162 J
. Stimson, S.T. Larned J. Exp. Mar. Biol. Ecol. 252 2000 159 –180
sediments are high enough to sustain the growth of macroalgae, and whether these rates are higher than on other tropical reefs that do not support high standing crops of
macroalgae. A number of observations from previous studies suggest that growth and persistence of macroalgae on Kaneohe Bay reefs is due in part to the efflux of sediment
nutrients. Dictyosphaeria cavernosa, Gracilaria salicornia and Kappaphycus striatum maintained in flow-through culture could not sustain net growth in unenriched seawater,
but did grow when provided with DIN-enriched seawater Larned and Stimson, 1996; Larned, 1998. Results from a field experiment indicated that D
. cavernosa can sustain net growth when exposed to sediment-derived nitrogen, but cannot grow when isolated
from the sediment Larned and Stimson, 1996. DIN concentrations measured within and below thalli of the three algal species were significantly higher than DIN
concentrations adjacent to the thalli Larned and Stimson, 1996; Larned, 1998. While these observations imply that the success of macroalgae is related to sediment-derived
nutrients, it is not known if efflux rates in Kaneohe Bay are elevated relative to those in other coral reef systems, and if so, whether the elevation is due primarily to sewage
release, which ended 20 years ago, or to the current influx of nutrients to the sediments. Smith et al. 1981 predicted that efflux from the organic matter-rich sediments would
continue to add nutrients to the water column for several years following sewage diversion, then the sediment nutrient reservoir would be depleted. This prediction does
not seem compatible with our observation that DIN efflux is presently high enough to support a substantial macroalgal standing stock on reef slopes. In the present study, we
report rates of DIN and phosphate flux across the surface of reef and lagoon sediments in southern Kaneohe Bay. We also report rates of sedimentation of particulate nitrogen to
the lagoon floor for comparison with rates measured shortly before sewage diversion Taguchi, 1982. Finally, we compare efflux and sedimentation rates in Kaneohe Bay
with rates reported at other coral reef sites that have not been subject to high levels of anthropogenic nutrient enrichment.
2. Methods
2.1. Study sites Samples for measurements of DIN efflux rates, porewater DIN concentrations and
water column DIN concentrations were collected at reef slope, reef flat and water column sites in southern Kaneohe Bay, Oahu, Hawaii, near the Hawaii Institute of
Marine Biology HIMB. Samples for the determination of DIN concentration in porewater and in water just above the sediments ‘near-substratum’ were taken from the
reef slope of the northeast windward fringing reef of Moku o Loe, the site of HIMB Fig. 1 between October 1993 and April 1998. Water column samples were taken from
two sites. Most water column samples were from a site 4 m windward of the northeast reef slope of Moku o Loe at a depth of 2 m Site 1, Fig. 1. These samples were
collected from January 1994 to January 1998. Additional water column samples were collected from August 1996 to April 1997 at a site 1 km windward of Moku o Loe, 1 km
J . Stimson, S.T. Larned J. Exp. Mar. Biol. Ecol. 252 2000 159 –180
163
Fig. 1. Map of Kaneohe Bay and Moku o Loe, Hawaii. Unstippled areas represent fringing reef, patch reefs and the barrier reef. Sites 1 and 2 are sites of water-column nutrient samples, efflux rate measurements and
sedimentation rate measurements.
from the nearest shoreline and at the same 2 m depth as the near-reef water column samples Site 2, Fig. 1.
2.2. Water column and porewater sampling Water-column samples and water samples from just above the sediment surface 1–15
cm were collected with acid-washed 60-ml syringes fitted with Pasteur pipettes. All water samples were filtered Whatman GF C into acid-washed polyethylene bottles and
frozen within 10 min of collection. Analyses of nitrate 1 nitrite, ammonium, phosphate and total dissolved nitrogen concentrations in water samples were performed on a
Technicon Autoanalyzer II by Analytical Services, University of Hawaii.
Porewater samples from the reef slope and reef flat sediments were collected using miniature wells similar to those of Sansone et al. 1988. Sample depths ranged from 5
to 100 cm. The reef sediments are very heterogeneous, with particles ranging from silt to coral branches. The presence of the large fragments of coral and shell precluded the
insertion of coring devices more than 5–10 cm, therefore, a 1.5-cm diameter sharp- tipped rod was pushed into the sediments to a predetermined depth to make a passage.
The rod was then withdrawn and an 8-cm long tubular intake and well casing of approximately the same diameter as the rod was inserted into the passage created by the
164 J
. Stimson, S.T. Larned J. Exp. Mar. Biol. Ecol. 252 2000 159 –180
rod Fig. 2. The intakes were constructed by wrapping 8 3 15 cm sheets of plankton netting and window screen around a 4-mm diameter plexiglass tube. Numerous small
holes had been drilled into the plexiglass tube prior to wrapping it with the screen. Both ends of the filter and the filter end of the plexiglass tube were sealed with polyolefin
Fig. 2. Design of well used for sampling sediment porewater.
J . Stimson, S.T. Larned J. Exp. Mar. Biol. Ecol. 252 2000 159 –180
165
adhesive ‘hot glue’. The plastic tube of the filter was attached to a length of flexible plastic tubing 3 mm internal diameter long enough to extend above the sediment
surface. The flexible tubing was then passed through a length of glass tubing, which was the same diameter as the filter. When the whole assembly was inserted into the hole in
the sediment, the glass tube formed a well casing and sealed the upper walls of the well. The tygon tubing extended 15–20 cm above the sediment and was capped until water
samples were to be drawn. The filter and tygon tubing were acid-washed prior to insertion into the sediment. To minimize the effects of disturbance created during
installation, the samples were taken 2 or more days after installation. To draw a sample, the cap on the tygon tubing was removed, an acid-washed syringe was attached to the
plastic tubing, a 5-ml volume was withdrawn and discarded, then a 30-ml sample was withdrawn. The sample was discharged through a GF C filter into an acid-washed
polyethylene bottle and frozen within 10 min.
