OS33C-1358
What drives the Azores Front dynamics?
The Azores Front is the northern boundary of the subtropical Gyre in the northeast Atlantic Ocean. This permanent thermohaline front forms a natural barrier between oligotrophic waters in the south and waters originating in the temperate North Atlantic. It is changing its position and thickness continuously because of a strong tendency to meandering. To locate the front CTD, XBT and XCTD casts were taken during two cruises in April 2007 and May 2008. The frontal system during spring 2007 will be characterized using hydrographical (T,S,ρ), biological (chlorophyll, pigments) and chemical (nutrients) data. A comparison of these results with similar hydrographical data from the period 2000 to 2006 shows moderate interannual variability in the position of the Azores Front. The front has no manifestation at surface throughout the year but its position can be determined in winter and spring season using the 18°C surface isotherm because the front is acting as the boundary of the 18°C water mass. Furthermore the frontal position will be compared to the position of climatological features like the Azores High and the Intertropical Convergence Zone using seasonal mean values of Sea Level Pressure and precipitable water. High lag correlations were found between the respective latitudinal and longitudinal position as well as the intensity of the Azores High and the position of the Azores Front with lags between one and two years.
OS33C-1359
Nearshore Circulation Over a Muddy Seafloor
The subtidal circulation over a muddy seafloor on the Louisiana Chenier-Plain coast is investigated using pressure and velocity measured for twenty days in about two and one half meters water depth. In contrast to sandy coasts, which erode during storms, prior Chenier-Plain studies in five to twenty meters water depth suggest that mud is transported shoreward during storms by onshore-directed, wind-driven cross-shore flows. During the initial ten days of the present study, the alongshore bottom stress was weakly correlated with the alongshore wind stress, and the cross-shore pressure gradient was correlated with the onshore wind stress, suggesting that flows are at least partly wind driven. Unlike shoaling waves observed outside the surfzone on sandy coasts, wave radiation stresses on the gently sloping, muddy coast decrease onshore owing to mud-induced dissipation. The cross-shore wave radiation stress gradient was correlated with the pressure gradient, suggesting that the mud-induced dissipation causes a 'setup' similar to that observed for breaking waves in the surfzone. Although the Coriolis term is similar in magnitude to the wind stress and wave radiation stress gradient terms, it was not correlated with the cross-shore pressure gradient. In the last ten days of the study, the alongshore bottom stress and cross-shore pressure gradient were uncorrelated with wind stress, indicating that the system was not always wind driven. Comparisons of estimated momentum balance terms suggest that spatial and temporal variations in stratification, bottom drag, and density gradients may be important. Funding was provided by the National Science Foundation and the Office of Naval Research.
OS33C-1360
Autonomous Glider Observations of Subducting Tongues of Warm, High-Chlorophyll Water on the Oregon Shelf
Year-round high resolution data from three Autonomous Underwater Vehicle Gliders from the Oregon shelf along the Newport Hydrographic (NH) Line (44.65°) are used to study small scale tongues of relatively warm, high-chlorophyll water subducting from near the surface to depth along isopycnal surfaces. Tongues are observed only during upwelling season and 15 tongues were observed over two years. Tongues are on average 7 km in length and 17 m thick with a temperature of 9°C, averaging almost a degree warmer than surrounding water. The average density is 1026.2 kg/m2 with an average density thickness of 0.4 kg/m2. One tongue is investigated in detail using wind and temperature data from locally moored instruments. The tested theory is that tongues are formed by sub-pycnocline water that has been heated by the sun and sinks along the upwelling front. The tongues seem to appear at the transition to relaxed conditions after a period of upwelling.
OS33C-1361
Quantifying Transport Associated With Non-Linear Internal Waves in Massachusetts Bay
Previous studies have shown that large-amplitude internal waves are significant agents in transporting water- borne materials, such as nutrients, larvae and sewage outfall effluent, across continental shelves towards shorelines. In August and September 2008, surface drifters, drogued at fixed depths, were deployed in Massachusetts Bay, a location where large amplitude internal waves are predictably observed to propagate towards the coast. The drifters were equipped with GPS devices to quantify the transport due to internal waves. Over the course of two ten-day cruises, drifters were deployed in front of the leading edges of non- linear wave packets and remained in the water for periods ranging from 4 to 24 hours. In addition, Acoustic Doppler Current Profilers (ADCPs) were affixed to moorings and tripods in order to measure the currents throughout the entire water column. The data garnered from these current meters were also used to estimate transport at fixed depths. Transport estimates from drifters and ADCPs were compared. During the study period, it was observed that drifters traveled more quickly onshore when drogued closer to the surface. Such analyses can be useful when quantifying the transport of materials in Massachusetts Bay, particularly those that have the ability to remain at a fixed depth, such as an oil spill or zooplankton.
