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 that release is sufficient to sustain a high total standing 
crop of plankton, that standing crop will 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 has *^*c productivity 
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, will 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 will 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). 
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