J . Stimson, S.T. Larned J. Exp. Mar. Biol. Ecol. 252 2000 159 –180
167
21
at flow-rates less than 5 cm s , conditions which should result in accurate estimates of
sedimentation rate Gardner, 1980. Traps were recovered after 24 h to minimize the effects of colonizing organisms. Sediment in the traps was filtered onto pre-combusted
GF F filters, rinsed, dried at 608C, and weighed. A subset of the filters were ground and 40-mg aliquots were used for CHN analysis with a Perkin-Elmer 2400 CHN Analyser.
Nitrogen content was only measured for samples collected between March and July 1997. The balance of the samples and the remainder of the ground samples were
weighed, combusted at 4508C 4 h and re-weighed for determination of AFDW. When sediment traps were collected, a 2-l sample of water was collected adjacent to each trap,
and was processed in the same way as the sediment trap samples. Suspended sediment
21
concentrations mg l in the traps were estimated from the 2-l water samples, and the
dry weights of sediment in the traps were corrected for the suspended material. Suspended sediment usually comprised , 4 of the trapped material. The amount of
material caught per day by sediment traps in this study is regarded as the trapping rate, consistent with the terminology of Taguchi 1982. This measure is distinct from the
sedimentation rate, which is the trapping rate corrected for the rate of trapping of resuspended material.
3. Results
The concentration of DIN in the water column adjacent to the windward reef slope of Moku o Loe site 1 was generally less than 0.5 mM throughout the year Fig. 3. The
Fig. 3. Water column concentrations of ammonium and nitrate 1 nitrite through time measured 2 m below the surface and 2–3 m horizontally from the windward reef slope of Moku o Loe.
168 J
. Stimson, S.T. Larned J. Exp. Mar. Biol. Ecol. 252 2000 159 –180
samples collected at site 2, 1 km windward of the reef slope, had significantly lower nitrate 1 nitrite values than those in the water column just off the reef slope site 1, and
both of these sites had significantly lower concentrations than the near-substratum samples Table 1. The concentration of ammonium within 15 cm of the sediments on
the reef slope near-substratum was significantly greater than the concentrations at site 1 and site 2 Table 1. Phosphate concentrations were less than 0.2 mM at all sites.
Ammonium concentrations in porewater samples from reef slope sediment on the windward Moku o Loe fringing reef increased with sediment depth and reached a
maximum value of 60 mM at a depth of 50 cm Fig. 4a. The sediment is anoxic a few centimeters below the surface and hence the concentration of ammonium is 10–40 times
greater than the concentration of nitrate 1 nitrite. Nitrate 1 nitrite concentrations were generally less than 1 mM in porewater samples. Phosphate concentrations reached a
maximum of 6 mM at a depth of about 20 cm.
Ammonium concentrations in the benthic chambers installed on reef slopes increased linearly over the first 6 h of each sampling period Fig. 5. After 24 h, the ammonium
concentration in the chambers with the most rapid rate of efflux had approached 2 mM, and the rate of increase in concentration had slowed. The rate of change in concentration
in each chamber was calculated by regressing concentration on time and this change was then compared to the change in concentration in each of the nine sets of control samples.
The average slope for ammonium concentration versus time in the chamber incubations
21
0.14 mmol h , n 5 24 was significantly greater than the average slope of ammonium
21
concentration versus time for the control runs 0.02 mmol h , n 5 9 t 5 2.24,
P , 0.05, df 5 31. The average slope of ammonium concentrations versus time in the ambient water samples was not significantly different from 0. The average slopes of the
nitrate 1 nitrite concentrations in control and chamber incubations were not significantly different from one another, and the average slope for the nitrate 1 nitrite concentration in
the control incubations was not significantly different from zero.
Ammonium and nitrate 1 nitrite efflux rates measured on the reef slope varied greatly
Table 1 Concentration of dissolved inorganic nitrogen mM in samples from the water column and positions close to
a
the sediments mean, standard deviation and sample size Water column
Water column 1–15 cm above
Central So. | 3 m above
sediments Bay
reef slope of reef slope
Site 2 Site 1
near-substratum NH
NO 1 NO NH
NO 1 NO NH
NO 1 NO
4 3
2 4
3 2
4 3
2
Mean 0.13
0.02 0.21
0.25 0.36
0.36 S.D.
0.10 0.02
0.12 0.29
0.17 0.21
Sample size 14
14 92
92 33
33
a
Comparisons of the means were performed using transformed concentrations, lnX 1 1. Differences among sites in ammonium and nitrate 1 nitrite concentrations were significant one-way ANOVAs, P , 0.005
for each comparison. A posteriori Tukey tests indicated that nitrate 1 nitrite concentrations were significantly different between all sites and that ammonium values differed significantly between the near-substratum zone
and the water column at sites 1 and 2 P , 0.05 for each comparison.
J .
Stimson ,
S .T
. Larned
J .
Exp .
Mar .
Biol .
Ecol .
