[Yellotang]

Try this one out, this is his article on pore water dynamics.

Effect of emersion and immersion on the porewater nutrient dynamics of an intertidal sandflat in Tokyo Bay
Tomohiro Kuwaem4.cor*m4.cor*, mailto:kuwae@ipc.pari.go.jpmailto:kuwae@ipc.pari.go.jp, a, Eiji Kibeb and Yoshiyuki Nakamurahit2hit2a

a Coastal Ecosystems Division, Port and Airport Research Institute, 3-1-1, Nagase, Yokosuka 239-0826, Japan
b Karatsu Port Construction Office, Ministry of Land, Infrastructure and Transport, 3-216-1, Futago, Karatsu, Saga 847-0861, Japan

Received 22 January 2002; accepted 20 November 2002. ; Available online 15 July 2003.


Abstract
Porewater nutrient dynamics during emersion and immersion were investigated during different seasons in a eutrophic intertidal sandflat of Tokyo Bay, Japan, to elucidate the role of emersion and immersion in solute transport and microbial processes. The water content in the surface hit1hit1sedimenthit3hit3 did not change significantly following emersion, suggesting that advective solute transport caused by water table fluctuation was negligible. The rate of change in nitrate concentration in the top 10 mm of hit2hit2sediment****4hit4 ranged from -6.6 to 4.8 mol N l-1 bulk sed. h-1 during the whole period of emersion. Steep nutrient concentration gradients in the surface hit3hit3sedimenthit5hit5 generated diffusive flux of nutrients directed downwards into deeper hit4hit4sediments,hit6hit6 which greatly contributed to the observed rates of change in porewater nutrient concentration for several cases. Microbial nitrate reduction within the subsurface hit5hit5sedimenthit7hit7 appeared to be strongly supported by the downward diffusive flux of nitrate from the surface hit6hit6sediment.hit8hit8 The stimulation of estimated nitrate production rate in the subsurface layer in proportion to the emersion time indicates that oxygenation due to emersion caused changes in the hit7hit7sedimenthit9hit9 redox environment and affected the nitrification and/or nitrate reduction rates. The nitrate and soluble reactive phosphorus pools in the top 10 mm of hit8hit8sedimenthit10hit10 decreased markedly during immersion (up to 68% for nitrate and up to 44% for soluble reactive phosphorus), however, this result could not be solely explained by molecular diffusion.
Author Keywords: interstitial nutrients; microbial processes; diffusive fluxes; oxygenation; nitrogen and phosphorus cycles; eutrophication; Banzu tidal flat


