Patterns in morphology, pigment concentration, and light saturation kinetics of Ecklonia radiata reveal great morphological and physiological variability among individuals from sites spanning strong gradients in topographic shading and wave exposure among the 14 fjords in southwestern New Zealand. Morphology of E. radiata varies from relatively narrow (85 6 4.7 mm) (mean 6 standard error), thick (3.2 6 0.30 mm) blades from the well-illuminated, wave-exposed outer coast sites to wide, undulate (460 6 36.8 mm,) and thin (0.46 6 0.059 mm) blades from quiescent, topographically shaded inner fjord sites. Chlorophyll a (Chl a) concentration of blades (0.084-1.34 mg g 21 of tissue) and the ratio of fucoxanthin to Chl a (0.33 to 0.56) also increased along this gradient, indicating photoacclimation within the inner fjord populations. In situ measurements of light saturation kinetics indicate maximum photosynthetic rates at lower irradiance (I max 5 43.7 vs. 257 mmol quanta m 22 s 21 ) for algae at inner fjord sites relative to well-lit outer fjord locations. Individuals exhibiting characteristically photoacclimated relative electron transfer rate curves had more deplete d 13 C (213.35% to 222.35%) than individuals with higher I max . There was no significant association between the kelp morphology or geographic location and the observed recombinant DNA diversity of ITS sequences that would indicate the presence of two Ecklonia species in the fjords. E. radiata occupies a wide range of habitats in Fiordland and displays variability in morphology and photo-physiological responses to low light that coincide with gradients in wave exposure and topographically shaded light conditions.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. In the research presented here, we examine the effects of water velocity and canopy morphology on rates of nutrient uptake by seagrass communities. Ammonium uptake rates for two types of seagrass communities, Halodule wrightii and Thalassia testudinum, are measured over a range of velocity using a field flume. The field flume allows independent measurements of uptake by communities of natural composition and condition. We compare our results with those estimated using empirically derived engineering equations that describe transport processes to rough surfaces in order to explore the possibility that uptake rates can be predicted from these equations. We also investigate the possibility that the seagrass canopy alters the characteristics of water flow within the community, which is reflected by the friction imposed by the canopy (the friction coefficient) on the moving water. Our results indicate that ammonium uptake by seagrass communities is dependent on water velocity. Further, seagrasses affect characteristics of water flow within the community that are reflected in rates of ammonium uptake. Empirically derived engineering equations used with measured friction coefficients yield expected Stanton numbers (a nondimensional ratio of flux to a surface to advection by a surface) that are within 95% of those measured in the field flume. Thus, the capacity of these communities to remove ammonium from the water column can be predicted using empirically derived engineering equations that describe the transport of chemicals between a fluid and a rough surface.

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Nutrient dynamics of aquatic communities are dependent on the flux of nutrients to organisms within the communities. Flux is dependent on water column concentration and hydrodynamic factors that affect both advection of nutrients through the community and rates of diffusion at the surfaces of organisms. In this study, we measured rates of ammonium uptake for a seagrass community under various hydrodynamic conditions and determined the effects of water velocity and oscillatory flow on uptake rates. Experiments were conducted using a portable flume deployed in natural Thalassia testudinum communities. Uptake rate constants ranged from 9.9 to 25.4 × 10 -5 m s -1 and were ~1.5 times higher in oscillatory flow than in unidirectional flow. Uptake rate constants were positively dependent on both water velocity and turbulent energy in the water column. These results demonstrate the importance of hydrodynamics on biogeochemical cycling in seagrass beds and provide evidence of the efficacy of merging research on hydrodynamics and biogeochemistry in understanding nutrient processes in complex nearshore communities.

