C. S. Duke

BIOCHEMICAL STRATEGIES FOR GROWTH OF GRACILARIA TIKVAHIAE (RHODOPHYTA) IN RELATION TO LIGHT INTENSITY AND NITROGEN AVAILABILITY1

The main effects and interactions between light (Io, full incident sunlight to 0.07 Io) and NO3− loading (0.4 to 4.3 mmol · g dry weight−1· d−1) on growth rate, photosynthesis and biochemical constituents of Gracilaria tikvahiae McLachlan were studied using a factorial design experiment in outdoor, continuous‐flow seawater cultures. Incipient nitrogen limitation in the low NO3− loading, Io and 0.57 Io treatments occurred after 2.5 weeks of growth under the experimental conditions and resulted in decreased tissue NO3− and R‐phycoerythrin. Tissue NO3− and R‐phycoerythrin accounted for up to ca. 15 and 20%, respectively, of the total N in G. tikvahiae suggesting a N reserve role for these N pools. Under light and NO3− limitation, growth rate was a parabolic function of the C:N ratio. As light limitation increased, growth rate and the C:N ratio decreased as levels of Chl‐a, R‐phycoerythrin, percent N and percent protein increased. As NO3− limitation increased, growth rate and levels of Chl‐a, R‐phycoerythrin, percent N and percent protein all decreased with parallel increases in the C:N ratio. In contrast to the inverse relationship between pigment content and light, ribulose bisphosphate carboxylase (RuBPCase) activity (on both a protein and dry weight basis) varied directly with light. This biochemical acclimation of G. tikvahiae to light and N availability appears to be a process directed towards maximizing photo synthetic capacity and growth.

EFFECTS OF TEMPERATURE, NITROGEN SUPPLY, AND TISSUE NITROGEN ON AMMONIUM UPTAKE RATES OF THE CHLOROPHYTE SEAWEEDS ULVA CURVATA AND CODIUM DECORTICATUM1

Nitrogen uptake rates of Ulva curvata (Kütz.) de Toni (Ulvales) and Codium decorticatum (Woodw.) Howe (Caulerpales) grown under several N addition regimes were determined by perturbation and continuous mode techniques, and as N demand, by the product of growth rate and tissue N. Uptake rates are reported as the slope of rate vs. concentration curves in each case. N uptake rates of U. curvata were inversely correlated with tissue N and affected only slightly by temperature. There was no correlation of N uptake rate with tissue N in C. decorticatum. N uptake rates of C. decorticatum were affected by temperature but to a lesser degree than were growth rates. Neither N addition per se nor light affected N uptake capacity of either species. The proximal mechanism for seaweeds accumulation of N at low light and temperatures may be that N uptake is less limited by light and temperature than is growth. This in turn may partially compensate for the effects of reduced light and temperature on growth by increasing pigment and enzyme levels. Perturbation uptake rates were higher than continuous mode or N demand rates in Ulva but not in Codium. N uptake rates of Ulva were higher than those of Codium, but N storage capacities were lower. These two observations suggest that Ulva experiences a fundamentally more variable N supply than does Codium. This is consistent with the clarification of Ulva as an ephemeral form and of Codium as persistent. A seaweeds functional form therefore appears to influence the spectrum of resource variability available to it as well as its ability to persist in the environment.

Effects of light on growth, RuBPCase activity and chemical composition of Ulva species (Chlorophyta)

The effects of light on growth, RuBPCase activity, and chemical composition of Ulva curvata (Kütz.) De Toni and U.Lactuca L. were examined at a range of temperatures and N‐supply levels. Groeth of Ulva speices becomes more light‐dependent with increasing temperature and N. The effect of light on RuBPcase is N‐dependent, with a positive correlation under N‐sufficient and a negative correlation under N‐limited conditions. Light effects on pigment levels and ratios may be independent of effects on growth rate. These interactions uncouple growth rate from RuBPCase and pigments, and thus from tissue%N. The limits of variability of the growth‐%N relationship can be described by a parabola. Under relative light or temperature‐limitation, %N is negatively, growth increase with increasing %N. Tight coupling of seaweed wrowth and chemical composition may therefor be relatively rare in natural waters where growth can be simultaneously limited by light, temperature, and N.