OS33C-1362
Analysis of Acoustic Wave and Current Data Offshore of Mytle Beach, South Carolina
Two bottom boundary layer (BBL) instrument frames have been deployed on the shoreface and inner-shelf of Long Bay, South Carolina offshore of Myrtle Beach as part of a South Carolina Sea Grant funded project to measure sediment transport over two hardbottom habitats. The inshore instrument frame is located on an extensive hardbottom surface 850 meters offshore. The second instrumented frame is secured to a hardbottom surface on the inner-shelf at a distance of approximately 2.5 km offshore. The nearshore BBL observing system is composed of a downward-looking RDI/ Teledyne 1200 kHz Pulse-Coherent Acoustic Doppler Current Profiler, an upward-looking Nortek Acoustic Wave and Current Profiler (AWAC), and an Aquatec Acoustic Backscatter Sensor. As part of this larger study, the wave and current data from the AWAC have been analyzed. Long-term continuous time series data include wave height, wave period, directional wave spectra, and the magnitude and direction of currents in the water column. Within the data set are several wave events, including several frontal passages and Tropical Storm Hanna which hit the Myrtle Beach area in early September. Wave data have been correlated with meteorological data, and a comparison of shoreface wave characteristics during each type of event are presented.
OS33C-1363
A summertime study of mixing between Walker Creek and Tomales Bay waters
During dry Californian summers, inflow to estuaries is very low and estuarine processes are only found in small tributary estuaries in which weak freshwater inflow can influence the predominant evaporative and tidal processes. The freshwater influence of Walker Creek on Tomales Bay was studied in summer 2008. The primary freshwater source at the seaward end of Tomales Bay is Walker Creek, a westward flowing stream, with a catchment area of 73 square kilometers. Watershed management through a dam and reservoir on upper Walker Creek provide summertime freshwater inflow that would have otherwise been absent. However, inflows are weak and there is minimal observed influence beyond the creek delta in Tomales Bay. The focus of this research is on the mixing dynamics within the narrow estuarine channel (drowned river valley) linking the Creek to the Bay proper. Salinity variations are primarily tidal and little vertical stratification is observed due to the shallowness of this tributary estuary. Longitudinal tidal mixing thus dominates the flux of freshwater from creek to bay receiving waters, with evaporative losses also being an important term. The resultant distribution of salinity provides a first-order approximation of dissolved constituents derived from the Walker Creek watershed, including barium which is used as a proxy for paleosalinity when incorporated in the calcite tests of estuarine foraminifera. The summer conditions are compared with limited historical data for winter and with projected conditions in pre-dam summers when inflow was lower.
OS33C-1364
The estuary part of low-inflow estuaries: stratification and residence in a Tomales Bay tributary estuary.
Flow continues in Lagunitas Creek throughout the summer, delivering freshwater to the hypersaline Tomales Bay estuary. The freshwater-saltwater interface is found in the narrow estuarine channel that connects the creek and the bay, with minimal freshwater influence being found seaward of the broad shallow deltaic region at the head of the deeper sections of Tomales Bay. Observations of salinity, temperature, waterlevel and dissolved oxygen describe a stratified system, interrupted by mixing during flood tides. Denser waters are trapped in deeper sections of the estuary, but only transiently. The estuarine channel is about 3km long and constrained by dykes used to convert salt-marshes to agricultural lands. These dykes are now being removed to allow restoration of over 500 acres of salt-marsh in the vicinity of the study region. Longitudinal surveys were conducted and moored sensors were deployed to obtain a detailed view of tidal variations. Estimates of vertical and horizontal mixing, as well as residence times, will be compared with results from a post-restoration study to assess the impacts of restoration on the pelagic estuarine environment.
