work was initiated in a year of unusual 

 hydrological conditions (no tidal flushing for 

 most of the year, no rainfall in winter of 

 1984), because field breakdown rates of 

 Milorganite are unknown, and because 

 fertilizers were applied in water solution, it 

 is not clear how broadly the findings can be 

 generalized. Finally, if some plant species 

 become more palatable upon urea fertilization 

 than others, what mechanism is involved? Is 

 it a difference in cordgrass vs. pickleweed 

 herbivores, or a difference in the grass vs. 

 succulent plant chemistry? Clearly, there 

 are multiple routes of research to be pursued 

 in understanding how nitrogen and other 

 nutrients influence salt marsh functioning. 



More recently, Johnson (1991) fertilized 

 plots of cordgrass with urea in two San Diego 

 Bay Marshes, Paradise Creek Marsh and the 

 Chula Vista Wildlife Reserve (a man-made 

 dredge spoil island). She followed foliar 

 nitrogen levels and insect populations in an 

 attempt to determine if herbivory increases 

 with soil nitrogen amendments. The results 

 were complicated, but they do help explain 

 variations in field observations. In the man- 

 made marsh, predatory arthropods were 

 absent, and herbivorous insects responded to 

 fertilization and increased foliar nitrogen 

 levels. In the natural marsh, there were 

 more predators, such as beetles and spiders. 

 Adding nitrogen must have stimulated 

 herbivore populations (e.g., Prokelisia 

 dolus), but the higher densities could not be 

 measured. Instead, the effect was apparent as 

 an increase in predatory beetle (Coleomegilla 

 fucilabris) populations. Johnson's work 

 documents the variability and complexity of 

 insect responses to nitrogen additions. 

 Whether or not nutrients increase herbivore 

 damage depends in part on how rapidly the 

 predators locate and crop the herbivores. 



4.5 ENERGY FLOW 



For many years, salt marshes have been 

 viewed as food producers that subsidize coastal 

 bays and nearshore waters (Odum 1971). 

 Haines (1979) and Nixon (1980) challenged 

 that dogma for Atlantic Coast marshes, as did 

 Winfield (1980) for Tijuana Estuary. As a 



result, investigators have concluded that 

 coastal marshes display a high degree of 

 individuality. Their ability to fix carbon at 

 remarkable rates remains unchallenged, but 

 the ecological fate of that carbon is highly 

 variable. Systems with large river flows are 

 likely to transport large fractions of their net 

 primary production during spring runoff; 

 systems with broad tidal amplitude may be 

 highly susceptible to exporting organic matter 

 year round; marshes experiencing rapid sea- 

 level rise may accumulate plant matter in the 

 sediments; small semi-enclosed wetlands may 

 use the energy of photosynthesis and recycle 

 large portions of their fixed carbon. The high 

 productivity, then, is either exported 

 (usually as detritus), accumulated (as peat) 

 or released in respiration (energy lost as 

 heat; carbon recycled as carbon dioxide). 



Studies at Tijuana Estuary add another 

 complication--that of tremendous temporal 

 variability in the processes that determine 

 the fate of organic carbon. This section 

 reviews research that has evaluated detrital 

 production (Winfield 1980), feeding and 

 growth rates of estuarine animals (Williams 

 1981; Boland 1981), carbon fluxes 

 (Winfield 1980), and variations in the 

 system's ability to "filter" materials from 

 incoming waters (Zedler and Onuf 1984). 



4.5.1 Detrital Production 



As live plant material dies and is 

 transformed into small particles, detritus is 

 "produced." It is much more than a 

 mechanical process, because fungal and 

 bacterial decomposers are integrally involved 

 in the transformation. As they help to break 

 down the fixed carbon, they simultaneously 

 enrich the particles with organic nitrogen 

 gleaned from tidal waters. Two lines of 

 evidence indicate that detritus production is 

 far from constant at Tijuana Estuary. The 

 first is Winfield's (1980) work on litter 

 dynamics (dead organic matter beneath the 

 marsh canopy); the second is his direct 

 measurements using litter bags. 



The seasonal changes in litter (Figure 

 4.7) were highly variable. Litter accumu- 



92 



