Temporal and Spatial Variability of Freshwater Plumes in a Semienclosed Estuarine–Bay System

Abstract

While the physical forcing mechanisms that govern the outflows of major rivers throughout the world are well documented in the literature, comparably less research has been done to examine the mechanisms that govern the contributions of small rivers and streams to coastal ocean systems. These rivers and streams provide a direct means for the transport of anthropogenic and terrigenous materials from watersheds to coastal oceans. This study describes the temporal and spatial variability of freshwater plumes from Kaneohe Stream, Hawaii, USA, after storm events in the Kaneohe Bay watershed. Freshwater plumes were examined using a combination of fixed moorings, synoptic shipboard surveys, and lagrangian surface drifters. Data sets were collected over the course of 19 months from August 2005 to March 2007 with particular attention paid to storms during the boreal winters. Stream discharge and duration were found to exert a primary control on plume persistence in the southern Kaneohe Bay system. Time series data show a strong coherence between wind forcing and surface currents, which, in combination with data derived from shipboard and aerial surveys, indicate that the spatial variability of freshwater plumes is primarily determined by atmospheric forcing.

haracterizing the Effects of Two Storms on the Coastal Waters of O‘ahu, Hawai‘i, Using Data from the Pacific Islands Ocean Observing System

“…The Ala Wai Canal watershed extends from the crest of the Ko'olau Range Ocean and Earth Science and Technology, Honolulu, HI, USA. out to a depth of ~ 30 m. Based on this atlas and our observations (Pawlak et al, 2009) (Giambelluca et al, 1986) (Macdonald et al, 1983;Wong, 1994;Tomlinson and De Carlo, 2003;Ostrander et al, 2008). During heavy storms, 24-hour rainfall can exceed 250 mm over coastal areas and 500 mm over the mountainous interior of the Ko'olau Range (Giambelluca et al, 1984).…”

“…During heavy storms, 24-hour rainfall can exceed 250 mm over coastal areas and 500 mm over the mountainous interior of the Ko'olau Range (Giambelluca et al, 1984). Runoff from these storms has pronounced effects on the physics, chemistry, and biology of Kāne'ohe Bay on the windward side of the island (De Carlo et al, 2007;Ostrander et al, 2008;Hoover and Mackenzie, 2009;Drupp et al, 2011). The effects of runoff in Māmala Bay on the south shore of O'ahu appear to be less dramatic (Laws et al, 1999), in part because of the lower rainfall and greater flushing.…”

Are diurnal fluctuations in streamflow real?

“…Our results are relevant depending on the range of the water level fluctuation and the frequency of data capture. For instance, Kröger et al (2008) and Ostrander et al (2008), with the same logger model that we used, measured streamflows and tide amplitudes, and placing the external logger at 1.5 or 4 m above ground, respectively. Both the purpose of those research studies and the temporal scale of measured water fluctuations did not show any of the possible instrumental and methodological uncertainties.…”

Fluvial Fluxes of Water, Suspended Particulate Matter, and Nutrients and Potential Impacts on Tropical Coastal Water Biogeochemistry: Oahu, Hawai‘i

“…Dissolved nutrients are likely to be taken up most efficiently under baseflow conditions when freshwater inputs occur at low but steady rates. Although storms result in large inputs of dissolved nutrients to coastal waters, inputs occur as large pulses and storm conditions frequently are unfavorable for nutrient uptake [e.g., low light due to cloud cover and elevated turbidity, rapid mixing of plumes into lownutrient coastal waters by wind and waves (Ostrander et al 2008) resulting in dilution of nutrients], so changes in storm fluxes of dissolved nutrients may not impact coastal ecosystems as much as changes in baseflow fluxes. When storm plumes persist for extended periods, elevated nitrogen concentrations can support phytoplankton and zooplankton blooms (Ringuet and Mackenzie 2005;Hoover et al 2006), and the increased SRP concentrations found in urban runoff might extend bloom durations for longer periods than would otherwise be observed (e.g., Ringuet and Mackenzie 2005).…”