Resultaten van het plankton onderzoek met behulp van pompmonsters in het kader van Cicar (Cooperative Investigations in the Caribbean and Adjacent Regions) project

From 1970 up to 1973 plankton sampling was executed as part of the “Cooperative Investigations in the Caribbean and Adjacent Rerions” (Cicar-project). For the stationlist one is referred to Van der Spoel & Koperdraat (1974). During the cruises 21-23, 27-29, 31-35 and 37-38 a 3 PK Stork/ Pelger vacuum pump was used for sampling. The samples examined in this study are all pump samples. The plankton collected from 4500 litre filtered water was settled during 24 hours. The volumina were divided into six classes: 0-0.5 ml; 0.5-2 ml; 2-5 ml; 5-10 ml; 10-25 ml and 25 ml. The colour of the samples was distinguished as green, red, white and brown. Slides made from subsamples showed that the red colour of the preserved samples stands for a high percentage (up to 93%) of zooplankton; green means that the sample consists mainly (78-94%) of diatoms with large volume, like Coscinodiscus species. The white colour is an indication of small diatoms and other phytoplankton, while the brown coloured samples contain more detritus (average 22.5%) than other samples. The results of the volume and colour measurements are presented in figure 2-12. These maps show in the coastal waters of the Guyana’s a high primary production. This production is in March (Fig. 4) as well in April (Fig. 5) high at a certain distance from the coast. Near the coast and further seaward near the continental slope the production is far lower. This also becomes clear in figure 13 up to 20, in which the ratio of sand, detritus, phyto- and zooplankton is given. Hulbert a.o. (1969) and Cadée (pers.comm.) found in those months near the coast a small strip with a high primary production. The high production is caused by the upwelling of nutrientrich water at the continental slope, because the Amazon outflow contains hardly phosphates and nitrates (Ryther a.o., 1969). This upwelling also explains the high quantity of sand found in the samples before the continental shelf (Fig. 13 and 17). It appears that the diatoms which are dominant in the samples (Fig. 21) -taken in a short period and rather close- belong to different groups. This could be an indication for different watermasses in which the circumstances are optimal for the different diatoms. It was not possible to discover a relation between the different phytoplankton populations and the physical and chemical data (Fig. 22). It is therefore improbable that the different phytoplankton populations are caused by different watermasses. They form rather different stages in the succession of one bloom. The diversity of the phytoplankton (Tabel 1 and 2) is very low. Although this is normal during the bloom, the main reason for this low diversity must be found in the hydrographical situation; in regions with upwelling the diversity drops (Margalef, 1967). In figure 23 the diatoms and dinoflagellates are divided into littoral, neritic and oceanic species. The coastal samples contain a lot of littoral species, in the region with a high primary production mainly neritic species are found and beyond the continental shelf the population is typical oceanic. On this ground the supposition that in this region different watermasses don’t mix is untenable. The suggestion of Ryther a.o. (1967) that the outflow of the Amazon mixes fast with the Guyana current seems to be right. The variation in average length of two Coscinodiscus species in relation to hydrographical circumstances neither gave further information about different watermasses (Fig. 24 and 25). Noteworthy is the relation that seems to exist between the length of both species and the salinity and Si-content of the sample. Coscinodiscus asteromphalus Ehr, has greater cells at a high salinity and smaller cells with increasing Si-content. With Coscinodiscus concinnus Schm. the relation is more clear, but just the reverse. About the situation in the eastern Caribbean hardly any information can be offered. The sampling method was too incomplete. In October primary production is found in the Gulf of Maracaïbo. The quantity of settled material in these regions is considerably lower than off the coast of the Guyanas. During the whole year blue algae and dinoflagellates are dominant (Fig.37). Only in the sample of 18 June Chaetoceros was dominant. This indicates the oceanic character of the region. A total impression of the samples shows a production period in January and in August-October.