Lawrence W. Harding

Retrieval of diffuse attenuation coefficient in the Chesapeake Bay and turbid ocean regions for satellite ocean color applications

are generally applicable for clear open ocean waters. Our results show that for the Chesapeake Bay, Kd(490) data from the existing models are significantly underestimated by a factor of � 2–3 compared with the in situ data. In this paper, new Kd(490) models for the Chesapeake Bay and coastal turbid waters are derived using a relationship relating the backscattering coefficient at the wavelength 490 nm, bb(490), to the irradiance reflectance just beneath the surface at the red wavelengths. For coastal turbid ocean waters, bb(490) can be more accurately correlated to the irradiance reflectance at the red bands. Using the in-situ-derived bb(490) relationship in the Chesapeake Bay, Kd(490) models are formulated using the semianalytical approach. Specifically, two Kd(490) models using the MODIS-derived normalized water-leaving radiances at wavelengths 488 and 667 nm and 488 and 645 nm are proposed and tested over the Chesapeake Bay and other coastal ocean regions. Match-up comparisons between the MODIS-derived and in-situ-measured Kd(490) and Kd(PAR) products in the Chesapeake Bay show that the satellite-derived data using the proposed models are well correlated with the in situ measurements. However, the new models are mostly suitable for turbid waters, whereas existing empirical and semianalytical models provide better results in clear open ocean waters. Therefore, we propose to use a combination of the standard (for clear oceans) and turbid Kd(490) models for more accurate retrieval of Kd(490) (or Kd(PAR)) products for both clear and turbid ocean waters.

An iron‐based ecosystem model of the central equatorial Pacific

The central and eastern equatorial Pacific region is characterized by lower than expected phytoplankton biomass and primary production given the relatively high ambient nitrate concentrations. These unusual conditions have spawned several field programs and laboratory experiments to determine why this high nitrate-low chlorophyll pattern persists in this region. To synthesize the results from these field programs, as well as providing additional evidence in support of the iron hypothesis, we developed a one-dimensional, nine-component ecosystem model of 0°N 140°W. The model components include two phytoplankton size fractions, two zooplankton size fractions, two detrital size fractions, dissolved iron, nitrate, and ammonium. The model was run for 5 years (1990–1994) and was forced using an atmospheric radiative transfer model, an ocean general circulation model (GCM), and in situ data. To our knowledge, this is the first ecosystem model at 0°N 140°W to synthesize the Joint Global Ocean Flux Study Equatorial Pacific Process Study (JGOFS EqPac) data set, as well as to use both in situ and modeled physical data to drive the model. Modeled phytoplankton, zooplankton, and iron all varied on interannual timescales due to El Nino events. Total phytoplankton biomass increased by as much as 40% from early 1992 (El Nino warm) to 1993 (normal). The results also indicate that the biomass increase during a cool period is not constant for each phytoplankton component, but instead the increase is most evident in the netphytoplankton (>10 μm). Netphytoplankton increase from a low of 0.1% of the total chlorophyll in 1992 to a high of 30% of the total in 1993. Microzooplankton grazing rates fluctuated in response to changes in nanophytoplankton growth rates, whereas mesozooplankton grazing was unrelated to netphytoplankton growth rates. The magnitude and temporal variability of phytoplankton chlorophyll agreed well with in situ data collected during 1992. Modeled primary production was lower than measured during El Nino but agreed with observations during normal conditions. The low primary productivity was probably a result of downwelling produced by the physical model. New production was calculated from total and recycled iron rather than nitrate-based production and was more variable in general and almost 3 times the nitrate-based new production during non-El Nino conditions.

Modeling particles and pelagic organisms in Chesapeake Bay: Convergent features control plankton distributions

A two-dimensional Lagrangian particle trajectory model is described and used to study how surface currents transport particles and, by analogy, plankton in Chesapeake Bay, United States. It is shown that persistent patches of high particle concentration develop in well-defined regions along the eastern shore and in the lower reaches of some western shore tributaries due to a combination of passive accumulation of particles in areas where the flushing rate is low and convergence. In the model the highest particle concentrations in the Chesapeake consistently develop in the lower bay (latitude 37.1°–37.7° N) in two specific regions near the shore of Cape Charles where convergence and downwelling occur. It is shown that one of these is associated with a strong and persistent, residual cyclonic eddy located over an abrupt change in bottom topography. Recent bay wide field surveys reveal that various planktonic groups, including phytoplankton, zooplankton, and bay anchovy eggs and larvae, have maximum abundances in the vicinity of this eddy. It is argued that these convergent areas are important features that have a strong influence on plankton distributions and that they provide consistently high food concentrations for higher trophic levels.