OS33C-1365
Paleoproductivity of the Southern Chilean Margin Over the Past 30,000 Years
Productivity on ocean margins is highly sensitive to climate change, including changes in macro-and micro nutrient inputs from land, upwelling intensity, and nutrient content of upwelled waters. Coastal productivity may also contribute to climate change, through impacts on carbon sequestration, sediment and water-column redox state (and hence rates of denitrification), and production of dimethyl sulfide. Reconstructing margin productivity can thus provide insight into the nature and causes of past climate change. We examined productivity off southern Chile at ODP Site 1233. At 41°S, 838 m depth, the site is at the core of Antarctic Intermediate Water (AAIW). It is at the southern limit of the Peru-Chile upwelling system, where the northern extent of the Antarctic Circumpolar Current (ACC) impinges on the South American continent. Paleoproductivity was reconstructed by normalizing biogenic fluxes to the flux of 230Th. Over the last 30ky, maximum organic carbon, opal and carbonate fluxes occurred during the last glacial interval (26-20 ky BP). A steady decline is seen in carbonate flux from the middle Holocene (~8 ky BP) to present, organic carbon flux increases from the late Holocene (~5 ky BP) to present while opal flux is essentially unchanged from 20 ky BP to present. The pattern of reconstructed productivity is consistent with a more northerly position of the ACC during the last glacial interval, bringing the core of high-nutrient waters to 41°S. However, maximum prodctivity is observed well prior (~5 ky) to the deglacial increase in alkenone- reconstructed SST at this site. The productivity maximum is coincident with a maximum in glacier extent, and in the flux of terriginous material to the site. High glacial productivity may therefore have been supported by enhanced macro or micro-nutrient delivery from land. The pattern of reconstructed productivity implies that the less reducing conditions during the glacial interval, inferred from authigenic metal accumulation, were driven by increased ventilation rather than changes in local productivity.
OS33C-1366
Aptian Carbon and Sulfur Isotope Curves From ODP Site 765
δ13C and δ34S ratios measured on samples from various ODP Sites suggest that major
shifts in the partitioning between the reduced and oxidized reservoirs of carbon and sulfur occurred during
the Early Cretaceous (Paytan 2004, Wortmann & Chernyavsky 2007). However, the Cretaceous
δ13C and δ34S signals are derived from datasets from different localities. The lack of a
comprehensive dataset and the vastly different residence times of C and S makes identifying coupling of the
C and S cycle difficult. Here we attempt to construct a high resolution record of the δ13C and
δ34S signals, where we obtain both measurements from the same samples.
312 Samples from ODP Site 765 were milled and divided in half. One half was decarbonated, and
measurements of δ13C were performed on the remaining bulk organic carbon. Our δ13C
data show the typical Aptian carbon isotopic signature, a negative spike followed by a positive peak. A marine
barite separation following the procedure of Paytan (1996) was performed on the other half of a subset of 48
of the samples from before and after the negative excursion. δ34S is currently being measured on
24 barite samples of nondiagenetic origin. Obtaining a coupled and highly resolved δ13C and
δ34S dataset will help constrain the temporal relationship between the C and S cycles on timescales
of ~1 million years.
Paytan, A. (1996). Marine barite: a recorder of oceanic chemistry, productivity and circulation. Ph.D. Thesis,
Univ. of Calif., San Diego, California.
Paytan, A., M. Kastner, et al. (2004). Seawater sulfur isotope fluctuations in the Cretaceous. Science
304(5677): 1663-1665.
Wortmann, U. G. and B. M. Chernyavsky (2007). Effect of evaporite deposition on Early Cretaceous carbon
and sulphur cycling. Nature 446(7136): 654-656.
OS33C-1367
Late Holocene Regression of the Northern Peruvian Coast Near Rio Chicama
Many Peruvian archaeological sites lie at the interface between an arid coastal desert, a rich marine ecosystem, and some of the tallest mountains in the Western Hemisphere, providing several unique environments within a small geographic area. While the region has supported civilizations since at least 6000BP, it has also been subject to a complex history of environmental impacts evident in the stratigraphy of the surrounding coastal environment. Most notable in the stratigraphy are El Nino flood events, providing the majority of sediment input to the coast, and tsunami events that are occasionally marked in the stratigraphic record. Such evidence for a paleotsunami appears to exist within a sequence of regressive Holocene shoreline deposits. This possible event is characterized by a planar erosional surface, dipping shallowly seaward, truncating the entire sequence of Holocene shorelines. The surface also consists of a lag of gravel that has been subsequently weathered by subaerial exposure to salt and sun. In addition there appears to be residual evidence of a similar, earlier event, most of which has been eroded from the record by the younger event. This entire sequence of shoreface deposits is situated approximately 2m above present mean sea level, and is suspected to be younger than 3000 years (pending radiocarbon dates), suggesting a rapid, recent Holocene regression in this region.
