Donald G. Redalje

Nutrients, irradiance, and mixing as factors regulating primary production in coastal waters impacted by the Mississippi River plume

Relationships among primary production, chlorophyll, nutrients, irradiance and mixing processes were examined along the salinity gradient in the Mississippi River outflow region. A series of six cruises were conducted during 1988–1992 at various times of year and stages of river discharge. Maximum values of biomass and primary production were typically observed at intermediate salinities and coincided with non-conservative decreases in nutrients along the salinity gradient. Highest values of productivity (>10 gC m−2 d−1) and biomass (>30 mg chlorophyll a m−3) were observed in April 1988, July–August 1990 and April–May 1992; values were lower in March and September 1991. Rates of primary production were apparently constrained by low irradiance and mixing in the more turbid, low salinity regions of the plume, and by nutrient limitation outside the plume. Highest values of primary production occurred at stations where surface nutrient concentrations exhibited large deviations from conservative mixing relationships, indicating that depletion of nutrients was related to phytoplankton uptake. Mixing and advection were important in determining the location and magnitude of primary production maxima and nutrient depletion. In addition to growth within plume surface waters, enhanced growth and/or retention of biomass may have occurred in longer residence time waters at the plume edge and/or beneath the surface plume. Vertical structure of some plume stations revealed the presence of subsurface biomass maxima in intermediate salinity water that was depleted in nutrients presumably by uptake processes. Exchange between subsurface water and the surface plume apparently contributed to the reduction in nutrients at intermediate salinities in the surface layer. DIN (=nitrate+nitrite+ammonium) : PO4 (=phosphate) ratios in river water varied seasonally, with high values in winter and spring and low values in late summer and fall. Periods of high DIN : PO4 ratios in river nutrients coincided with cruises when surface nutrient concentrations and their ratios indicated a high probability for P limitation. N limitation was more likely to occur at high salinities and during late summer and fall. Evidence for Si limitation was also found, particularly in spring.

The Relationship Between Primary Production and the Vertical Export of Particulate Organic Matter in a River Impacted Coastal Ecosystem

As part of the National Oceanic and Atmospheric Administration’s (NOAA) Nutrient Enhanced Coastal Ocean Productivity program, we have conducted four research cruises, July–August 1990, March 1991, September 1991, and May 1992, in the Mississippi River plume and adjacent shelf regions. Over this time period, photic-zone-integrated primary production varied significantly in both the river plume and shelf study regions, with greatest variability observed in the river plume region. In the river plume and the adjacent shelf, highest production occurred during July–August 1990 (8.17 g C m−2 d−1 for the plume and 1.89–3.02 g C m−2 d−1 for the shelf) and the lowest during March 1991 (0.40–0.69 g C m−2 d−1 for the plume and 0.12–0.45 g C m−2 d−1 for the shelf). The vertical export of POC from the euphotic zone, determined with free-floating MULTITRAP sediment trap systems, also varied temporally in both study regions, with highest values occurring in May 1992 (1.80±0.04 g C m−2 d−1 for the plume and 0.40±0.02 g C m−2 d−1 for the shelf) and the lowest values occurring during July–August 1990 (0.29±0.02 g C m−2 d−1 for the plume and 0.18±0.01 g C m−2 d−1 for the shelf). The fraction of production exported out of the photic zone was highly variable and was dependent, in part, on phytoplankton species composition and on the grazing activities of microzooplankton and mesozooplankton. The lowest ratio of export to production coincided with the time when production was greated and the highest ratios occurred when production was the lowest.

New production at the VERTEX time-series site

Particulat organic carbon and nitrogen fluxes measured with free-floating seediment traps deployed six times over an 18-month period were combined with 14C primary production and 15N uptake measurements in order to obtain annual estimates of new production (NP) and associted f-ratios at the VERTEX time-series site. The site, located in the northeast Pacific Ocean at 33°N, 139°W, was occuped at trimonthly intervals to conduct water column studies and to recover/redeploy the sediment traps. The upper 250m of the VERTEX site exhibited considerable variability in some biological properties over seasonal time scales. While integrated photoautotrophic biomass remained relatively constant (0.57 ± 0.1 g C m−2) during the 18-month period, both integrated primary production and particulate ATP varied approximately 2.5-fold, ranging from 220 to 550 mg C m−2d−1 and 0.6–1.5 g C m−2, respectively. There was also considerable variation in both NP and f-ratios over the 18-month sampling period, although most of the NP variability was associated with short-term (i.e <72 h) estimates. Despite the relatively large range in NP values derived from 72 h sediment trap deployments and the 15N tracer work (18–179 mg C m−2d−1), no clear relationship between NP and primary productivity was found. However, the f-ratio appeared to be inversely related to primary production, with lowest estimates obtained during the period of highest productivity. Values of annual NP derived from various estimates were remarkably similar ranging from 13–17 g C m−2y−1. The average annual f-ratio ranged from 0.11 to 0.14.