Short-term environmental variability and phytoplankton abundance in a shallow tidal estuary. I: Winter and summer

Fixed-point sampling of a shallow tidal estuary was performed hourly for 14 d in summer of 1982 and again in winter of 1983. This sampling regime was of statistically appropriate duration to allow characterization of the variability between periods of 2 to 96 h by spectral analysis of the time-series. The project (PULSE) took place in the Newport River Estuary, located behind the Outer Banks of North Carolina, USA. In all, twentyeight parameters were monitored, encompassing the meteorology, hydrology, water chemistry and phytoplankton-production physiology. Although the annual cycle was monitored, only the winter and summer seasons are compared here, i.e., the lowest water temperatures with the highest water temperatures. The physics, chemistry and biology of the estuary at the hourly scale were highly variable and non-random. The estuary is riverine in winter; growth-limiting nitrogen is supplied as nitrate (NO3-) and ammonium (NH4+) by runoff from the drainage basin. In summer, the estuary is lagoonal; nitrogen is supplied as NH4+by biological regeneration. Chlorophyll a biomass varies primarily at the ∼4 d period in winter and at the diel period in summer. Although finely tuned to environmental variability, phytoplankton abundances were at equilibrium insofar as daily chlorophyll production was balanced by losses, i.e., grazing, export and deposition. Most important, high-frequency processes, here periods at the scale of cell-division times, can be very important in phytoplankton ecology.

Effect of temperature on nitrogen-limited growth rate and chemical composition of Ulva curvata (Ulvales: Chlorophyta)

Ulva curvata (Kutz.) de Toni growing in shallow estuaries experiences a highly variable supply of dissolved inorganic N, which can limit growth rates. The effects of N supply variability and annual temperature and light variation on growth rates and chemical composition were assessed in plants grown in outdoor tanks supplied with running seawater and either pulsed or continuous N additions for 8 d. Environmental variables were measured every 12 h, growth rates every 2 d, and plant nitrogenous constituents every 4 d. The experiment was repeated 7 times over the course of a year. The slopes of growth rate-tissue N and growth rate-N supply plots increased with temperature and varied at periods at least as short as 2 d. Temperature explained 44% of variation in growth rate, and temperature, light, and N supply together explained 53% of variation in growth rate. N-limitation occurred only above 20°C, the approximate temperature for the maximum growth rate. Growth rate and tissue N were not affected by N supply frequency. Tissue N, ribulose-bisphosphate carboxylase activity, and chlorophyll contents were correlated with each other and with environmental variables but not with growth rate. U. curvata thus does not maintain balanced growth in the strict sense, but rather stores nutrients supplied as pulses for use when they become growth-limiting. Since the relative importance of any single growth-limiting factor is highly dynamic, assessing the importance of multiple limiting factors requires longterm, high-frequency sampling of principal limiting factors and seaweed growth rates.

Community oxygen metabolism in a shallow tidal estuary

Abstract Comprehensive sampling of a shallow tidal estuary was performed hourly for 14 days, at three-month intervals throughout the annual temperature cycle. The project took place in the Newport River estuary located inside the Outer Banks of North Carolina, U.S.A. In all 26 parameters were monitored at a single station, including meteorology, hydrology, water chemistry and phytoplankton production physiology. The estuarine character in colder months was dominated by riverine input, while in warmer months the estuary was lagoonal, having limited input and exchange with the sea. A holistic evaluation of estuarine community O2 metabolism was developed using diffusion-corrected changes in O2 concentration. These data indicated positive integrated diel production (autotrophy) only in winter (February). The estuary was functionally heterotrophic during warmer months (May, August, November). Integrated short-term radiocarbon productivity estimates compared favorably with measurements of oxygen based diel gross production rates. Community O2 based respiration rates (heterotrophy) exhibited a strong positive relationship with water temperature. During summer, rates of heterotrophic metabolism and associated nutrient regeneration appeared to control autotrophic production.

Seasonal variation in RuBPcase activity and N allocation in the chlorophyte seaweeds ulva curvata (kutz.) de toni and Codium decorticatum (Woodw.) Howe

Abstract Seasonal variation in ribulosebisphosphate carboxylase (RuBPCase) activity and cellular N allocation in Ulva curvata (Kutz.) De Toni and Codium decorticatum (Woodw.) Howe was followed over two annual cycles. Because apparent seasonal variation may result from random sampling of a coherent pattern of shorter-term variation, estimating the magnitude of seasonal variation in seaweed N allocation requires sampling over a range of time scales. Such sampling reveals annual ranges of variation in N allocation to be greater than those measured at time scales of hours to days. RuBPCase activity in both U. curvata and C. decorticatum peaks in winter and summer, with fall and spring lows. Total N and C exhibit pronounced winter highs and summer lows in C. decorticatum , with less predictable variation in U. curvata . Growth is controlled by temperature in both species, while N uptake is only loosely coupled to temperature or light. Seasonal variations in N allocation are driven by interactive effects of N supply, temperature, and light on growth and N uptake. These effects are largely independent of morphological and genetic differences between species.