OS33C-1368
Controls on the Remineralization Profiles of Sinking Organic Matter
As organic carbon matter descends down the water column, its remineralization profile with depth to inorganic nutrients may be influenced by a number of factors including the pelagic zooplankton and bacterial communities, temperature, oxygen concentrations, aggregation and disaggregation processes, and mineral ballast fluxes. In this study, we attempt to evaluate the impact of these factors and their significance to the carbon particle flux and the marine biological pump. We analyze a database of sediment trap sinking fluxes in conjunction with output from the Biogeochemical Elemental Cycling (BEC) ocean model. Understanding what influences the sinking particulate organic flux (POC) with depth can help predict how the biological pump and marine biogeochemistry will respond to global warming. The sediment trap database is used to evaluate the remineralization parameterizations in the BEC model, which incorporates the mineral ballast hypothesis of Armstrong et al. (2002). Through statistical analysis of the sediment trap data, in conjunction with BEC model output, we hope to constrain the impacts of ballast, temperature, and oxygen concentrations on the remineralization profiles of sinking organic matter in the oceans.
OS33C-1369
Sedimentary Iron Source Fuels Phytoplankton Blooms in the Southern Ocean near South Georgia Island
Phytoplankton blooms in the Southern Ocean are known to be fueled by iron sources. Here we examine the region of high chlorophyll in the southwest Atlantic near South Georgia Island. It appears that as the Antarctic Circumpolar Current (ACC) flows through this region it picks up iron from the North Scotia Ridge, which includes South Georgia Island. Ocean color data from the SeaWiFS sensor are used to examine chlorophyll and phytoplankton carbon concentrations in relation to ocean bathymetry in this region. There is a sharp increase in chlorophyll concentrations downstream of shallow areas along the North Scotia Ridge. There is also a decrease in phytoplankton C/chlorophyll ratio, this is indicative of a reduction in iron stress. The northern boundary of this high chlorophyll region is largely determined by the location of the Antarctic Polar Front. We also examine the effects of wind-stress curl and upwelling from Ekman pumping in an attempt assess the contribution to phytoplankton blooms in this region.
OS33C-1370
Denitrification and Oxygen Cycling in Saanich Inlet
Saanich Inlet, British Columbia is a seasonally anoxic fjord and therefore an ideal location to study seasonal changes in productivity and respiration. We have collected monthly profiles of dissolved oxygen and oxygen/nitrogen/argon ratios in this estuary since April 2008. These measurements allow us to explain deviations of gas concentrations from equilibrium with the atmosphere by separating the effects of physical processes, which affect all three gases, from biological processes, which only affect oxygen and nitrogen. The bottom water in this fjord becomes anoxic due to a sill that restricts water inflow and high rates of primary productivity at the surface that result in aerobic decomposition at depth. This profile is reset on an irregular basis by flushing events that allow oxygenated water from outside Saanich Inlet to enter the basin. An increase in oxygen concentrations below 90 m between sampling in April and May 2008 suggested that a renewal occurred between these cruises. Data from the VENUS project, which has deployed a CTD at 96 m in Saanich Inlet, confirms this hypothesis. Although aerobic respiration resulted in a continuous depletion in oxygen at intermediate depths during the summer, the dissolved nitrogen/argon profile over the full depth of the inlet remained remarkably constant from May through August. A persistent maximum in the nitrogen/argon ratio of 5% above the expected ratio at equilibrium was observed in the deepest 25 m at our station. The stability of the profile over the summer implies that a steady state was quickly established following the renewal. The excess nitrogen gas was produced by denitrifying and/or anammox bacteria that decompose organic matter in suboxic environments. Denitrification involves the use of nitrate to oxidize organic carbon and anammox utilizes nitrite to oxidize ammonium. Despite denitrifying bacteria preferentially reducing 14N-NO3- to produce isotopically light N2, the maximum δ15N- N2 of 1.2‰ coincided with the maximum nitrogen gas excess. This observation suggests that when denitrification occurred, all available nitrate was converted to N2 and that the nitrate consumed was substantially enriched in 15N relative to natural isotopic abundances. We will explore our data set using a mass balance approach.
