Appendix E. Estimates of algal biomass and relative productivity based on a two-source C:N mixing model.
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| FIG. E1. Estimates of algal biomass and eventually relative productivity were made with a two-source mixing model using C:N end-members for sediment and algae. |
This Appendix considers effects of varying the end-member C:N ratios assumed to be 6.6 for algae and 15 for sediment.
Algae. We checked the assumption of a 6.6 C:N ratio for algae by analyzing offshore POM samples which should represent a fairly pure local algal community. Samples collected between December 2001 and July 2002 along offshore transects (Fig. 1) had an average C:N ratio of 7.0 (±0.9 SD, n = 68; Wissel, unpublished data). This value was very close to our assumed value of 6.6, and using 7 instead of 6.6 did not significantly affect calculated source contributions from algae and sediments.
Sediments. Here we consider extreme values measured in this study for POM as possibly representative of sediment. The highest C:N ratios for POM observed during the study period generally occurred during strong wind events, which likely resuspended large amounts of local sediment into the water column. The average C:N ratios for POM in Barataria Bay (BB) and Breton Sound (BS) were 14.5 and 13.5, respectively, slightly below the assumed sediment value of 15. Using these 14.5 and 13.5 values, average algal contributions to POM in BB and BS would be 63 and 68%, down from 65 and 75%, respectively. Beyond this average decrease in phytoplankton contribution, the effects of changing end-members seemed rather random (Fig. 5), so that overall results fell near a 1:1 line. (In Fig. 5, BB stations are shown as black triangles and BS samples are represented by gray circles; the graph shows a comparison of the calculated algal fraction (%) assuming a constant sediment C:N ratio of 15 (Fig. 5, x-axis) vs. a sediment C:N ratio that is equivalent to the highest observed C:N ratio for POM for each station throughout the sampling period (Fig. 5, y-axis)). Overall patterns of algal contributions were similar down-estuary (not shown) regardless of choice of sediment end-member C:N value in the 13.5 to 15 range, so that using a sediment C:N value of 15 provided a reasonable estimate of algal contributions. Higher C:N values of 20–25 are also reported for estuarine sediments (Matson and Brinson 1990), and using those values rather than 15 would give higher contributions for algal biomass.
LITERATURE CITED
Matson, E. A., and M. M. Brinson. 1990. Stable carbon isotopes and the C:N ratio in estuaries of Pamlico and Neuse Rivers, North Carolina. Limnology and Oceanography 35:1290–1300.