Table 3.64. Permissible, and excessive loading rates for phosphorus as an index of eutrophication. 



Reference 



Rate 



Permissible 



Excessive 



Shannon and Brezonik 

 (1971) 



Shannon and Brezonik 



(1971) 



VoUenweider (1968) 

 for lakes < 5m 



Craig and Day (1979) 



Volumetric 



(g/m'/yr) 



Areal 

 (g/m^/yr) 



Areal 



(g/m^/yr) 



.^eral 



(g/m^/yr) 



0.12 



0.28 



0.07 



0.4 



0.22 



0.49 



0.13 



0.40 



These levels are useful indices for many different kinds 

 of water bodies. The stage of eutrophication in an en- 

 tire water body is influenced, however, by the flush- 

 ing or replacement rate of the water body, retention 

 of nutrients in bottom sediments, the previous his- 

 tory of eutrophication, the water depth, and the total 

 water volume. These factors should be considered in 

 site-specific analyses. 



Output and storage (retention). A portion of the 

 nutrients discharged into a lake is later discharged 

 from the lake in the stream outflow. Inorganic P can 

 be transformed to organic forms within minutes and 

 subsequent chemical changes depend on cycling rates 

 of the biota and on sedimentation rates. Eventually 

 some of the P entering the water body leaves down- 

 stream, although it may not be in the same form. The 

 losses of P increase as the areal water load increases. 

 The areal water load of a body of water (m/yr) is de- 

 fined as the ratio of the outflow volume[(m /yr to its 

 surface area (m^)]as shown by the empirical relation- 

 ship developed by Kirchner and Dillion (1975) in fig- 

 ure 3-31. 



Retention and outflow are influenced by the pre- 

 vious history of the water body (Craig et al. 1979). In 

 fresh waters with a previous history of low P loading 

 rates, the sediment acts as a sink and traps P efficiently . 

 But if excess nutrients are introduced, the sediments 

 gradually become saturated with P and are able to 

 store new P only at slow rates related to the net rate 

 of sedimentation. Estuarine sediments naturally trap 

 P (Pomeroy 1970). In shallow water areas such as 

 those in the Chenier Plain, where wind, dredging 

 activities, or other factors stir up these enriched sedi- 

 ments, P is released into the water column. Thus, the 

 concentration in the water is buffered by the under- 

 lying sediments. Tidal flux is an additional factor that 

 influences the export of P from estuarine waters. In 

 tidal areas, the areal water load, based on freshwater 

 flow througli the lake, underestimates the dilution of 

 a pollutant. This was demonstrated by Ketchum 

 (1969) for the Hudson River estuary. His findings 

 suggest that (1) retention values from Kirchner and 

 Dillion (1975) are probably overestimated in estuaries 

 with significant tidal action and (2) pollutant dis- 

 charge into tidal waters can be expected to influence 

 upstream as well as downstream areas. An example of 

 the latter is the discharge from menhaden plants in 



the lower Calcasieu River that resulted in closure of 

 the oyster beds upstream in Calcasieu Lake. 



P loading rates in Chenier Plain estuaries were de- 

 termined from the total water discharging into a basin 

 and the P concentration in runoff entering each basin 

 from its drainage area. The analysis was perfonned 

 for the entire watershed area of each basin, not just 

 the area within the Chenier Plain boundaries. The P 

 loading rates were determined by multiplying the 

 Shannon and Brezonik (1971) coefficients of P run- 

 off from different land types (urban, industrial, agri- 

 culture, forests, wetlands, etc.), by the area of each 



A'*si Wal^rload q^ m yr 



Figure 3-3 1 . The relationship between the areal water- 

 load (qs) and phosphorus retention (Rp) 

 in fifteen southern Ontario lakes, from 

 Kirchner and Dillion (1975) as shown in 

 Craig etal. (1979). 



type. The total loading rate was them compared with 

 P loading rates from eariier studies to determine the 

 sensitivity of the basin to eutrophication. Details of 

 the methodology are given in appendix 6.4. The values 

 obtained across the Chenier Plain are shown in table 

 3.65. In all cases the heaviest contributor to P runoff 



92 



