Taxon-specific growth and loss rates for dominant phytoplankton populations from the northern Gulf of Mexico

Abstract

Taxon-specific growth and sedimentation rates of dommant phytoplankton were measured during 2 cruses (summer 1990 and spring 1991) in the northern Gulf of Mexico as part of the NOAA Nutrient-Enhanced Coastal Ocean Productivity (NECOP) program. Microzooplankton grazing rates also were measured during the summer cruise. During each of the cruises, a series of stations from the Mississippi River mouth to the hypoxia region (located ca 50 to 100 km west) were sampled to examme variability of growth and loss processes along a strong environmental gradient. Significant taxa-and group-specific differences were noted for both growth and loss rates Growth rates ranged from <0.1 to 3.0 d-I with highest rates in the plume region during the summer cruise, where surface rates were close to or exceeded previous urnax values for several taxa. For all taxa, growth rates were lower in the hypoxia region (mean = 0.5 d l ) than in the plume region (mean = 1.1 d l ) ; soluble nitrogen concentrations explained over 50 % of the variability in growth rates. Diatom growth rates were similar to non-diatoms in the plume region, but were significantly lower in the hypoxia region, which suggests that silica limitation may exist in this region. The fate of phytoplankton appeared to be controlled by size and by the degree of silicification. Significant mcrozooplankton grazing loss rates were noted only for small taxa (<20 pm). For microflagellates, microzooplankton grazing rates averaged 82 % (range 42 to 214 %) of the growth rate; sedimentation rates were always < I % of the growth rate. Sedimentation was an important loss for several diatoms, with significant taxon-specific and seasonal differences noted. Large colonial diatoms, such as Skeletonema costatum and Thalassiosira rotula, exhibited the highest sedimentation rates in the plume region dunng the spring cruise(0.2 to 1.0 d l ) , whereas the lowest rates (<0.01 d l ) were noted for Rhizosolenia fragilissirna and Ceratulinapelagica in the hypoxia region dunng the summer cruise. Our results suggest that in the northern Gulf of Mexico, phytoplankton rate processes proceed very rapidly, with growth rates primarily controlled by the supply of nitrogen via the Mississippi River and the fate controlled primarily by size and density (silicification).

Interactions between nutrients, phytoplankton growth, and micro- and mesozooplankton grazing in the plume of the Mississippi River

“…Except at the far-field station, phytoplankton growth rates were all higher than 1 doubling per day, with the growth rate of the > 20 µm phytoplankton higher than 2 doublings per day at some stations. These rates are within the ranges of previous reports (Fahnenstiel et al 1995, Strom & Strom 1996 which also showed nutrient limitation of phytoplankton growth in the far-field or offshore sites, with diatoms the most severely affected.…”

“…Under far-field conditions slightly farther to the west, phytoplankton growth rates were nutrient-limited (Strom & Strom 1996) and the microzooplankton grazing to phytoplankton growth ratio was typically between 0.3 and 0.9. In contrast to Fahnenstiel et al (1995), Strom & Strom (1996) observed significant consumption of large diatoms by microzooplankton (heterotrophic dinoflagellates) at some stations. It appears that a large fraction of the river nitrate incorporated into nanophytoplankton, and a smaller fraction of nitrate incorporated into larger phytoplankton, are grazed by microzooplankton.…”

“…Within the mid-salinity region of the plume, Fahnenstiel et al (1995) reported that microzooplankton grazing rates on cells < 20 µm averaged 82% of algal growth rates in the summer, although no grazing was observed on cells > 20 µm. Under far-field conditions slightly farther to the west, phytoplankton growth rates were nutrient-limited (Strom & Strom 1996) and the microzooplankton grazing to phytoplankton growth ratio was typically between 0.3 and 0.9.…”

A coupled physical-biological model of the Northern Gulf of Mexico shelf: model description, validation and analysis of phytoplankton variability

“…These observed growth rates varied considerably between taxa with lowest values of <0.1 d −1 and maximum rates of 3 d −1 . Since our model considers only one phytoplankton group, which represents the whole phytoplankton community, we compare the simulated rates with the observed mean and median growth rates of Fahnenstiel et al (1995). In March 1991 the observed mean and median rates were 0.5 and 0.4 d −1 , respectively, and agree …”

Phytoplankton community structure and productivity along the axis of the Mississippi River plume in oligotrophic Gulf of Mexico waters

“…The Mississippi River is the 7th largest freshwater input into the world's oceans (Milliman & Meade 1983), and strongly affects the biological productivity of the northern Gulf of Mexico shelf (Walsh 1988, Fahnenstiel et al 1995, Rabalais et al 2002. The region surrounding the Mississippi Delta is in fact so productive that it accounts for 70 to 80% of Gulf of Mexico (GOM) fishery landings (Lohrenz et al 1999, Grimes 2001, the US portion, of which alone was valued at ca.…”