AUTOTROPHIC GROWTH AND PHOTOACCLIMATION IN KARLODINIUM MICRUM (DINOPHYCEAE) AND STOREATULA MAJOR (CRYPTOPHYCEAE)1

We compared autotrophic growth of the dinoflagellate Karlodinium micrum (Leadbeater et Dodge) and the cryptophyte Storeatula major (Butcher ex Hill) at a range of growth irradiances (Eg). Our goal was to determine the physiological bases for differences in growth–irradiance relationships between these species. Maximum autotrophic growth rates of K. micrum and S. major were 0.5 and 1.5 div.·d−1, respectively. Growth rates were positively correlated with C‐specific photosynthetic performance (PPC, g C·g C−1·h−1) (r2=0.72). Cultures were grouped as light‐limited (LL) and high‐light (HL) treatments to allow interspecific comparisons of physiological properties that underlie the growth–irradiance relationships. Interspecific differences in the C‐specific light absorption rate (EaC, mol photons·g C−1·h−1) were observed only among HL acclimated cultures, and the realized quantum yield of C fixation (φC(real.), mol C·mol photons−1) did not differ significantly between species in either LL or HL treatments. The proportion of fixed C that was incorporated into new biomass was lower in K. micrum than S. major at each Eg, reflecting lower growth efficiency in K. micrum. Photoacclimation to HL in K. micrum involved a significant loss of cellular photosynthetic capacity (Pmaxcell), whereas in S. major, Pmaxcell was significantly higher in HL acclimated cells. We conclude that growth rate differences between K. micrum and S. major under LL conditions relate primarily to cell metabolism processes (i.e. growth efficiency) and that reduced chloroplast function, reflected in PPC and photosynthesis–irradiance curve acclimation in K. micrum, is also important under HL conditions.

MODULATION OF POLYUNSATURATED FATTY ACIDS IN MIXOTROPHIC KARLODINIUM VENEFICUM (DINOPHYCEAE) AND ITS PREY, STOREATULA MAJOR (CRYPTOPHYCEAE) 1

We examined whether fatty acid (FA) composition changed when Karlodinium veneficum (D. Ballantine) J. Larsen (Dinophyceae) was grown phototrophically or mixotrophically on Storeatula major Butcher ex D. R. A. Hill (Cryptophyceae). We hypothesized that the FA composition of mixotrophic K. veneficum would not change relative to the FA composition of phototrophic K. veneficum. As in other phototrophic dinoflagellates, octadecapentaenoic acid (18:5n3) represented 9% to 20% of total FA in K. veneficum and was enriched within chloroplast‐associated galactolipid classes. The 18:5n3 content showed a highly significant positive correlation (r2 = 0.95) with chl a content and a highly significant negative correlation with growth rate (r2 = 0.88). A previously undescribed chloroplast galactolipid molecular species, digalactosyldiacylglycerol (DGDG; 18:5n3/18:5n3), was a dominant structural lipid in K. veneficum. Docosahexaenoic acid (22:6n3) represented 14% to 19% of total K. veneficum FA and was enriched within phospholipids. In the prey S. major, 18:5n3 was not present, but octadecatetraenoic acid (18:4n3) and α‐linolenic acid (18:3n3) represented approximately 50% of total FA and were enriched within chloroplast‐associated galactolipid classes. Eicosapentaenoic acid (20:5n3) and 22:6n3 represented approximately 18% of total FA in S. major and were enriched within phospholipids. The FA profile of mixotrophic K. veneficum, compared to phototrophic K. veneficum, showed elevated levels of 18:3n3, 18:4n3, and 20:5n3, and lower but persistent levels of 18:5n3. Production to ingestion (P:I) ratios >1 for major polyunsaturated fatty acids (PUFAs) indicated that direct assimilation from prey under balanced growth could not support rates of PUFA production in mixotrophic K. veneficum. These data suggest that the plastid plays a continuing and essential role in lipid metabolism during mixotrophic growth.