Biophysical Interactions: Influence of Water Flow on Nutrient Distribution and Nitrate Uptake by Marine Algae

Abstract

Inorganic nutrients are required by primary producers for photosynthesis. Their distribution and availability therefore underpin the success of ecosystems. The relationship between primary production and inorganic nutrient concentration and speciation is complex and variable, based not only upon differences in algal physiologies, but also upon physical environmental factors. For example, physical factors such as water flow rate can impact transport of nutrients to algal uptake surfaces. In order to understand survival strategies employed by the algae, it is essential to understand the different stages and drivers of the nutrient uptake and assimilation processes, including the interplay between nutrient concentration and hydrodynamics. The series of studies described in this dissertation represent a threetiered investigation into the nitrate uptake response of marine algae to variable water flow rates. First, the interaction of an individual organism with its local flow environment is assessed in the field; next, the response of an epiphyte community to variable water flow rates is evaluated in an experimental study. Finally, a combined field-modeling study scales up data obtained from smaller-scale experiments and timeseries observations, focused on individuals and communities, to an ecosystem level study. In this field-modeling study, the impact of interplay between nutrient concentrations and hydrodynamics on the rates of, and capacity for, nitrate uptake by the algal community is examined. All phases of the study take place in the southern portion of Kāneʻohe Bay, O‘ahu, in the waters surrounding Moku o Loʻe and in Heʻeia Fishpond, an ancient Hawaiian fishpond. Investigation into the role that local hydrodynamics can play in nutrient uptake by the specific benthic algae targeted in this study reveals that each benthic component displays flow-mediated nitrate uptake. Field studies reveal that Gracilaria salicornia, an invasive Rhodophyte in Hawaiʻi that is characterized by a particularly rigid canopy, is effective at forming microhabitats within its canopy understory, where inorganic nutrient concentrations are significantly elevated above the water column exterior to the v canopies. Measurement of nitrate reductase (NR) activity in the tissue of this alga also suggests that it can respond quickly to its immediate nutrient concentration environment, on spatial scales of at least 2 cm. Thus, vertical gradients in NR activity within G. salicornia canopies are likely driven by the nutrient microenvironment that the alga, itself, creates. Model results indicate that within Heʻeia Fishpond, G. salicornia is the main driver of nitrate drawdown from the water column. Assessment of an epiphyte community, hosted by benthic alga resident in Kāneʻohe Bay, reveals that the cumulative responses of individual epiphyte species to elevated nitrate concentrations and nitrate flux can translate into shifts in primary producer community structure. Finally, successful application of a nitrate distribution model developed for Heʻeia Fishpond reveals that the total biomass of each of the members of the primary producer community is an important determining factor in nitrate distribution.