OS33C-1371
Inhibition of Gypsum Precipitation for Subsurface Barrier Formation
As coastal and arid climate regions around the world experience population growth and changing climates, they face the issue of seawater intrusion into freshwater aquifers. Seawater intrusion is the result of overexploitation of freshwater resources and leads to contamination of freshwater aquifers rendering them unfit for drinking water without treatment. It is most common in locations such as Yucatan Peninsula, Mediterranean, Southeast and Southwest areas of the United States, Middle East, and many arid to semi-arid islands. Induction of gypsum precipitation for the creation of an in-situ subsurface barrier may be effective to efficiently prevent the seawater intrusion into coastal aquifers. Gypsum precipitation in the barrier must occur slowly, however, over a wide area. Laboratory research was conducted for the purpose of finding an inhibitor which decreases the rate at which gypsum (CaSO4*2H2O) precipitates in high salinity solutions at favorable temperatures. A series of Non-Inhibited experiments were performed at temperatures 10° C, 15° C, 20° C and 25° C to serve as a comparison to inhibited experiments. Organic Polymers, PEO (Poly(ethylene-oxide)) and PMAA (Poly(methyl-acrylic acid)), were shown in prior laboratory research to be most effective, therefore they were used in this study. Samples were taken periodically to determine average number of gypsum particles per mL and crystal size and geometry. Alteration of crystal habit occurred due to the adsorption of the inhibitors on the 111 (Ca) face of nucleating crystals. This resulted in precipitation of more tabular than needle-like (orthorhombic) crystals. Inhibition times of days could be achieved with less than 100ppm of the better inhibitors (i.e. PEO, PMAA) in 50:50 Seawater: Freshwater solutions.
OS33C-1372
Oceanic Gas Bubble Measurements Using an Acoustic Bubble Spectrometer
Gas bubble injection by breaking waves contributes significantly to the exchange of gases between atmosphere and ocean at high wind speeds. In this respect, CO2 is primarily important for the global ocean and climate, while O2 is especially relevant for ecosystems in the coastal ocean. For measuring oceanic gas bubble size distributions, a commercially available Dynaflow Acoustic Bubble Spectrometer (ABS) has been modified. Two hydrophones transmit and receive selected frequencies, measuring attenuation and absorption. Algorithms are then used to derive bubble size distributions. Tank test were carried out in order to test the instrument performance.The software algorithms were compared with Commander and Prosperetti's method (1989) of calculating sound speed ratio and attenuation for a known bubble distribution. Additional comparisons with micro-photography were carried out in the lab and will be continued during the SPACE '08 experiment in October 2008 at Martha's Vineyard Coastal Observatory. The measurements of gas bubbles will be compared to additional parameters, such as wind speed, wave height, white cap coverage, or dissolved gases.
OS33C-1373
Spatial and temporal variability of phytoplankton chlorophyll and carbon in the equatorial Pacific, 2005 to 2008: Observations from ships and satellites.
A database of chlorophyll fluorescence, particulate backscatter and beam attenuation was constructed from 17 cruises spanning the equatorial Pacific between August 2005 and February 2008. These optical measurements serve at least two important purposes. First, they can be used to document changes in phytoplankton abundance and physiology in a globally significant ecosystem. Second, they represent an important validation database for satellite observations that form the core of emerging primary productivity models. The data consist of CTD profiles from the surface to 1000m at least every degree of latitude between 8N and 8S, from near the Galapagos to beyond the date line. The optical data were calibrated with in situ samples of chlorophyll and particulate organic carbon (POC) from 4 of the 17 cruises. Chlorophyll concentration was derived from a multiple linear regression of chlorophyll fluorescence, time of day and depth, to account for photoinhibition of the fluorescence signal near the surface during the day. POC was derived from both particulate backscatter and beam attenuation. The optical data were then used to produce maps and latitude-depth sections of chlorophyll and POC for cruises where no in situ samples exist. In the eastern and central equatorial Pacific, phytoplankton chlorophyll to carbon ratios decreased by 30 to 50 percent during the weak El Nino conditions of 2006-2007. This change was due mostly to a decrease in chlorophyll, while POC remained relatively constant. In the western Pacific, the decrease in chl:C was absent, but an increase occurred in early 2008 when the system recovered from El Nino. Changes in chl:C, mostly indicative of photoadaptation, were also observed with depth and latitude as upwelled waters from the equator move poleward. Satellite-based maps of chlorophyll, phytoplankton C and chl:C were also produced and compared with the in situ optical measurements, with mostly good agreement.
