Introduction
Coral reefs provide critical shoreline protection and important ecosystem
services, such as marine habitat, and support local economies through
tourism, fishing, and recreation (Hughes et al., 2003; Ferrario et al.,
2014). However, coral reefs are being threatened by global climate change
processes, such as increasing temperatures, sea-level rise, and ocean
acidification (OA), caused by uptake of atmospheric carbon dioxide into the
ocean (Orr et al., 2005). These effects are often compounded by local
stressors such as over-fishing, sedimentation, land-based sources of
pollution, and coastal acidification (Knowlton and Jackson, 2008) that can
result from freshwater inflow, eutrophication, and/or coastal upwelling.
These stressors can lead to a decrease in reef health by removing grazing
fish, decreasing calcification rates, and increasing nutrient and contaminant
concentrations, thereby shifting the balance between reef accretion and
erosion. However, isolating the effects of these stressors is difficult
without establishing the biological and physical controls on community
calcification and production. This is particularly challenging for coral
reefs adjacent to densely inhabited shorelines, where freshwater fluxes can
deliver excess nutrients. In turn, this can lead to coastal acidification
caused by eutrophication and enhanced respiratory processes that release
CO2 and increase coastal water acidity (e.g., Cai et al., 2011; Strong
et al., 2014), outbreaks of harmful algal blooms (Anderson et al., 2002), and
decreased coral abundance and diversity (Fabricius, 2005; Lapointe et al.,
2005). In many cases, eutrophication can alter ecosystem function and
structure by shifting reefs from coral to algae dominated (Howarth et al.,
2000; Andrefouet et al., 2002; Hughes et al., 2007). Changes in community
structure can have profound impacts on coral reef metabolism and reef carbon
chemistry dynamics (e.g., Page et al., 2017), which are ultimately linked to
reef health and the ability to predict future responses to rising pCO2
levels (Andersson and Gledhill, 2013). Understanding the local drivers of
ecosystem function and reef community metabolism is critical for gauging the
susceptibility of the reef ecosystem to future changes in ocean chemistry.
Numerous efforts have been conducted along West Maui, Hawaii, USA, to
characterize and quantify submarine groundwater discharge (SGD) and
associated nutrient input (Dailer et al., 2010, 2012; Glenn et al., 2013;
Swarzenski et al., 2013, 2016; Silbiger et al., 2017), which may influence
reef metabolism and community composition by changing coastal water quality.
Building upon these studies, we present a comprehensive study to characterize
the carbonate system parameters from the reefs in this area. The carbonate
chemistry system is sensitive to changes in photosynthesis, respiration,
calcification, and calcium carbonate (CaCO3) dissolution, and can be
characterized by measuring total alkalinity (TA), dissolved inorganic carbon
(DIC), pH, pCO2, nutrients, salinity, and temperature. Analysis of
these parameters yields valuable information on ratios of net community
calcification and production, and can be used to identify biological and
physical drivers of reef health and ecosystem function (Silverman et al.,
2007; Shamberger et al., 2011; Lantz et al., 2014; Albright et al., 2015;
Muehllehner et al., 2016; DeCarlo et al., 2017; Richardson et al., 2017;
Cyronak et al., 2018). This is particularly important given growing concern
that coastal and ocean acidification may shift reef ecosystems from
calcification to dissolution by the middle to end of the century (Silverman et
al., 2009; Andersson and Gledhill, 2013) with an overall reduction in
calcification rates and increase in dissolution rates (Shamberger et al.,
2011; Shaw et al., 2012; Bernstein et al., 2016) that can contribute to reef
collapse (Yates et al., 2017).
The health of many of Maui's coral reefs has been declining rapidly (Rodgers
et al., 2015), with recent coral bleaching events leading to increased coral
mortality (Sparks et al., 2016). The decline in coral cover along the shallow
coral reef at Kahekili has been observed for decades (Wiltse, 1996; Ross et
al., 2012), along with a history of macro-algal blooms (Smith et al., 2005).