Vacuum filtration of sediment samples collected near the porewater samplers with a piston corer 2.5 cm diameter, 60 cc volume yielded 22–24 ml of water per 100 ml of
undrained sediment, giving a minimum estimate of porosity of 23. Particle size distributions of these sediments were obtained using graded sieves; the modal size of
particles was retained on the 0.125-mm sieve 26 of weight, 8 was retained on the 0.625-mm screen and 2.5 was less than 0.625 mm. Given the small volume of the
water samples and the porosity of these sediments, it is unlikely that the shallowest wells 5–10 cm depth contained water from above the sediment surface. Measurements of
salinity have been conducted on porewaters drawn from wells on a nearby reef. These showed high salinities at depths of 1 and 2 m in four wells over a 300-h period,
suggesting little groundwater input to the system Tribble et al., 1992.
2.3. Efflux rates Efflux rates of DIN, phosphate and dissolved organic nitrogen DON from the
sediments of the windward reef flat of Moku o Loe, the windward reef slope of Moku o Loe and the Bay bottom were estimated by measuring the change in nitrate 1 nitrite,
ammonium, phosphate and DON concentrations in open-bottomed benthic chambers placed over sediment patches. These samples were collected between January 1994 and
April 1998. The sediment patches used on the reef flat and slope occur between limestone and coral outcrops. Chambers were installed on the windward reef slope of
Moku o Loe at 1–5 m depth. This depth range spanned the zone of highest densities of Dictyosphaeria cavernosa. Bay-bottom chambers were installed in very fine homoge-
neous sediment at a depth of 14 m at site 2 Fig. 1. Reef flat chambers were installed on patches of coarse sediment at | 1 m depth.
The chambers were created by pushing 30 cm long, 30 cm diameter sections of PVC pipe 5–10 cm into the sediment. After allowing 2 or 3 days for the sediments to recover
from the disturbance, the inside walls of the chamber were wiped clean, and 30 min later a clear plexiglass lid was clamped over the top of the chamber. The lid contained a
sampling port, an incurrent port for water replacement and a manually operated propeller, which was turned to stir the water prior to taking samples i.e. once per hour.
Stirring did not disturb the sediments. The small diameter of the chambers was dictated
166 J
. Stimson, S.T. Larned J. Exp. Mar. Biol. Ecol. 252 2000 159 –180
by the difficulty in finding large patches of unobstructed sediment on the reef flat and
2
slope. Chambers enclosed 0.075 m of sediments and, depending on the distance the pipe was inserted into the sediment, approximately 11 l of water. All sampling was done
during daylight hours and was started in the late morning hours. Water samples were withdrawn from the chambers by attaching an acid-washed
syringe to the excurrent tube, un-crimping the tubes on both ports and withdrawing approximately 60 ml from the chamber. The samples were taken to the surface, filtered,
and frozen as described above. The sediment enclosed by the chambers contained up to ten macroinvertebrate burrows, some of which were as large as 1 cm in diameter. The
numbers and pumping activity of these animals presumably influenced the concen- trations of dissolved nutrients in the chambers. Ambient water samples were collected
during nine of the efflux measurements by drawing water samples each hour from a position 10–15 cm above the sediment and approximately 1 m away from the chamber,
i.e. in water equivalent in depth and position to that in the chambers. The estimates of efflux into the chambers were computed by calculating the slope of the nutrient
concentration in the chambers versus elapsed time over the first 3 or 4 h of operation. This change in concentration and the volume and area of the chamber, were used to
22 21
calculate efflux rates in mmol m day
. This rate is regarded as a net rate of exchange. Additional measurements of efflux were made at two exposed sites on the windward
coast of Oahu southeast of Kaneohe Bay, Lanikai 1578439 W, 218249 N and Waimanalo 1578419 W, 218199 N. These sites are in lagoon-like settings near patches of coral and
macroscopic algae, with depths of 2–3 m. Each lagoon is protected by fringing reefs 100–300 m seaward, but wave action is stronger at these sites than in Kaneohe Bay and
the sediments are correspondingly coarser. Measurements at Lanikai and Waimanalo were made in April and September 1997.
2.4. Sedimentation rate Rates of sedimentation of particulate organic nitrogen PON from the water column
to reef slopes in southern Kaneohe Bay were estimated indirectly by placing cylindrical sediment traps at various depths in open water 1 km from the nearest reefs at site 2 Fig.
1. The traps were placed away from patch and fringing reefs in order to minimize the amount of material in traps that had been created on reefs and or resuspended from
reefs. An additional reason for estimating sedimentation at site 2 was that sedimentation rates had been measured at this site for a year in 1981, shortly after the diversion of
sewage effluent from southern Kaneohe Bay. A pilot study of sedimentation rates at different distances from the slope of the fringing reef of Moku o Loe indicated that
sedimentation rates at distances from 1 to 10 m off the reef were not higher than at comparable depths at site 2. The weight of sediment in traps placed directly on the reef
slope were two times those of traps . 1 m off the reef at the same depth. The water depth at site 2 is 14 m. The traps were positioned in the upper 10 m of the water column
2, 4, 7 and 10 m depth to reduce the contribution of sediment resuspended from the Bay bottom Taguchi, 1982. The depth range used for sediment traps corresponded with
the depth distribution of Dictyosphaeria cavernosa on reef slopes.
Sediment traps were cylindrical 7.3 cm diameter and 21 cm long and were deployed
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
