PREDICTED BIOLOGICAL RESPONSES TO SEWAGE 

 DIVERSION 



The post-sewage diversion delivery of land-derived nutrients to the bay will 

 decrease to about 10 percent of the present delivery (Table 23-2). The 

 sediment reservoir will temporarily continue to release nutrients, but that 

 reservoir must eventually be depleted as a fraction of the released material is 

 constantly lost to advection. The sediment nutrient release to the water 

 column is diffuse; while tliat release is sufficient to sustain a high total standing 

 crop of plankton, that standing crop v^ll not be as locally concentrated as the 

 crop presently sustained by the point-source sewage input (Figure 23-5, 

 chlorophyll). 



The central and northwest sectors of the bay presently have '■*€ produc- 

 tivity rates of about 5 mg c m'^ hr'', in comparison with about 9 mg C m'^ 

 hr"' in the southeast sector and 24 mg C m '^ hr~' near the sewer outfall. 

 There is relatively little inorganic nutrient export from the southeast sector 

 to the other sectors (Table 23-4), so those sectors are not directly affected by 

 the sewage. They are indirectly affected, because particulate material 

 produced from the sewage nutrients is swept from the southeast sector and 

 is sedimented in the other sectors, where it then releases nutrients back to 

 the water column. Without the sewage point-source "new" nutrient input to 

 the southeast sector, phytoplankton productivity there will stabilize near 

 that of the other sectors. Except in the immediate vicinity of the present 

 sewage plume, actual planktonic biomass decrease associated with the 

 diversion should be small. Compositional shifts of both phytoplankton and 

 zooplankton will undoubtedly occur, but we do not anticipate a significant 

 change in the number of species present. We do anticipate a decrease in the 

 abundance of certain meroplankton (e.g., barnacle larvae). 



The benthos will also respond to sewage diversion, but more slowly than the 

 plankton. The relatively large biomass, longevity, and relative immobility of 

 the benthic organisms provide a substantial nutrient pool which is not as 

 efficiently removed from the system as are suspended and dissolved materials. 

 Some of the filter-feeding benthic animals, particularly those immediately 

 within the sewage plume, vail not survive lowered food availability. Biomass 

 may gradually drop, but plant-animal symbioses and other relatively "tight" 

 pathways of nutrient cycling within the benthos community are mutualistic 

 strategies which v^ll tend to preserve the status quo. Efficient internal cycling 

 of phosphorus has been demonstrated in shallow reef benthos communities 

 elsewhere (6,7). Nitrogen is not as efficiently retained as phosphorus (11); 

 however, experiments we have performed suggest that, given adequate 

 phosphorus reserves, the reef benthos community can rely on nitrogen fixation 

 and nitrification to supplement fixed nitrogen losses (see also 12, 13). 



357 