The shift in benthic cover from abundant corals to turf or macro-algae
(primarily Ulva fasciata) and increased rates of coral bioerosion
have been linked to input of nutrient-rich water via wastewater injection
wells (Dailer et al., 2010, 2012; Prouty et al., 2017a). Treated wastewater
is injected through these wells into groundwater that flows toward the coast
where it emerges on the reef through a network of small seeps and vents
(Glenn et al., 2013; Swarzenski et al., 2016). Changes in coastal water
quality observed off West Maui can impact the balance of production of
CaCO3 skeletons by calcifying algae and animals on the reef, cementation
of sand and rubble, and CaCO3 breakdown and removal that occurs through
bioerosion, dissolution, and offshore transport. Here, a high-resolution
seawater sampling study was conducted to constrain the carbonate chemistry
system and evaluate the biological and physical processes altering reef
health along the shallow coral reef at Kahekili in Kaanapali, West Maui,
Hawaii, USA (Fig. 1). This study characterizes the diurnal and multi-day
variability of coral reef carbonate chemistry along a tropical fringing reef
adjacent to a densely inhabited shoreline with known input from land-based
sources of pollution, and identifies the controls on carbon metabolism.
Ultimately, understanding carbonate system dynamics is essential for managing
compounding effects from local stressors.
Methods
Study site
The benthic habitat along the shallow reef at Kahekili in Kaanapali, West Maui
(Fig. 1), consists of aggregate reef, patch reef, pavement, reef rubble,
and spur and groove (Cochran et al., 2014), with persistent current flow to
the south (Storlazzi and Jaffe, 2008). Only 51 % of the hardbottom at
Kahekili is covered with at least 10 % live coral, with the remaining
hardbottom consisting of aggregate reef, spur and groove, patch reefs,
pavement, and reef rubble (Cochran et al., 2014). The shallow fore reef
experiences algae blooms in response to inputs of nutrient-rich water via
wastewater injection wells (Dailer et al., 2010, 2012). Groundwater inputs
occur from both natural sources (rainfall and natural infiltration) and from
artificial recharge (irrigation and anthropogenic wastewater). The inland
Wailuku Basalt, consisting of a band of unconsolidated sediment along the
coast and a small outcrop of Lahaina Volcanics, dominates the geology of the
area surrounding the study site, controlling the flow of groundwater (Langenheim and Clague, 1987; Gingerich and Engott, 2012). Mean
annual precipitation rates are up to 900 cm yr-1 (Giambelluca et al.,
2013), with natural recharge the greatest in the interior mountains.
Location map of the island of Maui, Hawaii, USA, and the study area
along West Maui. Bathymetric map (5 m contours) of study area showing
seawater sampling locations (blue closed circle) along Kahekili Beach Park,
and the primary seep site (blue open circle) superimposed on distribution of
percent coral cover versus sand.
