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.