The impact of hydrodynamic regime on rates of nutrient uptake for a seagrass community and for individual components of the community (Thalassia testudinum, epiphytes, phytoplankton, microphytobenthos) was quantified through the deployment of a field flume and application of 15 N-labeled ammonium and nitrate tracers. Ammonium uptake rates for the community and for seagrass leaves and epiphytes were enhanced with increased bulk velocity (uptake rate ∝ U b 0.57 to 0.70 ) and Reynolds shear stress at the top of the canopy (uptake rate ∝ τ R 0.32 to 0.40 ); thus, relationships expected for masstransfer limitation apply for the entire assemblage and individual components that form the canopy. Nitrate uptake rates for the community and for epiphytes < 35 µm were also enhanced with increased bulk velocity (uptake rate ∝U b 0.40 to 0.67 ) and Reynolds shear stress (uptake rate ∝ τ R 0.19 to 0.32 ), but less so than ammonium uptake rates. For all components, uptake rates for NO 3 -were lower than those for NH 4 + , suggesting that nitrate uptake was affected by a biological factor (e.g. availability of nitrate reductase). Epiphytes and phytoplankton each accounted for 40 to 45% of the total ammonium and nitrate recovered; however, these components contributed the least to total particulate nitrogen in the community. Uptake by seagrass leaves and sediments containing microphytobenthos each represented < 5% of the ammonium and <10% of the nitrate recovered, but contained the majority of particulate nitrogen. Our results emphasize the importance of epiphytes and phytoplankton in nitrogen uptake from the water column over the short term, and reaffirm that seagrasses and sediments play an integral role in the long-term retention of nitrogen within the canopy.

Field surveys and laboratory experiments were used to investigate the influence of the physical environment on variability in δ13C and δ15N signatures of Ulva pertusa, an abundant macroalgae inhabiting the low salinity layer (LSL) of Doubtful Sound, a New Zealand fjord. Field surveys revealed significant spatial variability in δ13C (‐18% to ‐12%) and δ15N (0% to 6%). δ13C was enriched at high irradiance sites and depleted at the fjord’s wave‐exposed entrance. δ15N signatures increased from 0% at the fjord head where freshwater influence is greatest to an oceanic signature of 6% at the fjord entrance. δ15N also increased by up to 4% between 2‐m depth and the LSL‐seawater interface (4‐m depth); this pattern was less pronounced near the ocean. During laboratory experiments, δ13C of U. pertusa became significantly enriched under high levels of irradiance (>50 mmol quanta m−2 s−1). When exposed to high irradiance, increases in water motion rapidly depleted δ13C signatures by as much as 5%. Variability in δ13C of U. pertusa in Doubtful Sound is largely a function of the light regime, which influences rates of photosynthesis and in turn the algae’s dependence on HCO3−, an enriched source of carbon. However, increased water motion at the fjord entrance counteracts the influence of irradiance, leading to enhanced flux of CO2 and depleted δ13C signatures. Variation in δ15N of U. pertusa is less dependent on the physical environment and instead is driven by the source pool signature, which in turn varies between freshwater and marine sources of nitrogen.

Field and laboratory velocity profiles were used to quantify boundary layer dynamics within communities of a small (< 0.2 m tall), dense canopy-forming seaweed Adamsiella chauvinii (Rhodophyta) in a soft-sediment habitat and to examine the role of hydrodynamics in modulating nutrient supply. At the 'canopy scale' there was a mixing layer at the fluid−canopy interface where turbulent kinetic energy was greatest, potentially enhancing nutrient uptake in this region. In the lower half of the canopy, a drag-dominated area of very low water velocity (< 0.01 m s −1 ) occurred. Spectral analysis revealed a reduction in energy within the canopy of around 1 Hz. The hydrodynamic parameters obtained from flume measurements were in good agreement with those recorded in the field. To understand the implications of the hydrodynamic environment on nutrient uptake, a flushing ratio was developed that compares the time for macroalgae to remove all nutrients from the canopy volume relative to the timescale for flushing. Results suggest that when nutrient demand is low, the canopy is well flushed and not mass-transfer limited. However when nutrient demand is high, the canopy can deplete nutrients more quickly than they can be replenished by ambient flows and are potentially mass-transfer limited.

In a series of flume experiments, 15N‐labeled ammonium was used to isolate the effects of water velocity on ammonium uptake by epiphytes from that of an assemblage of organisms that included seagrass leaves, epiphytes, and phytoplankton. Rates of NH4+ uptake for epiphytes, seagrass leaves, and the total assemblage were dependent on water velocity. Ammonium uptake rates for epiphytes, normalized to chlorophyll a, increased by an order of magnitude (0.65 to 6.8 × 1028 g N removed [mg Chl a]−1 s−1) over a range of velocity (0.02–0.20 m s−1) and were correlated to uptake by the entire assemblage. The relationship between NH4+ uptake and velocity for the epiphytes was within the range expected for mass transfer limited uptake, which suggests that water flow strongly influences NH4+ uptake by this important component of seagrass communities. Our results demonstrate that isotopically labeled nutrients can be used to isolate the effects of water velocity on rates of nutrient uptake by an individual component of a community and to evaluate how uptake rates for the component compare to those of the community as a whole.