Spatial and Temporal Variations of Photosynthetic Parameters in Relation to Environmental Conditions in Coastal Waters of the Northern Gulf of Mexico

On a series of eight cruises conducted in the northern Gulf of Mexico, efforts were made to characterize temporal and spatial variability in parameters of the photosynthesis-irradiance saturation curve (PmasB, αB, Ik) and to relate the observed variations to environmental conditions. Experiments to examine the importance of diel variation in upper mixed layer populations were conducted in July–August 1990 and March 1991. During July–August 1990, PmaxB and Ik showed significant increases and αB decreased during the photoperiod in both river plume and shelf-slope populations. During March 1991, no consistent covariance of P-I parameters with local time was found, although highest values of αB in the river plume were observed in early morning. Seasonal variation in PmaxB and αB were correlated with temperature. Spatial variations of photosynthetic parameters in the upper mixed layer ranged from twofold to threefold within any given cruise. Variations of photosynthetic parameters in the upper mixed layer were related to principal components derived from environmental variables, including temperature, salinity, nutrients, mixed layer depth, attenuation coefficient, and daily photosynthetically available radiation (PAR). Greater than 70% of the variation in the environmental variables could be accounted for by two principal components; the majority of this variation was associated with the first principal component, which was generally strongly correlated with salinity, nutrients, mixed layer depth, and attenuation coefficient. Correlations of PmaxB, αB, and Ik with the first principal component were found to be significant in some cases, an indication that spatial variability in P-I parameters was related to river outflow. Variation of P-I parameters in relation to depth and PAR were evaluated by regressions with principal components derived from depth, temperature, and mean daily PAR. For most cruises, PmaxB and Ik were negatively correlated with the first principal component, which was strongly positively correlated with depth and negatively correlated with daily PAR. This was consistent with a decrease in both PmaxB and Ik with depth that could be related to decreasing daily PAR. Positive correlations of αB with the first principal component for two cruises, March 1991 and April 1992, indicated an increasing trend with depth. In conclusion, relationships between P-I parameters and environmental variables in the region of study were significant in some cases, but variation between cruises made it difficult to generalize. We attributed this variation to the physically dynamic characteristics of the region and the possible effects of variables that were not included in the analysis such as species composition. Our findings do support the view that a limited set of observations may be adequate to characterize P-I parameter distributions in a given region within a restricted period of time.

Nitrogen dynamics at the VERTEX time-series site

Abstract Euphotic zone concentrations and fluxes of nitrate, ammonium, particulate and dissolved organic nitrogen were measured over an 18 month period at the VERTEX time-series site in the oligotrophic northeast Pacific (33°N, 139°W). Variations in all N-forms were significant but not clearly linked to the temporal hydrographic cycle. Inorganic-N uptake (nitrate + ammonium) from 15 N tracer experiments generally paralleled primary productivity variations, peaking in summer; ammonium accounted for most of the uptake (∼90%) and temporal variability. Comparisons of 15 N results with estimates of autotropic N-uptake from 14 C incorporation into protein suggest that as much as 40% of the annual inorganic-N uptake was due to microheterotrophs; peak heterotrophic N-uptake occurred in summer when heterotrophic biomass was at its maximum. Nitrate uptake (new production) was less variable than ammonium uptake and annually equivalent to particulate nitrogen export from sediment traps. Dissolved organic-N (DON) represented the largest and most variable N-pool, accounting for 80–90% of the total nitrogen in the euphotic zone. Vertical DON gradients, however, were small and temporally invariant, implying little contribution to the biogenic nitrogen export from the euphotic zone.

VARIABILITY IN RATIOS OF PHYTOPLANKTON CARBON AND RNA TO ATP AND CHLOROPHYLL A IN BATCH AND CONTINUOUS CULTURES1,2

The feasibility of estimating phytoplankton carbon and RNA concentrations from measurements of ATP and chlorophyll a (chl a) concentrations was studied using chemostat populations of the marine diatom Thalassiosira weissflogii (Grunow) Fryxell & Hasle (= T. fluviatilis Hustedt). C:ATP and RNA:ATP ratios were studied for six additional marine species in batch culture representing five classes of phytoplankton. Statistical analyses revealed that both the growth rate and the factor limiting growth (NO3‐, NH4+, PO43‐ or light) could alter C:ATP, RNA: ATP, C:chl a and RNA:chl a ratios by amounts which were large compared to measurement error. An analysis of variance of the batch culture results indicated that both species and the source of inorganic nitrogen (NO3‐, or NH4+) had a significant effect on C:ATP and RNA:ATP ratios. Light had less of an influence on C:ATP and RNA:ATP ratios than on C:chl a and RNA:chl a ratios, and for this reason we feel that phytoplankton C and RNA concentrations can be estimated with greater reliability from ATP than from chl a measurements. The range of C:ATP and RNA:ATP values found for T. weissflogii under a variety of growth conditions was similar to that for the six additional species grown in batch culture, suggesting that this range of values is indicative of the extremes likely to occur in living cells. Our results and additional data in the literature indicate that phytoplankton C and RNA concentrations can be estimated to within a factor of two by multiplying ATP concentrations by 311 and 35, respectively, in N limited systems, and by 341 and 36, respectively in PO43‐ limited systems.

