Cd. Lambert et al., Cross-shelf changes in phytoplankton community composition in the Gulf of Mexico (Texas shelf/slope): the use of plant pigments as biomarkers, CONT SHELF, 19(1), 1999, pp. 1-21
Plant pigments were used as tracers to document changes in the composition
of phytoplankton assemblages in relation to nutrient distributions across t
he Texas shelf/slope in the northwestern Gulf of Mexico (GOM) on three crui
ses in June 1992, January 1993 and October 1993. Diatoms, prymnesiophytes,
and cyanobacteria dominated the Texas shelf and shelfislope break waters in
January 1993 (ca. 85%), with diatoms and cyanobacteria as the dominant tax
a in October 1993 (ca. 90%). In general, diatoms and cyanobacteria dominate
d the shelf waters while cyanobacteria, pelagophytes, and prymnesiophytes w
ere dominant in slope waters. The highest concentrations of prymnesiophytes
, based on 19' hexanoyloxyfucoxanthin (19-hex) concentrations, were found i
n slope waters in January 1993 and were likely related to the presence of a
warm-core ring at that time.
Water column N:P and Si:N ratios in the dissolved phase for the three cruis
es averaged 11.44 +/- 4.46 and 8.67 +/- 5.61, respectively. Cyanobacterial
abundance was positively correlated with total nitrogen (TN) concentrations
in slope waters on all three cruises, indicating that nitrogen-fixing bact
eria may have contributed to TN concentrations in Texas slope waters. Based
on the range of the ratio of chlorophyll a:diagnostic pigments (F(p)ratios
) (0.09-0.34) the overall trophic status of surface waters across the Texas
shelf/slope was characterized as oligotrophic. Nutrient concentrations in
shelf waters indicated that these waters were not oligotrophic and that the
F-p-derived trophic status was incorrect. One problem with the application
of F-p, ratios in nearshore systems is that new production may be represen
ted by classes other than dinoflagellates and diatoms, such as cyanobacteri
a, which if ignored will underestimate new production resulting in an incor
rect characterization of eutrophication. (C) 1998 Elsevier Science Ltd. All
rights reserved.