SPATIAL AND TEMPORAL VARIATION IN C N/P RATIOS, DELTA-N-15 AND DELTA-C-13 OF EELGRASS ZOSTERA-MARINA AS INDICATORS OF ECOSYSTEM PROCESSES, TOMALES BAY, CALIFORNIA, USA/

The nutrient content, delta(15)N, and delta(13)C of seagrasses in Toma les Bay (California, USA) vary in both space and time. The variations in these measurements were not random, but followed spatial and season al patterns that provide insight into the sources and processing of nu trients in the Tomales Bay ecosystem. Wide ranges of carbon, nitrogen, and phosphorus content of green leaves of Zostera marina were found i n 72 samples collected over the 2 yr of sampling. Carbon content varie d from 29.0 to 40.9% of dry weight, nitrogen from 1.13 to 3.79%, and p hosphorus from 0.11 to 0.90%. Stable isotopic composition was variable as well: the mean delta(15)N for all samples collected was 9.7 +/- 0. 3 parts per thousand with a range of 6.2 parts per thousand. Carbon is otopic content had a range of 7.5 parts per thousand, with a mean delt a(13)C Of -9.6 +/- 0.2 parts per thousand. There was a strong spatial trend in the N content, but not the P content, of Z. marina leaves fro m Tomales Bay. The C:N ratio was around 15 near the mouth of the bay a nd increased Linearly with distance into the bay to similar to 25 near the head of the bay; there was a concomitant pattern of increasing de lta(15)N Of seagrass leaves, from 7 parts per thousand near the mouth of the bay to near 12 parts per thousand at the head of the bay. The s patial pattern in N content was only present during summer months, but the spatial pattern in stable isotopic composition was present in bot h summer and winter. The patterns in the N and delta(15)N content of t he seagrasses indicate the importance of denitrification in the C and N cycles in Tomales Bay. The analysis of patterns in variation of elem ental content and isotopic composition of seagrasses is a powerful too l for investigating ecosystem-scale processes in coastal marine system s. Similar analyses of seagrasses from less well studied ecosystems sh ould provide data to generate hypotheses about spatial and temporal va riation in processes in these other systems.