Field sampling
Two intensive sampling periods were carried out during the 6-day period from
16 to 24 March 2016 along the reef flat with live coral cover. Seawater
nutrients and carbonate chemistry variables were collected every 4 h during
each sampling period from the primary vent site and in adjacent coastal
waters along the shallow reef at Kahekili (Fig. 1). The first sampling period
was from 15:00 HST on 16 March 2016 to 15:00 HST
on 19 March 2016, and the second sampling period was from 15:00 HST on
21 March 2016 to 11:00 HST on 24 March 2016 (all reported times in local time).
There were five sampling sites: two shallow (< 1.5 m) sites (S1 and S2)
located approximately 10 m offshore, two deeper (5 m) sites (S3 and S4)
located approximately 115 m offshore, and a shallow site located
approximately 20 m offshore and within 0.25 m of an active SGD vent (vent
site; < 1.5 m) (Glenn et al., 2013; Swarzenski et al., 2016). Sampling
tubes (ranging from approximately 100 to 200 m in length) were installed at
each site by affixing the tube to a concrete block located approximately 20 cm
above the seafloor or by attaching the tubing directly to dead reef
structure using zip ties. Tube intakes were fitted with a stainless-steel
screen cap to prevent uptake of large particulates. The remaining length of
each tube was positioned along the seafloor to the adjacent beach by
weighting the tube with a 1 m piece of chain or by weaving the tube through
dead reef structure approximately every 20 m. The tube outflow ends were
labeled for each sampling site, bundled in a common location, and located
above the high water line on the beach for sampling access. A peristaltic
pump was used to pump seawater from the seafloor. Sampling tubes were flushed
for a minimum of 20 min to remove residual seawater before collecting data
and water samples. Sampling tubes were inspected upon extraction and no
significant algal growth was observed. Temperature, salinity, and dissolved
oxygen of water samples were measured using a YSI ProPlus multimeter that was
calibrated daily with an accuracy of ±0.2 ∘C, ±0.1 psu, and
±0.2 mg L-1, respectively. However, due to temperature change
during water transit time within the sampling tube, in situ temperatures were
also recorded from Solonist conductivity–temperature–depth (CTD) divers installed at the intake of each
sampling tube. An upward-looking 2 MHz Nortek Aquadopp acoustic Doppler
profiler (ADP) was deployed at the southern deeper site (S4). The ADP sampled
waves at 2 Hz for 17 min every hour and currents at 1 Hz every 10 min in
1 m vertical bins from 1 m above the seabed up to the ocean surface.
Seawater analyses
Samples for dissolved nutrients (NH4+, Si, PO43-, and
[NO3-+NO2-]) were collected in duplicate by filtering water
with an in-line 0.45 µm cellulose nitrate filter and
0.20 µm polyethersulfone syringe filter, and were kept frozen until
analysis. Nutrients were analyzed at the Woods Hole Oceanographic
Institution's nutrient laboratory and University of California at Santa
Barbara's Marine Science Institute Analytical Laboratory via flow injection
analysis for NH4+, Si, PO43-, and
[NO3- + NO2-], with precisions of 0.6–3.0,
0.6–0.8, 0.9–1.3, and 0.3–1.0 % relative standard deviations,
respectively. Select samples were collected and analyzed for nitrate isotope
(δ15N and δ18O) analyses at the University of California
at Santa Cruz using the chemical reduction method (McIlvin and Altabet, 2005;
Ryabenko et al., 2009) and University of California at Davis' Stable Isotope
Facilities using the denitrifier method (Sigman et al., 2001). The isotope
analysis was conducted using a Thermo Finnigan MAT 252 coupled with a
GasBench II interface; isotope values are presented in per mil (‰)
with respect to AIR for δ15N and Vienna Standard Mean Ocean Water (VSMOW) for δ18O with a
precision of 0.3–0.4 and 0.5–0.6 ‰ for δ15N-nitrate and
δ18O-nitrate, respectively.
Seawater samples for determining carbonate chemistry variables (pH on the
total scale, TA, and DIC) were collected from the five sampling sites using a
peristaltic pump and pressure filtering seawater through a 0.45 µm
filter. Samples for pH (0.007 ± 0.017) were filtered into 30 mL
optical glass cells and analyzed within 1 h of collection using
spectrophotometric methods (Zhang and Byrne, 1996), an Ocean Optics USB2000
spectrometer, and thymol blue indicator dye. Samples for TA and DIC were
filtered into 300 mL borosilicate glass bottles, preserved by adding
100 µL saturated HgCl2 solution, and pressure sealed with
ground glass stoppers coated with Apiezon grease. TA samples were analyzed
using spectrophotometric methods of Yao and Byrne (1998) with an Ocean Optics
USB2000 spectrometer and bromocresol purple indicator dye. DIC samples were
analyzed using a UIC carbon coulometer model CM5014 and CM5130 acidification
module fitted with a sulfide scrubber, and methods of Dickson et al. (2007).
In situ temperatures recorded from Solonist CTD divers were reported and used
to temperature correct pH and perform CO2SYS calculations as described below.