1. Introduction
Semi-diurnal movement of tidal water alters biotic and abiotic environments in intertidal sediments over short time intervals ([Alongi, 1998]). When sediments are exposed to air, the water table drops due to drainage and evaporation ( [Anderson and Howell, 1984, Agosta, 1985 and Howes and Goehringer, 1994]). During tidal flooding, in turn, vertical infiltration of tidal water controls interstitial water levels ( [Hemond and Fifield, 1982]). Rhythmic emersion and immersion can also mediate the below-ground transport of nutrients and metabolic products ( [Harvey and Odum, 1990 and Dolphin, Hume and Parnell, 1995]). The role of advective solute transport in the distribution of nutrients has mainly been reported for salt marsh creekbanks (e.g. [Agosta, 1985, Yelverton and Hackney, 1986 and Howes and Goehringer, 1994]) and sandy beaches ( [McLachlan and Illenberger, 1986 and Uchiyama, Nadaoka, Rölke, Adachi and Yagi, 2000]). In contrast, the movement of water and solutes associated with a fall in the water table has been reported to be minor in non-vegetative intertidal flats due to the development of a capillary fringe ( [Drabsch, Parnell, Hume and Dolphin, 1999]).
The biogeochemistry of intertidal sediments during immersion has been well studied in relation to the sediment–water column exchange of nutrients (e.g. [Falcão and Vale, 1990, Middelburg, Klaver, Nieuwenhuize and Vlug, 1995, Asmus et al., 1998, Kuwae, Hosokawa and Eguchi, 1998, Mortimer et al., 1999, Cabrita and Brotas, 2000 and Falcão and Vale, 1998]). However, little is yet known of the dynamics of porewater nutrients in such sediments during emersion or transitional periods ( [Rocha, 1998, Rocha and Cabral, 1998 and Usui, Koike and Ogura, 1998]). During emersion, the penetration of oxygen into sediments may increase ( [Brotas, Amorim-Ferreira, Vale and Catarino, 1990]), causing changes in the redox environment ( [Koch, Maltby, Oliver and Bakker, 1992]). This oxygenation affects the rates and pathways of nutrient flow ( [Kerner, 1993]) related to, e.g. aerobic nitrifiers and anaerobic denitrifiers ( [Henriksen and Kemp, 1988, Seitzinger, 1988 and Parkin, 1990]). In addition, the absence of overlying waters indicates no efflux of nutrients from the sediment, which will either accumulate or be consumed within the sediments. [Rocha, 1998] has shown that total (dissolved and exchangeable) sedimentary ammonium accumulated during emersion. [Usui, Koike and Ogura, 1998] have reported that porewater nitrate decreased remarkably during the initial 3–4 h after the onset of emersion. On the other hand, at immersion, mixing of porewater with overlying water can result in drastic changes in the interstitial nutrient pool ([Rocha, 1998 and Rocha and Cabral, 1998]). [Rocha and Cabral, 1998] have shown that approximately 80% of the nitrate pool was flushed during immersion.
This paper reports the dynamics of porewater nutrients induced by tidal cycles in a eutrophic intertidal sandflat of Tokyo Bay, Japan. To our knowledge, this is the first report dealing with tide-induced temporal changes in the concentrations of three porewater nutrient species (nitrate, ammonium, and soluble reactive phosphorus) during different seasons. Special emphasis is placed on (1) the influence of diffusive fluxes and advective transport on the pool size of nutrients during emersion and immersion; and (2) the role of emersion in the microbial processes, which affect porewater nutrient concentration.
2. Materials and methods

2.1. Study site
The Banzu intertidal sandflat is located on the east coast of Tokyo Bay (Japan) and covers an area of 7.6 km2 (Fig. 1). Tokyo Bay receives a nutrient loading from a population of ca. 26 million humans (320 t N day-1 for total nitrogen and 26 t P day-1 for total phosphorus, [Nakanishi, 1993]), and is subjected to heavy eutrophication and anoxia in bottom waters over a wide area during the summer. Tides are semi-diurnal with amplitudes from 0.5 to 1.6 m ([Guo and Yanagi, 1994]). The Obitsu River has a watershed area of 267 km2 and 2.5–3.0 m3 s-1 of normal discharge. The sampling site (35°24.2'N, 139°54.2'E) is 30 cm above mean sea level, experiencing emersion and immersion during each tidal phase. The slope of the seabed at the sampling site is very low (0.07 cm m-1). Sediments are characterized by well-sorted fine sand (99.6% sand and 0.4% silt) with a median grain size of 170 m ([Kuwae and Hosokawa, 1999]). Organic carbon and total nitrogen contents at 0–20 mm depth, measured from August 1998 to September 1999, were 0.981±0.047 mg C g-1 dry wt. (mean±SE, n=45) and 0.214±0.013 mg N g-1 dry wt. (mean±SE, n=45), respectively. There is no macro-vegetation, and pennate diatoms dominate the epibenthic microalgal flora. Gross primary production in September 1997, measured by sediment core incubation under both light and dark conditions was 151.9±17.1 mg O2 m-2 h-1 (mean±SE, n=3) ([Kuwae, Hosokawa and Eguchi, 1998]).

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Fig. 1. Location of the study site (Banzu intertidal sandflat, Tokyo Bay). Dotted line indicates the lowest tidal level.