252 2000
159 –
180
169 Fig. 4. a Comparison of ammonium and nitrate 1 nitrite porewater concentrations in sediments on the windward reef slope and reef flat of Moku o Loe. Values are
means, bars are 1 S.E., numbers refer to sample sizes. Reef flat values are from the present study, and Haberstroh 1994, Sansone et al. 1988 and Tribble et al. 1988. b Porewater ammonium concentrations at other tropical sites. The values plotted have been computed by averaging across studies within 3 depth classes. The
values in the 1.5–4.5 m depth class are equivalent in depth to Kaneohe Bay reef slopes, and are from Boon 1986, Corredor and Morell 1985, Entsch et al. 1983, Johnstone et al. 1989 and Williams et al. 1985. The values in the 10–13 m depth class are equivalent in depth to the Kaneohe Bay lagoon floor and are from Boon
1986, Capone et al. 1992, Boucher and Clavier 1990 and Charpy-Roubaud et al. 1996. The 20–40 m depth values are from Capone et al. 1992, Corredor and Capone 1985, Pigott 1977 and Szmant and Forrester 1996. Error bars are not shown for some data points representing results from depths less than 5 cm. Points
without error bars at depths greater than 5 cm represent single observations.
170 J
. Stimson, S.T. Larned J. Exp. Mar. Biol. Ecol. 252 2000 159 –180
Fig. 5. Average concentration of NH in domes over time 61 S.E.. Concentration values were included in
4
this figure if they were measured within 20 min of the plotted time. The concentration values are from the operation of 25 domes on the reef slope. Numbers on the figure represent the sample sizes on which the
statistics are based.
among runs, the standard deviations were large and some values were negative, but the frequency distribution of efflux values for both ammonium and nitrate 1 nitrite were not
skewed Fig. 6, Kaneohe Bay reef slopes, , 5 m depth. The mean ammonium efflux
22 21
22 21
rate was 490 mmol m day
equivalent to 6.9 mg N m day
, Table 2 and was significantly different from zero. Nitrate 1 nitrite efflux rates were generally lower than
22 21
ammonium efflux rates Fig. 6, and averaged 123 mmol m day
equivalent to 1.7
22 21
mg N m day
, Table 2. This rate was not significantly different from zero. Water samples from 8 of the reef slope chambers were analyzed for total dissolved nitrogen
TDN, and DON efflux rates were then calculated by subtracting DIN from TDN
22 21
concentrations. The average DON efflux rate, 1757 mmol m day
, was nearly three times greater than the average DIN efflux rate. The average efflux rate of all three forms
22 21
of nitrogen combined was 32.2 mg N m day
. Efflux rates varied widely, because they were assessed at sites with heterogeneous sediments, over a range of depths on the
slope, in different seasons, and varying numbers of burrowing animals occupied the sediments enclosed by the chambers.
Efflux rates were also measured in three other environments for comparison with Kaneohe Bay’s reef slope environment: the Bay bottom at site 2, the Moku o Loe reef
flat, and the lagoons at Lanikai and Waimanalo. DIN efflux rates measured at the Bay bottom were much greater than those measured on the Moku o Loe reef slope Table 2.
DIN efflux from the shallow, coarse sediments of the Moku o Loe reef flat 1 m depth, and Lanikai and Waimanalo 2–3 m depth were substantially lower than those
measured on the reef slope or the Bay bottom Table 2.
The average sediment trapping rate at site 2 in South Kaneohe Bay over a 1-year
J . Stimson, S.T. Larned J. Exp. Mar. Biol. Ecol. 252 2000 159 –180
171
Fig. 6. Comparison of efflux rates in south Kaneohe Bay with efflux rates measured at other tropical reef sites. Reef slope values are from this study and can be compared to the values for ‘Other reefs , 5 m’ reported in
Hansen et al. 1987, Johnstone et al. 1989, Iizume cited in D’Elia and Wiebe 1990 and Williams et al. 1985. Deep Kaneohe Bay lagoon values are from Harrison 1981 and this study and can be compared with
the vales for ‘Other reefs 10–16 m’ reported in Boucher and Clavier 1990, Boucher et al. 1994, Fisher et al. 1990, Hansen et al. 1987 and Harrison 1983. Reef flat values are from Haberstroh 1994 and the
present study.
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period was linearly related to depth trapping rate g dry sediment m day
5 0.62 depth m 1 3.51, standard error of slope 5 0.19, t-test of H
b 5 0: t 5 3.24, P , 0.001,
o
n 5 224 over a range of depths from 3 to 10 m. The slope of the relationship between organic content of the sediments and depth was not significantly different from zero for
a subset of 134 of the samples. The mean organic content was 29.2 standard deviation 5 14.1, n 5 134. PON concentrations were measured in 41 trap samples
collected from 3 to 10 m depth at site 2; there was no significant regression between
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PON concentration and depth, and the average trapping rate was 61 mg N m day
standard deviation 5 33, n 5 41 Table 3. Trapping rates potentially overestimate rates of PON sedimentation in Kaneohe Bay because resuspension may be high in the shallow
water column. The estimated rate of sedimentation of new material is much lower
22
according to Taguchi’s 1982 formula, which corrects for resuspension: 12 mg N m
21
day .
4. Discussion
