Current observations at Station 3 indicate 

 that water drains out of the Sasanoa system for 

 about an hour after the tide has begun to flood in 

 the Sheepscot, producing a swift surface current 

 with an easterly set. The peak current from 

 this source occurs about three hours after high 

 tide with the same direction at all depths. Dur- 

 ing the last three hours of the flooding cycle, 

 the dominant component of the current flows 

 northerly up the Sheepscot while a lesser 

 westerly moving current flows into the 

 Sasanoa system. The latter component is 

 strongest in the deeper layers. 



Plankton 



Plankton sampling was done with the ob- 

 jective of providing general information about 

 the abundance, composition and distribution of 

 plankton stocks in the estuary at different times 

 of the year . Although plankton was sampled 

 regularly over a two-year period at approxi- 

 mately monthly intervals only, some generaliza- 

 tions for the purpose of this report can be made 

 without much risk. 



Volumetrically the plankton consists pre- 

 dominantly of copepods . Cladocerans, diatoms 

 and dinoflagellates also make up substantial por- 

 tions of the total volume at certain times. Four 

 characteristic species of copepod predominate: 

 Pseudocalanus minutus , Acartia longiremis, 

 Oithona similis , and Microsetella norvegica . 

 The last occurs chiefly during the winter months 

 and is most abundant near Station 3 . Other 

 common species of copepod are Eurytemora 

 herdmani, Centropages hamatus, and Temora 

 longicornis . 



Pelagic larval stages of non-planktonic 

 species are abundant seasonally: pluteus larvae 

 of brittle stars (Ophiuroidea), barnacle nauplii, 

 crab zoeae, and worm trochophores in spring 

 and early summer; bivalve veligers and various 

 worm larvae in middle and late summer. Larg- 

 er planktonic species were often taken in the 

 hauls made with the 1/2 meter net, but these 

 (mostly medusae and ctenophores) were not 

 specifically identified. The jellyfish Aurelia 

 aurita and Cyanea arctica are abundant in the 

 early part of the summer, the former often 

 appearing in dense concentrations . 



Phytoplankton blooms occur from time to 

 time, some extend in from the Gulf of Maine and 

 others are local populations inside the estuary. 

 Diatoms of several species, chiefly of the genera 

 Thalassiosira, Thalassiothrix, and Chaetoceras 

 appear in the spring, mostly in the most seaward 

 parts of the estuary and outside it. Skeletonema , 

 Asterionella and Rhizosolenia appear during the 

 summer, often in local concentrations within the 

 estuary. Coscinodiscus (several species) is 

 nearly always present. 



Ceratium tripos, C . longipes, C . fusus, 

 and one unidentified species are the most abun- 

 dant of the dinoflagellates but several species of 

 Peridinium are common. Bigelow (1924) has 

 described the phytoplankton populations of the 

 Gulf of Maine and the times of seasonal maxima, 

 and the observations made in the Sheepscot agree 

 in general with his account. 



The abundance of all plankton decreases 

 during the winter and early spring from November 

 to May. With the exception of a substantial in- 

 crease in the local population of Acartia late in 

 the summer and occasional local diatom blooms 

 the plankton abundance diminishes as one pro- 

 ceeds up the estuary. The plankton of the upper 

 estuary contains scattered elements from the 

 more seaward regions and several species that 

 are indigenous to the area, e.g. Acartia longi- 

 remis, Eurytemora herdmani, the nauplii of the 

 brackish water barnacle Balanus improvisus, 

 and numerous mollusc and worm larvae. Marine 

 and estuarine plankton can be found as far into 

 the estuary as any salt is detectable. Harpacti- 

 coid copepods (except for Microsetella ) are 

 more common in the plankton of the upper 

 estuary than that of the lower. 



Plankton abundance may vary from year 

 to year as can be seen in fig. 6. There also 

 may be variations in its taxonomic composition . 



Other populations 



Important environmental variables exert- 

 ing an influence on the biota in the Sheepscot 

 estuary are temperature, salinity, tides, depth 

 of water and the nature of the substrate . Com - 

 binations of these variables create a variety of 

 environments or habitats, differing enough in 



