. Allen of the
Department of Marine Biology, University of Liverpool,
Port Erin, Isle of Man, and Dr G. Cadée of the NIOZ
for providing time series on chlorophyll and primary
production in the Irish Sea and the Marsdiep, respectively. This work was carried out within the NOWESP
project in the Marine Science and Technology
programme (MAST): MAS2-CT93-0067.
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