Biological And Hydrodynamic Regulation Of The Microbial Food-Web In A Periodically Mixed Estuary
We examined the natural variation of nitrification potentials (NPs) of surface sediments and macrofaunal tubes and burrow walls in relation to sediment NH4+ level, season, and macrofaunal species. NP (the ability of a unit of sediment to oxidize NH4+ when NH4+ and O-2 are not limiting) is an index of the abundance and activity of nitrifying bacteria which we measured in slurries with the chlorate block technique (nmol NO2--N produced g(-1) dry weight sediment h(-1)). The NP of the tubes of the polychaete Loimia medusa was positively related to sediment NH4+ (KCl-extractable) concentration at 3 sites where tubes were collected in June 1990 (Spearman rank correlation coefficient I-s = 0.90, p = 0.03), as was the NP of surface (0 to 1 cm) sediment (I-2 = 0.92, p = 0.002). The degree to which tube NP exceeded the NP of surface sediment was, however, negatively associated with sediment NH4+ (I-s = -0.84, p = 0.05). Tube NP of L. medusa did not vary significantly with date (February, April, and June 1990). Tubes or burrow walls of Macoma balthica (bivalve), Leptocheirus plumulosus (amphipod), and the polychaetes Macroclymene zonalis, Pectinaria gouldii, L. medusa, and Diopatra cuprea had NPs significantly greater (2 to 20 times) than that of adjacent sediment from the same depth interval, indicating that these species stimulated nitrification. Except for burrows of M. balthica, the NPs of these structures were significantly (p less than or equal to 0.05) greater (1.5 to 61 times) than that of surface sediment. The duration of macrofaunal irrigation activity, but not irrigation rate, was positively associated (I-s = 0.72, p = 0.01) with the enhancement of NP in tubes and burrow walls relative to surface sediment. These findings indicate that macrofaunal tubes and burrows tend to be sites of enhanced NP and that this enhancement varies among species due to Variations in irrigation behavior. The NP of macrofaunal structures also varies among sites in relation to sediment NH4+ concentrations.