Effects of surface roughness and oscillatory flow on the dissolution of plaster forms: Evidence for nutrient mass transfer to coral reef communities

Abstract

We dissolved plaster forms in seawater to examine the effects of surface roughness and flow conditions on masstransfer rates. Plaster blocks with varying roughness were dissolved under both steady and oscillatory flows between 7 and 43 cm s Ϫ1 yielding calcium mass-transfer coefficients (S Ca ) that varied from 0.5 to 3 m d Ϫ1. S Ca measured in a flume was 30-40% greater under oscillatory flow than under steady flow at flow speeds Ͻ10 cm s Ϫ1 ; this difference decreased with increasing flow speed. Plaster blocks with added millimeter-, centimeter-, and decimeter-scale roughness were dissolved under oscillatory flows between 2 and 26 cm s Ϫ1 on three different coral reef flats. Plaster dissolution rates in the field were linearly proportional to surface area regardless of the roughness scale. Variation in S Ca across different reef environments was less affected by whether the flow was steady or oscillatory (a factor of 1.2-1.6) than it was by flow speed alone (a factor of 4.7).Water motion is an important factor regulating the exchange of metabolites between reef communities and their surrounding environment. Rates of nutrient uptake (Atkinson and Bilger 1992; Thomas and Atkinson 1997), photosynthetic production (Dennison and Barnes 1988;Carpenter et al. 1991), and nitrogen fixation (Williams and Carpenter 1988;Carpenter et al. 1991) by coral and algae increase with greater water motion. This occurs as a result of changes in the rate at which dissolved compounds are physically transferred across concentration boundary layers adjacent to the surfaces of autotrophic organisms. Prior studies of metabolite mass transfer have been conducted using reef communities with morphologies incorporating multiple scales of roughness (10 Ϫ3 to 10 Ϫ1 m); however, little attention has been paid to the relative importance of varying roughness scales to total rates of metabolite mass transfer to reef communities. It is well known that autotrophic reef organisms exhibit morphologies that incorporate multiple scales of roughness (Kaandorp 1999). However, the success of correlating nutrient mass-transfer rates to bottom drag or frictional dissipation have led some investigators to suggest that small roughness scales are unimportant to total rates of metabolite mass transfer (Baird and Atkinson 1997;Hearn et al. 2001). In addition, most studies of water motion on coral and algal metabolism have been conducted under steady, unidirection-1 To whom correspondence should be addressed. Present address: MSB 205, 1000 Pope Road, Honolulu, Hawaii 96822 (falter@ hawaii.edu). AcknowledgmentsThanks to Jim Fleming and Eric Hochberg for suggestions and help in doing field work.

Effects of nonlocal turbulence on the mass transfer of dissolved species to reef corals

“…where MW gyp is the molecular weight of gypsum (172.2 g mol 21 ), Dt is the time taken to dissolve the form, A f is the surface area of the form, and DC is the concentration difference across the boundary layer (20.8 mol m 23 ; Falter et al 2005). For cylinders, A f was calculated on the basis of the average height and diameter of the cylinders measured before and after being dissolved.…”

“…Nutrient uptake is considered mass-transfer limited when the concentration at the surface (C s ) is very small in comparison to the concentration in the bulk fluid (C s ,, C b ), and thus, S R b (Eq. 1) The correlations proposed by Bilger and Atkinson (1992) have been shown to be effective in predicting rates of nutrient mass transfer to both experimental (Atkinson and Bilger 1992;Thomas and Atkinson 1997) and naturally occurring (Falter et al 2004) coral reef communities. Falter et al (2004) showed that given the range of flows and roughness heights found for most reef communities, and the values of Sc for dissolved metabolites (.400), these heat and mass transfer correlations could be simplified into the following relationship:…”

“…When the biochemical demand for a particular metabolite by an organism is high enough, the convective transfer of dissolved metabolites across the concentration boundary layer becomes the rate-limiting step or is ''mass-transfer limited'' (Bilger and Atkinson 1992). Estimates of nutrient fluxes to natural reef communities have been made in the field using principles of convective mass transfer derived from both theory and controlled experimentation (Atkinson 1992; Baird et al 2004;Falter et al 2004). Our ability to make such in situ predictions is essential for understanding how rates of nutrient uptake, photosynthesis, respiration, and other metabolic processes vary within and between reefs.…”

“…The expression shown in Eq. 8 can be rearranged to show the dependency of b on the important dimensional quantities of interest (see Falter et al 2004)…”

The effect of hydrodynamics on the mass transfer of dissolved inorganic carbon to the freshwater macrophyte Vallisneria americana

“…The mass transfer of these solutes is a complex process, and aside from diffusion, the transport of solutes to and from the water column and organisms is affected by water velocity, solute concentration, chemical reactions, and the geometry of the organism (Riebesell et al 1993;Larned et al 2004;Falter et al 2005). Hence, the ecophysiology of aquatic organisms is directly influenced by the surrounding environment and the rates of mass transfer have a profound influence on processes such as photosynthesis, respiration, and nutrient uptake (Ploug et al 1999(Ploug et al , 1999(Ploug et al , 2002.…”