ECOLOGY OF BUZZARDS BAY: An Estuanne Profile 1 07 



As might be expected from the outfall data, ni- 

 trogen from on-site septic treatment of wastewater 

 is the major nonpoint source with a total contribu- 

 tion of 320 t/year. Nitrogen inputs from septic sys- 

 tems have been quantified within the Buzzards Bay 

 watershed (Weiskel and Howes 1991 ) to calibrate 

 loading models. Septic disposal treats the waste- 

 water from almost half of the population, and with 

 residential fertilizer usage (68 t/year) accounts for 

 almost 80% of the nonpoint source nitrogen inputs 

 originating within the watershed (i.e., disregarding 

 input from precipitation). Both of these sources reach 

 bay waters primarily by groundwater transport. Re- 

 gardless of the original form of the nitrogen, the form 

 of almost all nitrogen in groundwater is nitrate. For 

 example, although both organic and inorganic ni- 

 trogen enter septic systems, as a result of degrada- 

 tion and anaerobic conditions within tanks almost 

 all of the nitrogen released is as ammonium. Even at 

 the very high resulting concentrations (millimolar), 

 the ammonium is rapidly oxidized to nitrate by bac- 

 teria (nitrification) generally after a few meters of 

 infiltration. Once the nitrate reaches the groundwa- 

 ter it is transported nearly conservatively (i.e., con- 

 centration changed only by dilution) to the bay shores 

 (Weiskel and Howes 1 992). Even where large treat- 

 ment facility groundwater plumes occur the amount 

 of removal from the groundwater is quantitatively 

 relatively small compared to the loading (Smith et 

 al. 1991). 



Of the residential sources, septic system and 

 fertilizers, the role of lawn fertilizers is more difficult 

 to quantify because they are applied at low con- 

 centrations over wide areas. Estimates of lawn fer- 

 tilizer application within the watershed are thus vari- 

 able and subjective. Using data based on the num- 

 ber of dwellings per lot size (<0. 1 -0.2, and > 0.2 ha, 

 with 279, 465. and 1 ,394 nr of lawn, respectively) 

 a general application rate of 0.45 kg of nitrogen per 

 93 nr per year, and a 30% transport to groundwa- 

 ter, the estimated input of nitrogen is 68 t/year (SAIC 

 1 991 ). An understanding of the role of lawn fertil- 

 izers is important for management, as they are a 

 moderate-sized source but present an inexpensive 

 trade-off for controlling nitrogen inputs when 



compared to removing nitrogen loading from septic- 

 systems or agricultural sources. 



Agricultural inputs from cranberry bogs, dairy 

 farms, and cattle, and miscellaneous crops account 

 for the remaining land-based inputs ( 1 04 t/year). 

 Cranberry growing accounts for relatively little ni- 

 trogen, about 33 t/year (Howes and Teal 1992), 

 about the same as dairy farms and cattle or terres- 

 trial croplands (Buzzards Bay Project 1 990; Terkla 

 et al. 1 990). While they may be locally important 

 sources of nutrients to the associated embayments, 

 agricultural inputs do not represent a major source 

 to the bay proper and are probably even smaller 

 than stated since the inputs from dairy and cattle 

 fanning are based on the assumption of significant 

 runoff in these penneable soils. The conclusion that 

 low nitrogen loading results from agricultural prac- 

 tices is often hotly contested at the citizen and regu- 

 latory levels. The debate frequently arises from in- 

 tuitive awareness that fanning uses fertilizers and 

 from omitting alternative uses of the land from the 

 nitrogen loading equation. For example, while the 

 total input from agriculture is from an area of the 

 watershed about half the size of that covered by 

 residential lots, its contribution of nitrogen is only 

 about a quarter as much. The comparison with ur- 

 ban areas with sewage outfalls yields even greater 

 contrasts. The sewered area of New Bedford rep- 

 resents much less than half of the total agricultural 

 area of the watershed yet discharges 44% of the 

 total nitrogen load to bay waters compared to less 

 than 5% for agriculture (Terkla et al. 1 990). 



A frequently overlooked source of nitrogen to 

 coastal waters is atmospheric deposition, either di- 

 rectly on the bay or via groundwater recharge. At- 

 mospheric deposition takes two forms: dry (par- 

 ticle settling) and wet (dissolved in rainwater). The 

 nitrogen in atmospheric deposition is about equally 

 divided between organic and inorganic forms. In a 

 study of nitrogen inputs to a Buzzards Bay salt 

 marsh. Valiela and Teal ( 1 979) found a total annual 

 nitrogen deposition of almost 0.79 g/year at an av- 

 erage rainfall of 1 05 cm/year on each square meter 

 of the watershed. Dissolved inorganic nitrogen 

 deposition has been the topic of several regional 



