Grazing pressure of the calanoid copepod Temora longicornis on a Phaeocystis dominated spring bloom in a Dutch tidal inlet

Abstract

Between 30 March and 1 1 May 1990, total copepod abundance and the abundance. b~o m a s s and gut fluorescence of Temora longlcornls were determined and related to the abundance and succession of phytoplankton development in a Dutch tidal inlet Gut plgment values were h~g h e s t In females and lowest In young copepodites, but weight-spec~f~c pigment concentrations were about slrnilar Pigment levels measured In the guts were relatively high at the beginning and end of the penod of investigation when diatoms d o m~n a t e d the phytoplankton community, dnd low d u n n g the Phaeocystls d o m~n a t e d period, when ambient chlorophyll concentrations were h~g h e s t For the latter period, calculated ingestion rates In T longlcornls were low and estimated dally consumption amounted to less than 1 % of the phytoplankton standing stock, s u g g e s w g a negl~gible grazlng Impact on the development of the Phaeoc)/stls bloom In splte of the low grazrng on phytoplankton. T long]-corms biomass increased by one older of magnitude The discrepancy between low giazlng pressuie and copepod development is explained by assumlng that T long~cornls scvltched to heterotrophic food a bloom of ciliates present during the Phaeocystls dominated period

Trophic interactions between zooplankton andPhaeocystis cf.globosa

“…Based on the spring biomass increase of T. longicornis, Hansen & van Boekel (1991) calculated a required ingestion of 70 to 140 % body weight daily, which is well in agreement with results from laboratory growth experiments (Klein Breteler et al, 1990). In contrast to the results of the experiments in which only algal food was present,…”

“…The gut pigment content of males females and smaller copepods, did not resemble the seasonal course of the chlorophyll-a concentration in the water (Figs 3, 4) It was inversely correlated with the dominance of Phaeocystis (Figs 4,5). Calculated ingestion of phytoplankton was negligible during the Phaeocystis bloom and far too low to fulfil Temora's metabolic needs, which w~r~ ~timated from its biomass increase (Hansen & van Boekel, 1991}. It was concluded that Temora had switched to a heterotrophic food source and it has been snggest~d that this could have been a bloom of ciliate.~ present during the Phaeocystis-dominated period of the phytoplankton spring bloom (van Boekel et al, 1992). The hypothesis is put forward that 7", longicornis preferentially feeds on microzooplankton and by this may enhance, rather than depress, Phaeocystis blooms.…”

“…We have suggested that Temora fed on co-occurring ciliates instead (Hansen & van Boekel, 1991). There is much evidence that microzooplankton is a suitable food for copepods (Klein Breteler, 1980: Stoecker & McDowell Capuzzo.…”

“…As expected from the hypothesis, the reduction of the Phaeocystis biomass in the incubations with cihated a n d copepods was less than with ci~ates alone: that means a decrease in overall grazing-pressure on Phaeocystis by the copepods. April May (Hansen & van Boekel, 1991) …”

“…The applied methods were described in detail by Hansen & van Boekel (1991). Copepods were collected off Texel (53 ~ 00.2'N.…”