Certified reference materials (CRMs) for TA and DIC analyses were from the
Marine Physical Laboratory of Scripps Institution of Oceanography (Dickson et
al., 2007). TA and DIC sample accuracy were within 0.56 ± 0.55 and
1.50 ± 1.17 µmol kg-1 of certified reference material,
respectively. Precision for TA based on replicate sample analyses
was 0.76 ± 0.83 µmol kg-1. Precision for DIC based on
replicate sample analyses was 1.9 ± 1.5 µmol kg-1. The
full seawater CO2 system was calculated with measured salinity,
temperature, nutrients (phosphate and silicate), TA, DIC, and pH data using
an Excel Workbook Macro translation of the original CO2SYS program (Pierrot
et al., 2006). Given the enriched nutrient setting of the study site, TA
values were nutrient corrected in CO2SYS (Dickson, 1981). The aragonite
saturation state (Ωarag) and pCO2 are reported based
on DIC–pH pairs, with dissociation constants K1 and K2 from
Mehrbach et al. (1973) refit by Dickson and Millero (1987) and KSO4 from
Dickson (1990). The TA and DIC values were normalized to salinity (by
multiplying by a factor of 35/S, where S is the measured salinity value) to
account for variations in TA and DIC along the reef flat driven by
evaporation and/or precipitation (Friis et al., 2003) and are reported as
nTA and nDIC as previously established in reef geochemical surveys (e.g.,
Suzuki and Kawahata, 2003; Yates et al., 2014; Muehllehner et al., 2016)
where TA and DIC exhibit non-conservative behavior with respect to salinity.
However, at the vent site, the TA and DIC data were not normalized to salinity
given the contribution of TA and DIC from SGD.
Results of time series of seawater chemistry variables over a 6-day
period collected from the vent site located on the nearshore reef every 4 h.
(a) Salinity, (b) dissolved nutrient
(nitrate plus nitrite, phosphate, and silicate) concentrations
(µmol L-1), and nitrate stable nitrogen isotopes
(δ15N-nitrate; ‰), (c) total alkalinity (TA) and
dissolved inorganic carbon (DIC) (µmol kg-1),
(d) calculated carbonate parameters for aragonite saturation state
(Ωarag), and pCO2 (µatm; inverted) based on
DIC–pH pairwise and measured salinity, temperature, nutrient (phosphate and
silicate) data, (e) dissolved oxygen (DO; mg L-1), and
(f) temperature-corrected pH (total scale). End-of-century
projections are according to IPCC-AR5 RCP8.5 “business as usual” scenario for
pH (reduction by 0.4 units), Ωarag (2.0; blue dashed), and
pCO2 (750 µatm; red dashed).
Statistical analysis
Slope of salinity normalized total alkalinity (nTA) to salinity normalized
dissolved inorganic carbon (DIC), net community calcification to net community
production ratio (NCC : NCP = 2ΔDIC / (ΔTA-1))
(Suzuki and Kawahata, 2003), correlation coefficients (r2), analysis of
variance (ANOVA), and standard error of difference (SEdif) were
calculated in Excel v. 14.7.6. Histogram plots and cubic spline fits were
made in KaleidaGraph 4.1.3.
Carbonate chemistry parameters and sea surface temperature (SST)
composite from S1, S2, S3, and S4 along the shallow reef flat of Kahekili,
Maui, and cubic spline fits highlighting diurnal cycle for the first (16 to
19 March 2016; blue solid line) and second (21 to 24 March 2016; red dashed line)
sampling periods for (a) temperature, (b) pH,
(c) nDIC, (d) nTA (µmol kg-1),
(e) Ωarag, and (f) pCO2
(µatm).