A theoretical and experimental examination of the predictions of two recent models of phytoplankton growth

Abstract An analysis of the theoretical models of phytoplankton growth developed independently by Bannister (1979) and Shuter (1979) shows that the models are mathematically identical in their predictions regarding steady-state light limited growth rates, respiration rates and chlorophyll: carbon ratios if one takes n = 1 and m = ∞ in the Bannister model. The chief advantages of the Bannister model are its ability to predict short-term photosynthesis versus light curves, and its adaptability to L : D cycle growth conditions. The latter capability of the model is greatly facilitated if n = 1. The principal advantages of the Shuter model are its ability to describe nutrient-limited steady-state nitrogen: carbon and phosphorus: carbon ratios, and its ability to predict the interactive effects of temperature, light and nutrient limitation on phytoplankton composition. A comparison of the models with nutrient and light limited growth rate and composition data for the marine diatom Thalassiosira weissflogii shows good agreement in all cases except for RNA: C and ATP: C data. The latter two composition ratios do not appear amenable to description by the model. An examination of the Shuter model shows that it predicts a hyperbolic relationship between nitrogen :carbon (or phosphorus: carbon) ratios and nutrient-limited growth rate of exactly the form proposed on empirical grounds by Caperon & Meyer (1972), and that the hyperbolic function reduces to the form earlier proposed by Droop (1968) in the limit of high light intensity. The predicted light and temperature dependences of the parameters in the hyperbola are shown to be in qualitative agreement with recent experimental data.

Sewage diversion effects on the water column of a subtropical estuary

Abstract A study of the phytoplankton community and water column chemistry in Kaneohe Bay, Oahu, Hawaii, before and after the diversion of secondary treated sewage from the bay has shown that changes in total nutrient concentrations in the water column cannot be accurately predicted without taking into account water column-benthos interactions. During the first year after sewage diversion, the decomposition of about 400 tonnes of benthic organisms, primarily filter feeders, resulted in water column dissolved organic nitrogen and phosphorus concentrations roughly an order of magnitude higher than those expected in the absence of such interactions. The biomass of phytoplankton appears to have been N limited both before and after sewage diversion, with internal nutrient recycling accounting for 70–99% of phtyoplankton nutrient uptake. Both the biomass and growth rate of the phytoplankton declined as a result of the sewage diversion, with post-diversion growth rates evidently well below nutrient-saturated values. Since the principal stresses on the bays coral reef community as a result of the sewage discharges appear to have resulted from the elevated concentrations of plankton in the water, various measures of seston concentration appear to be the most ecologically significant indicators of nutrient enrichment in this system.

Primary production in the Gulf of Mexico coastal waters using “remotely-sensed” trophic category approach

Abstract Attempts to derive ocean-color based estimates of pigment and primary production in coastal waters have been complicated by the contributions of signals from non-pigment materials to the water leaving radiance. An ocean-color model to estimate primary production was evaluated for coastal waters of the northern Gulf of Mexico. The model utilizes C sat , (mg m −3 ) (a variable that accounts for the pigment sensed by the satellite sensor), photosynthetically available radiation (PAR, J m −2 day −1 ) and a parameter.ψ * m2 (g Chl) −1 , the water column chlorophyll specific cross-section for photosynthesis. C sat and PAR were treated as variables whileψ * was a site-specific parameter in the model. The model uses the approach outlined in Morel and Berthon (1989) Limnology and Oceanography , 34 , 1545–1562, but with site-specific statistical relationships to estimate the integrated pigment in the water column from C sat and site-specific trophic categories (oligotrophic to eutrophic) based on pigment concentration in the water column. The statistical relationships perform extremely well within the ranges of C sat and integral chlorophyll normally encountered in the coastal waters of the northern Gulf of Mexico.ψ * varies between 0.054 and 0.063 m 2 (g Chl) −1 and are comparable to values observed in other regions. The ability of the model to predict production usingψ * within each of the trophic categories was demonstrated. The overall performance of the model has been encouraging for two reasons: (a) the possibility of estimating production from future ocean-color sensors, and (b) the fact that the model performs well in a dynamic coastal area.

Phytoplankton Dynamics and the Vertical Flux of Organic Carbon in the Mississippi River Plume and Inner Gulf of Mexico Shelf Region

In this ongoing study, we are examining the degree to which elevated levels of nutrients carried to the coastal Gulf of Mexico waters in the Mississippi River Plume affect carbon cycling, phytoplankton growth and production, and photosynthesis-irradiance (P-I) properties. The river outflow enters the coastal Gulf of Mexico through several channels in the delta and gives rise to strong environmental gradients in salinity, nutrients, and light in the study region.