iOr 



S 



19 74 



D ' J F M 



Month and Year 



A M 

 1975 



Figure 5-37. 



o 



Monthly measurements of chlorophyll-a concentrations (mg/m ) 

 in upper Penobscot Bay for 1974 to 1975 (Bertrand 1977). 



In the Damariscotta River estuary, nutrient (nitrates) levels are related 

 inversely to cell densities of phytoplankton. In this estuary, the spring and 

 fall peaks in productivity are similar to those in coastal waters. 

 Apparently, nutrients are not available continuously at high enough levels to 

 sustain phytoplankton productivity throughout the summer. 



Much of the supply of nutrients in the Sheepscot River estuary is thought to 

 be due to the inflow of nutrient-rich sea water (Garside et al. 1978; see 

 "Plant Nutrients" above). Phytoplankton in this estuary are not nutrient 

 limited and populations may be limited by heavy grazing (Garside et al. 1978). 



Many nearshore sediments are rich in phosphorus and nitrogen compounds. These 

 nutrients show concentrations 10 to 100 times higher in the interstitial 

 spaces in sediments than in the water column (Yentsch, unpublished ) . Periodic 

 mixing of the sediments (i.e., resuspension) may be an important mechanism for 

 adding nutrients to the water column. As an example, tidal currents that mix 

 the water column bring nutrient-rich water into the euphotic zone and prevent 

 thermal stratification; however, Petrie (1975) believes that nutrients in the 

 Damariscotta estuary probably are mixed to the surface. This mixing also 

 entrains some phytoplankton down below the photic zone, reducing productivity. 



Tidal currents also probably help to retain the sediment load in the water 

 column. Increased turbidity due to tidal currents, which are particularly 

 prevalent in shallow bays, may have an inhibitory effect on phytoplankton 



5-79 



10-80 