Discussion
The diurnal pattern observed at the four sampling sites along the reef flat
is typical of a reef environment where biotic processes involving coral reef
community metabolism (e.g., respiration/photosynthesis and
calcification/dissolution) dominate the carbonate chemistry system (e.g.,
Smith, 1973). The non-linear relationship between salinity and carbonate
chemistry parameters further supports the notion that biotic processes are
driving carbonate chemistry variability along the reef flat (Millero et al.,
1998; Ianson et al., 2003). The lower-amplitude nTA diurnal signal supports
previous observations that the region was algal dominated (Smith et al.,
2005). In this case, the lower biomass of calcifying organisms leads to
conditions that favor respiration–photosynthesis processes relative to
calcification–dissolution (Jokiel et al., 2014). Elevated pH values during
midday, coincident with elevated sea surface temperature (SST) and peak
solar irradiance, are consistent with maximum photosynthetic activity. DIC
decreased during the day due to photosynthesis, whereas at nighttime, pH
decreased and DIC increased in response to respiration (Fig. 3). This pattern
is in stark contrast to the primary vent site where no diurnal pattern was
observed, and abiotic controls on the carbonate system dynamics explain the
strong linear relation to salinity. Variability at the vent site is driven by
SGD rates, which are elevated during low tide when hydraulic gradients are
the steepest (Dimova et al., 2012; Swarzenski et al., 2016). This spatial
pattern is consistent with offshore transects from Maunalua Bay where sites
closest to shore incorporated greater contribution of SGD-derived TA and DIC
than offshore sites (Richardson et al., 2017).
Seawater carbonate chemistry system along the reef flat off Kahekili
as a function of nDIC and nTA for the shallow sampling
sites (a) S1 and (b) S2, and two deeper sites
(c) S3 and (d) S4, for the first (blue) and second (red)
sampling periods and their respective slopes (solid lines) of nDIC and
nTA (Table 1) and theoretical slope (dashed lines) given the predicted net
effects of photosynthesis, respiration, calcification, and dissolution as
shown in panels (a–d), and the respective change in net community
calcification (NCC) and net community production (NCP). The relative
positions of the open-ocean nDIC and nDIC values are reported as 1977 ± 11 and
2304 ± 5 µmol kg-1 (adapted from Dore et al., 2009).
To further understand the temporal variability in carbonate chemistry over
the 6-day sampling period along the reef flat, diagrams of nTA versus nDIC
were plotted according to Zeebe and Wolf-Gladrow (2001), along with vectors
indicating theoretical effects of net community production (NCP) and net
community calcification (NCC) on seawater chemistry (Kawahata et al., 1997;
Suzuki and Kawahata, 2003) (Fig. 5). As presented here, NCP refers to the
balance of photosynthesis and respiration, and NCC refers to the balance
between calcification and dissolution (see review by Cyronak et al., 2018).
Diagrams of nTA–nDIC indicate the dominance of photosynthesis (+NCP)
and CaCO3 precipitation (+NCC) during the first sampling period
(16 March 2016 to 19 March 2016). The slope values of the nDIC–nTA plots
were used to calculate ratios of NCC : NCP (Table 1) using methods of
Suzuki and Kawahata (2003) to estimate the relative contribution of these
processes to reef biogeochemistry. In the absence of reliable water mass
residence time, ratios were used rather than metabolic rates. The NCC : NCP
ratios for the first sampling period ranged from 0.50 to 0.88, indicating a
dominance of NCP relative to NCC. Plots of nDIC–nTA (Fig. 5) indicate
that these sites were dominated primary by photosynthesis and calcification
during +NCP and +NCC. This pattern was observed at all four sites along
the reef flat. The lower NCC : NCP ratios at the shallow sites highlight
the greater vulnerability of the shallow sites to dissolution under lower-pH
conditions relative to the deeper. These results are in agreement with
Richardson et al. (2017) who found dissolution at reef sites closest to
groundwater vents in Maunalua Bay, Oahu. A shift occurred at all sampling
sites after the first sampling period. Elevated nDIC and nTA values from
21 to 22 March 2016 indicate a shift to respiration and dissolution in the
nTA–nDIC diagrams during -NCC and -NCP (Fig. 5). At the shallow
sites, S1 and S2 (Fig. 5a and b), the NCC : NCP ratios were 0.56 and 0.39
during the second sampling period (Table 1), respectively, indicating the
dominance of NCP relative to NCC. In comparison, at sites S3 and S4 located
further offshore, dissolution and respiration contributed nearly equally, with
NCC : NCP ratios near 1.0 during the second sampling. Given the salinity range
along the reef flat (34 to 36), traditional salinity normalization (e.g.,
Friis et al., 2003) could potentially overestimate the nDIC and nTA
concentrations by ∼ 20 to ∼ 10 µmol kg-1,
respectively, according to non-zero normalization described in Richardson et
al. (2017). However, rather than reflecting an artifact of the salinity
normalization, given the non-linear relation of DIC and TA to salinity along
the reef flat (Supplement, Fig. S1), this shift is interpreted as a reef community
response. As shown in Figs. 4 and 5, this change captures a shift from a reef
community dominated by calcification and photosynthesis to one dominated by
respiration and dissolution during -NCC and -NCP.