OS33C-1374
Regulation of a Novel Phosphatase Common in Marine Bacteria
The assessment of the in situ phosphorus (P) status of marine bacteria will help us understand the P cycle in the ocean and its effect on bacterial ecology and diversity. A possible marker of P status is phoX, a recently discovered, widespread phosphatase gene in marine bacteria. This gene has been shown to encode for the major P-regulated phosphatase in Silicibacter pomeroyi. However, further information about the regulation of this gene is required in order to use it as a molecular marker of P status. The aim of this study was to elucidate whether it PhoX is solely responding to P starvation or it is induced upon other conditions. We investigated the effect of carbon and iron limitation and the addition of organic P on phoX expression and activity. S. pomeroyi was grown in minimal media in the presence and absence of phosphate, glucose, and glucose-6-phosphate (G6P), and under low and high iron conditions. Growth and phosphatase activity were monitored and phoX gene expression was determined by RT-PCR techniques. phoX expression was induced in all the treatments that lacked phosphate, but not by the presence of G6P, indicating that PhoX is not regulated by organic P substrates. The effect of carbon limitation on phoX expression could not be assessed, since there was no growth in any of the treatments that lacked glucose. Iron limitation in the presence of phosphate had no effect on PhoX activity, but the effect on phoX gene expression is still unclear and requires further study.
OS33C-1375
Diatom Community Response to Global Change
Diatoms are ubiquitous components of marine and freshwater environments and are responsible for nearly a quarter of the world's primary production. These microscopic algae are excellent indicators of environmental change and are routinely used as indicators of water quality. Diatom frustules have also been used to infer past climate change. With anticipated increases in atmospheric CO2 and eutrophication, understanding the contribution by diatoms as sinks for carbon in the world's oceans and estuaries is crucial. Benthic diatoms are especially significant in this respect due to their interactions with both atmospheric and sedimentary carbon cycling. We investigated changes in marsh sediment diatom community structure in response to elevated atmospheric carbon dioxide and nitrogen input. Twenty plots of brackish marsh were enclosed in environmental chambers and exposed to two levels of atmospheric CO2 (ambient and elevated) crossed with a nitrogen-addition treatment (2 x 2 factorial) beginning in May 2006. DNA was extracted from sediment samples obtained from environmentally controlled marsh plots in June, 2008. Using diatom-specific primers, the diatom community was amplified by PCR and evaluated by denaturing gradient gel electrophoresis (DGGE). The diatom community composition was then compared across the four treatments (Amb, Amb+N, Elev, Elev+N) using multivariate statistical methods. Multidimensional scaling plots revealed clear grouping of samples according to treatment. A global analysis of similarity test was significant, as were all pairwise comparisons of treatments. The greatest changes in community structure occurred in the elevated CO2 group. In contrast, Amb+N was more similar to Elev+N, suggesting that nitrogen effects may mask elevated CO2 effects on diatom community structure in these plots.
OS33C-1376
The Market as an Institution for Zoning the Ocean
In recent years, spatial conflicts among ocean users have increased significantly, particularly in the coastal ocean. Ocean zoning has been proposed as a promising solution to these conflicts. Strikingly, most ocean zoning proponents focus on a centralized approach, involving government oversight, planning, and spatial allocations. We hypothesize that a market may be more efficient for allocating ocean space, because it tends to put ocean space in the hands of the highest valued uses, and it does not require public decision-makers to compile and analyze large amounts of information. Importantly, where external costs arise, a market in ocean space may need government oversight or regulation. We develop four case studies demonstrating that private allocations of ocean space are taking place already. This evidence suggests that a regulated market in ocean space may perform well as an allocative institution. We find that the proper functioning of a market in ocean space depends positively upon the strength of legal property rights and supportive public policies and negatively upon the number of users and the size of transaction costs.