We conducted four seasonal surveys on the spring tide: March 1998, May 1998, September 1998, and November 1998. One tidal cycle was sampled in each survey. The tidal range on the survey days ranged from 1.1 to 1.6 m. No rainfall was recorded during surveys of the tidal flat.
2.2. Hydrology
To track fluctuations in the water table depth, a small well was dug into the sediment a few days before each survey. A polyvinyl chloride pipe (4.5 cm internal diameter (i.d.)), with holes drilled and covered with a nylon mesh, was placed in the well to a depth of 25 cm. Water levels in the pipe were measured using a float.
The diffusive flux of nutrients from the sediment during immersion was calculated according to Fick's first law of diffusion ([Berner, 1980]):
J=-DS(dC/dx),
where J is the diffusive efflux, is the porosity at the sediment surface (0–10 mm), x is the vertical axis and DS is the whole sediment diffusion coefficient. DS was calculated from the temperature corrected diffusion coefficient in particle-free water (D0) ([Li and Gregory, 1974]) and tortuosity reported by [Sweerts, Kelly, Rudd, Hesslein and Cappenberg, 1991]. dC/dx is the concentration gradient across the sediment–water interface. dC/dx was calculated by linear interpolation between the overlying water concentration at the sediment surface and the porewater concentration in the 0–2.5 mm section assigned to 1.25 mm depth.
2.3. Sediment characteristics
At each survey, the photon flux of photosynthetically available radiation (PAR), sediment temperature, redox potential (Eh), porosity, water content, chlorophyll a, and macrofauna were measured. PAR was continuously measured during each sampling time using a Biospherical quantum sensor. Sediment temperature and Eh were measured in situ at intervals of 2–5 cm during emersion using a temperature probe (RT-10; Tabai Espec) and a platinum redox electrode (HM-14P; TOA). For the measurement of porosity and water content, core samples were taken to a depth of 1 cm with acrylic tubes (4.5-cm i.d.) during both emersion and immersion. Porosity and water content were determined by the weight loss after drying wet sediments at 90 °C for 24 h. The remainder of each emersion period core sample was used for the analysis of chlorophyll a in the sediment, extracted using 90% acetone solution, spectrophotometrically analyzed (U-3200; Hitachi) according to [Lorenzen, 1967]. For the measurement of macrofaunal abundance, core samples (n=8) were taken to a depth of 20 cm with acrylic tubes (25 cm long×8.6 cm i.d.). The sediment in each core was sieved (1 mm mesh) to retain macrofauna. Macrofauna were preserved in neutralized 10% formalin–seawater solution and stored for later counting.
2.4. Porewater solutes
On each survey day, sediment samples were taken every one to several hours from the onset of emersion to after immersion. Sediment cores (n=3) were collected randomly from the site (2 m×2 m) at each sampling, using an acrylic corer (8.6 cm i.d.×25 cm long). Sediment cores were immediately cut in situ into 2.5–10 mm segments, and macrofauna were removed from sliced sediments. The sliced sediments were immediately fixed by dried ice in order to stop biogeochemical reactions. Fixed sediments were thawed and filled in syringes (10 ml) and were centrifuged for 10 min at 2000 rpm (580×g) at ambient temperature. Extracted water was filtered through a Millipore HA filter. Filtered water was immediately frozen for the later analysis of nutrients and salinity. Ammonium, nitrate, nitrite, and soluble reactive phosphorus (SRP) were measured using standard colorimetric techniques ([Strickland and Parsons, 1972]) on an analyzer (TRAACS-800; Bran+Luebbe). Salinity was measured using a conductivity electrode (9382-10D; Horiba).
2.5. Statistical analysis
Linear regression was used to calculate the rate of change in porewater nutrient concentrations over the whole period of emersion. A one-way ANOVA was used to examine statistical differences in porewater nutrient concentrations between the last samples collected before immersion and the first samples collected after immersion. Data sets were tested for homogeneity of variances (Hartley test), and the log-transformed values were used if needed for a normal distribution.
3. Results

3.1. Water table dynamics
The sediment water table was measured by using a well dropped gradually following emersion (Fig. 2). Slightly before the onset of the next immersion, the water table dropped to its lowest level, and then rose steeply. This pattern was observed for all cases (not shown). The greatest water table drop (7.8 cm) occurred in summer (September 1998), and the smallest (2.7 cm) in winter (March 1998).

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Fig. 2. Representative data (May 1998) for the fall and rise of water table depth (cm) measured by using a small well during emersion.