Slope of salinity normalized total alkalinity (nTA) to salinity
normalized dissolved inorganic carbon (DIC), net community calcification to net
community production ratio (NCC : NCP = 2ΔDIC / (ΔTA-1)) (Suzuki and Kawahata, 2003), and correlation coefficients (r2).
Site
nTA–nDIC
NCC : NCP
r2
slope
16–19 March 2016
S1
0.88
0.78
0.94
S2
0.67
0.50
0.75
S3
0.93
0.88
0.89
S4
0.93
0.87
0.92
21–24 March 2016
S1
0.72
0.56
0.78
S2
0.56
0.39
0.77
S3
0.99
0.98
0.95
S4
1.04
1.08
0.94
The shift from photosynthesis (P) to respiration (R), as captured in the
ΔnDIC histogram plots (Fig. 4), suggests that the coral–algal
association consumed more energy than it produced during the second sampling
period. As a proxy for autotrophic capacity, the change in P : R ratio may
reflect an increase in coral heterotrophic feeding relative to autotrophic
feeding (Coles and Jokiel, 1977; Hughes and Grottoli, 2013). Typically,
stored lipid reserves in the tissue are utilized when the stable symbiotic
environment is disturbed (e.g., Szmant and Gassman, 1990; Ainsworth et al.,
2008). Although short-lived, thermally induced bleaching has been linked to
depletion of coral lipid reserves (e.g., Hughes and Grottoli, 2013), and excess
nutrient loading can also shift the stability of the coral–algae symbiosis,
thereby reducing stored tissue reserves (Wooldridge, 2016). According to
Glenn et al. (2013), up to 11 m3 d-1 of dissolved inorganic
nitrogen are discharged onto the West Maui reef as the result of receiving
and treating over 15 000 m3 d-1 of sewage. Using a SGD flux rate
of 87 cm d-1 at the primary seep site (Swarzenski et al., 2016), and
SGD nitrate endmember concentration of 117 µmol L-1 (Prouty
et al., 2017b), the nitrate flux from the primary vent site is
712 mol d-1, clearly demonstrating excess nutrient loading. Elevated
SGD endmember nutrient concentrations are consistent with those observed
from Black Point, Maunalua Bay, where effluent from proximal on-site sewage
disposal is linked to excess nitrogen loads (Nelson et al., 2015; Richardson
et al., 2017). As described above, an offshore gradient in nutrient
concentrations was observed with enriched nutrients at the shallow sites
compared to the deeper sites, consistent with a decrease in coral δ15N values away from the vent (Prouty et al., 2017a). Coral tissue
thickness was also negatively correlated to coral tissue δ15N
values (r=-0.66; p=0.08), with the latter serving as a proxy for
nutrient loading in algae samples along the reef flat (Dailer et al., 2010).
It is possible that a reduction in coral tissue reflects preferential
heterotrophic feeding under high nutrient loading, with nutrient enrichment
by sewage effluent increasing primary production and biomass in the water
column (e.g., Smith et al., 1981; Pastorok and Bilyard, 1985). While
assessing the impacts of nutrient loading on coral physiology may be long
term and subtle in some cases, results from our study highlight the potential
short-term impacts of nutrification.
