A HISTORY OF SUFFOLK 



Essex and Suffolk it also seems to show why some are good for the culture 

 of oysters and others unfit, and why they sometimes turn green and in 

 other estuaries lose greenness acquired elsewhere. 



Marine and Freshwater Plankton 



When carrying out researches in the Thames in 1882 in connexion 

 with the main drainage of London on behalf of the Board of Works it 

 occurred to me that it was desirable to ascertain the number per gallon 

 of water of such small animals as Cyclops, since their excrements were 

 often so abundant in the mud as to indicate that they might play an 

 important part in destroying the sewage material. Subsequently from 

 the early part of May to the end of September I systematically and regu- 

 larly carried out similar inquiries in the various estuaries and rivers of 

 Suffolk, Essex, and Kent, and in the more open seawater off the coast. 

 At that time little or no attention had been paid to this subject, which 

 is now looked upon as very important because such small animals form so 

 large a part of the food of larger species. 



By the method adopted I obtained in a small bulk of water all the 

 living animals too of an inch or more in diameter in a known volume of 

 water, collected near the surface, near the bottom, and half way between. 

 I found it most convenient to give the number per 10 gallons in order to 

 avoid fractions when dealing with mean results, though as a general rule 

 only 2J gallons were collected. No attempt was made to distinguish the 

 species, which would have made the study much more difficult and com- 

 plicated, but the different groups of animals could easily be recognized 

 by their manner of swimming. The numbers thus found varied enor- 

 mously from 10 gallons, in fact from only two or three up to 10,000 or 

 more. 



One object which I had in view was to ascertain the influence of 

 the varying amount of salts on the kind and number of the small free- 

 swimming animals in passing down the rivers from where the water was 

 fresh till we come to the sea water itself. This is a question of paramount 

 importance in studying estuaries, which in the case of Suffolk have so 

 great an extension. At the same time the results depend not merely on 

 the amount of salts, but also on associated conditions of different kinds. 

 In order therefore to properly elucidate the facts I determined the excess 

 in the weight of the volume of 1,000 grains of rain water.' The rivers 

 and estuaries I studied in 1884 were: in Suffolk the Aide, Butley, 

 Orwell, and Stour ; in Essex the Colne, Blackwater, Crouche, and 

 Roche; and in Kent the Medway. Since 1884 I have much extended 

 these observations. 



' The late Dr. H. C. Sorby evidently intended to give tables showing the distribution of animals 

 similar to those printed in F.C.H. Essex, i, 74, but no details on this point have been found among 

 the MS. notes relating to his article [Editor V.C.H.]. 



R8 