3.2. Sediment characteristics
The sampling days of March 1998 and May 1998 were cloudy, showing low PAR averages during the sampling time; the remainder showed high PAR values (Table 1). During emersion, Eh was >0 mV for all seasons and depths, except below 5 cm in summer, when the mean sediment temperature in the top 10 cm reached a maximum (30.1 °C) (Fig. 3). Although there was a measurable decline in the water table level during emersion, no statistical differences in the water content and porosity were observed between emersion and immersion for any season (P>0.05) (Table 1). Within the samples studied, the porosity and water content ranged from 44.6±0.4 to 47.7±1.0% (mean±SE) and 23.6±0.3 to 25.9±0.8% (mean±SE), respectively. Salinity of the porewater was close to that of tide water. The porewater salinity never varied more than 3.0 from the tide (surface) water salinity during the study. This strongly suggests that there was no significant ground water input. The chlorophyll a content in the top 10 mm of sediments ranged from 1.15±0.16 to 8.33±0.68 g cm-3 (mean±SE) (Table 1). The polychaete Armandia sp. and the bivalve Ruditapes philippinarum dominated infauna, whereas the gastropod Batillaria cumingii dominated epifauna (Table 2).

Table 1. Sediment water content during emersion and immersion, sediment chlorophyll a content, and photosynthetically available radiation (PAR) during sampling time
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Fig. 3. Vertical profiles of sedimentary temperature (°C) and redox potential (mV) during emersion. Seasons: March 1998 (); May 1998 (); September 1998 (); and November 1998 ().


Table 2. Total densities (mean±SE) and dominant species of macrofauna (>10%) at the sampling site (depth: 0–20 cm)
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3.3. Porewater nutrient profiles
Depth profiles of porewater nitrate+nitrite (hereafter, nitrate) and SRP during emersion showed marked concentration gradients with depth (Fig. 4); nitrate and SRP concentrations peaked in the uppermost layer of sediments for all the seasons, and below this they sharply declined until a depth of 30 mm. Nitrate concentrations in the upper layer peaked in November 1998 and were minimal in September 1998. Ammonium profiles either exhibited a stable pattern, or a gradual decrease in concentration from the sediment–water interface. All the nutrient concentrations in the uppermost sediments, except for nitrate in September 1998, were always higher than those in overlying waters.

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Fig. 4. Temporal changes in interstitial nutrient concentrations (M). Error bars indicate standard errors (n=3). Only data from the top 20 mm sediment are shown to improve clarity. Note that the depth interval for March 98 is different from the other seasons.


3.4. Nutrient dynamics during emersion
Remarkable changes in nitrate and SRP concentrations in the upper layers were observed during emersion, whereas deeper layers (>10 mm) showed only slight changes (Fig. 4). A linear regression analysis revealed that the rates of change in nitrate concentration during emersion were statistically significant (P<0.05) for most samples of the top 10 mm sediments, ranging from -6.6 to 4.8 mol N l-1 bulk sed. h-1 (Fig. 5). These rates were positive in May 1998, negligible in March 1998, and negative in both September 1998 and November 1998. However, the rates of change in nitrate concentration approached zero below 20 mm depth. Ammonium concentration decreased with time, except for the summer samples, where they increased (Fig. 4). The rates of change in ammonium concentration were near constant with depth, in contrast to the rates measured for other nutrient species ( Fig. 5). The maximum rate of decrease in ammonium concentration (-22.9 mol N l-1 bulk sed. h-1) was observed in the deeper layer (20–25 mm) in spring, whereas the maximum rate of increase (11.8 mol N l-1 bulk sed. h-1) was observed in the deepest layer (90–100 mm) in summer (not shown). Few statistically significant rates of change in SRP concentration were measured (Fig. 5); the greater changes were observed in the upper layers than in the deeper layers (>30 mm).

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Fig. 5. Rates of change in interstitial nitrate+nitrite (a), ammonium (b), and SRP (c) concentrations (mol N or P l-1 bulk sed. h-1) during the whole period of emersion. Seasons: March 1998 (); May 1998 (); September 1998 (); and November 1998 (). The rates were calculated using a linear regression analysis. Plots with asterisks indicate statistically significant rates of change at the 5% level.