Identifying the exact mechanism(s) responsible for driving this shift is
difficult given the complexity of the reef system. Possible explanations
include warmer SSTs and suspension of organic matter, as well as secondary
effects of nutrification from contaminated SGD (D'Angelo and Wiedenmann,
2014). Given that microbial communities rapidly take up inorganic nutrients
(Furnas et al., 2005), there could be increased respiration as a result of
increased microbial remineralization of organic matter in the nutrient-loaded
environment (Sunda and Cai, 2012). In other words, enhanced SGD-driven
nutrient fluxes during the second sampling period could have increased
microbial growth and remineralization, shifting the reef community
metabolism, as captured in a shift in the carbonate chemistry system. In
addition to community metabolism, local oceanographic effects such as the
wind and wave regime can also drive carbonate chemistry by altering air–sea
exchange and water mass residence times. During the first sampling period,
the wave height increased from 0.4 to 1.6 m over the first 2 days and mean
current speeds were 1.6 cm s-1 as wind speeds increased (Supplement, Fig. S2). In comparison, during the
second sampling period, wave height declined to less than 0.4 m and mean
current speeds were 1.0 cm s-1. Together, the reduced wave height and
reduced wind speeds favor slower release of CO2 generated by
calcification and respiration processes from the water column (Massaro et
al., 2012), resulting in higher pCO2 and lower pH.
Despite being situated in an oligotrophic region with naturally occurring,
low nutrient concentrations, anthropogenic nutrient loading to coastal waters
via sustained SGD is driving nearshore eutrophication (Dailer et al., 2010, 2012;
Bishop et al., 2015; Amato et al., 2016; Fackrell et
al., 2016), with algal δ15N signatures at Kahekili Beach Park
indicative of wastewater effluent (Dailer et al., 2010, 2012). In response,
there has been a shift in benthic cover from abundant corals to turf or
macro-algae over the last two decades. Areas of discrete coral cover loss up
to 100 % along the shallow coral reef at Kahekili have been observed for
decades (Wiltse, 1996; Ross et al., 2012), with a history of macro-algal
blooms (Smith et al., 2005). More recently, Prouty et al. (2017a) found
accelerated nutrient-driven bioerosion from coral cores collected along the
Kahekili reef flat in response to land-based sources of nutrients. This is
consistent with earlier work showing nutrification-mediated increase in
plankton loads can trigger increases in filter feeders and bioeroders that
endanger reef structure integrity (e.g., Fabricius et al., 2012).
Eutrophication from nutrient-enriched SGD may contribute to an already
compromised carbonate system (i.e., reduced pH and Ωarag) by
increasing respiration and remineralization of excess organic matter, and
increasing bioerosion. Therefore, secondary effects of nutrient-driven
increase in phytoplankton biomass and decomposing organic matter are also
important considerations for coral reef management (D'Angelo and Wiedenmann,
2014).
As discussed above, SGD rates are elevated during low tide when the relative
pressure head difference between terrestrial groundwater and the oceanic water column is
greatest (Dimova et al., 2012; Swarzenski et al., 2016). Relative SGD is
greater in the shallows close to shore where the tidal height is larger
relative to the depth of the water column. Higher islands, therefore, have
the potential for not only greater orographic rainfall, and thus submarine
groundwater recharge, but also greater potential pressure head and thus
enhanced SGD-driven nutrient fluxes. There is also greater potential for
enriched nutrient sources and reduced water quality with fast-growing
population and development (Amato et al., 2016; Fackrell et al., 2016). Thus,
SGD represents a key vector of nutrient loading in tropical, oligotrophic
regions (e.g., Paytan et al., 2006). At the same time, closer to shore,
current speeds are generally slower resulting in longer water mass residence
times (Storlazzi et al., 2006); longer residence times would also be expected
closer to the seabed, compared with upper water column flows (Storlazzi and
Jaffe, 2008). Together, these suggest that the resulting exposure
(the product of intensity multiplied by residence time) of coral reefs to nutrient-laden,
low-pH submarine groundwater is greater for coral reefs closer to shore off
high islands than along barrier reefs or on atolls. This heightened
vulnerability therefore needs to be taken into account when evaluating
vulnerability of nearshore fringing reefs to changes in carbonate chemistry
system given evidence of nutrient-driven bioerosion from land-based sources
of pollution.