250 L. HrLLIS-COLINVAUX 



the Halimedae were essentially absent, probably because of grazing 

 pressures. Outcrops of the back-reef and the moat of the reef crest 

 (which contained much wave-stirred sand) are also excluded. 



In Table XXII these results are compared with data for phyto- 

 plankton productivity supplied by Ryther (1969). The highest figure 

 Ryther cites, that for upwellings, is about one-third of the performance 

 of dense stands of Halimeda in a coral reef. Even where Halimeda plants 

 are thinly spaced in Thalassia beds their contribution to productivity 

 is still better than that of phy toplankton of typical coastal waters. Even 

 the average productivity of the Halimedae of the Glory Be reef is equal 



Table XXII. Comparison of Xet Productivity 



OF Halimeda Populations with Phytoplankton 



Production 



« Data from Ryther (1969). 

 " Data from Table XXI. 



to phytoplankton production in coastal waters, in spite of the fact that 

 large areas of reef are denied to Halimeda by grazing animals, Hving 

 corals and the s\u"f. 



The contribution of fixed carbon to a reef by Halimeda, therefore, 

 may be very high, and the primary productivity of a tropical area or 

 coral reef, taken as a mosaic of fleshy algae, calcareous algae, sea 

 grasses and photosynthetic corals, may be of the same approximate 

 order of magnitude as the most productive of Mann's (1972) Laminaria 

 forests in Nova Scotia. Accounts of the productivity of tropical waters 

 should include the high values encountered in these latitudes, as well as 

 the proverbial low ones, a point well made in the comparative study of 

 productivities of Antarctic and tropical-subtropical regions byEl-Sayed 

 and Turner (1977), and the significant contribution of benthic algae 

 should not be ignored as is done so frequently. Coral reefs occupy about 



