DRAFT SAC SUPPORT DOCUMENT_Clarity_REVISED 02-17-23

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NCNCDP SAC SUPPORT DOCUMENT
1. Use of light extinction coefficient factors
a. Why does the SAC feel that the light extinction coefficient is the appropriate measure to use to
protect SAV?
While there are several environmental parameters that can affect SAV in coastal waters (Koch 2001), there
is scientific consensus that the availability of light is one of the most critical factors controlling their
distribution and abundance (Kemp et al. 1984, Dennison et al. 1993, Carter et al. 2000, Biber et al. 2008).
Like all plants, SAV require light to photosynthesize, grow and reproduce. Compared to other aquatic
primary producers, e.g., microalgae & macroalgae, SAV have a larger proportion of non-photosynthetic
structural biomass with higher respiratory demands, including roots and rhizomes, often growing in
anaerobic sediments that must be aerated by the plants for them to survive. Consequently, SAV have
significantly higher light requirements than other aquatic primary producers (Kenworthy and Haunert
1991). Significant declines in SAV are routinely attributed to impaired water quality, which is directly
linked to the optical constituents that affect water clarity, such as chlorophyll, suspended sediments and
colored dissolved organic matter (CDOM) leached from soils, decaying plants and other sources of organic
matter (Kemp et al. 1994, Rybicki and Landwehr 2007).
Water clarity is determined by the absorption and scattering properties of the water itself and the
constituents suspended and dissolved in the water (e.g., Chl, TSS, CDOM) (Gallegos, 1994). As light is
transmitted through the water it is attenuated (extinguished) exponentially by these optical constituents.
The change in light with depth is described by the equation:
Iz = I0 e - kz ; (1)
where,
I = irradiance,
I0 = irradiance (light) just below surface
Iz = irradiance (light) at depth z
e = natural logarithm
k = attenuation coefficient (extinction coefficient)
i. Why use the extinction coefficients as opposed to the specific light levels at the specified depths
as identified in the Coastal Habitat Protection Plan CHPP?
ii. How do the extinction coefficients relate to the specific light levels at the specified depths?
With measurements of light at two or more depths in the water column, we can use equation (1) to
estimate the amount of light reaching any prescribed depth in the estuary (Iz) as a percentage of the
irradiance just below the surface (I0). Based on the known maximum depth of growth for low (1.5 m) and
high (1.7 m) salinity SAV and their respective minimum light requirements (low = 13% SI, high = 22%)
(Kemp et al. 2004; Biber et al. 2008), we can assess the deviation of the median growing season light
extinction coefficients from each of their respective thresholds (low salinity =1.36 per meter; high salinity=
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0.89 per meter). The table below summarizes these values by SAV category and explains the rationale for
how these values are applied.
SAV Category Maximum
Depth of
Growth
Minimum
Light
Requirements
Light
Extinction
Coefficient
Rationale
Low Salinity 1.5 m 13% 1.36 per meter Maximum depth of growth and
minimum light requirements are
fixed values. Due to the
variability in sampling depth, the
fixed values were converted to
light extinction coefficients,
respective to each salinity
category, that can be used to
equate the established
protection levels to variable
sampling depths.
High Salinity 1.7 m 22% 0.89 per meter

In conclusion, samples with light extinction coefficients greater than those listed in the above table are
not providing sufficient light to the SAV, meaning less than 13 or 22% light at depth. The details regarding
use of the median in comparison against the light extinction coefficients and a growing season are further
described later in this writeup.

2. Use of PAR measurements
a. Why is PAR the appropriate parameter for measurement
The preferred method for measuring light in the water column is to use a submersible sensor that
responds to visible light in the spectral range used by plants in photosynthesis (400-700 nm), referred to
as photosynthetically active radiation (PAR). PAR sensors significantly reduce the sources of variability
experienced when estimating light extinction with a Secchi disc. PAR sensors are designed so they can be
easily mounted on a lightweight portable frame to obtain fixed equidistant depth measurements or
multiple depth measurements in a profile of the water column. Most PAR sensors are equipped to
electronically record and store light data collected during monitoring.

3. Use of different extinction coefficients for different salinities
a. Why are different extinction coefficients needed?
There are two distinct community types of SAV in North Carolina distributed according to the estuarine
salinity gradient; 1) high salinity, salt tolerant meadow-forming SAV commonly referred to as seagrasses,
and 2) low salinity canopy-forming SAV (Thayer et al. 1984, Kemp et al. 2004, NCDEQ 2021). This is an
important distinction because each of the SAV communities has different light requirements. These
differences are mostly the result of their morphological responses to the environments in which they
grow. In high salinity regions of the estuary, seagrasses grow in relatively higher energy wave and tidally
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influenced environments with coarse grained, unstable sediments. To survive in these environments, the
plants have proportionately smaller leaf canopies that minimize drag forces and tidal exposure. They also
produce relatively more non-photosynthetic root and rhizome tissue to anchor them in place and to
acquire nutrients from the sediments. The root to shoot biomass ratios of the seagrasses are generally >1
(Stevenson 1988). In most low salinity environments SAV experience less tidal energy and water level
fluctuations. These conditions favor the development of taller and more robust canopies, which occupy
more volume and often grow to the surface of the water (Kemp et al. 2004). The root to shoot biomass
ratios in low salinity SAV communities are generally < 1 (Stevenson 1988). With proportionately more
photosynthetic tissue available, the low salinity canopy forming SAV have a lower light requirement (13%
of surface irradiance) and can tolerate a higher extinction coefficient (1.36 per meter) than the meadow
forming seagrasses which have a higher light requirement (22% of surface irradiance) that dictates a lower
extinction coefficient (0.89 per meter) (Kemp et al 2004, Biber et al 2008).
b. Why is the CHPP appropriate for defining the different salinity areas?
The CHPP represents a North Carolina specific analysis of the habitat and respective species of SAV in the
state’s waters. As per our understanding of the SAV species composition in the eight CHPP regions with
known historical and recently documented extent of SAV designated in Table 4.5 of the 2021 Amendment
(Luczkovich and Zenil 2015, Luczkovich 2016, Luczkovich 2018, Speight 2020, NCDEQ 2021, Field et al.
2021), the SAC agrees that regions 1,2, 3 and 4 accurately represent the low salinity SAV communities and
regions 5, 6, 7, and 8 accurately represent the high salinity SAV communities.

4. Use of a growing season median
a. Why is a growing season appropriate?
b. Why is the selected growing season appropriate?
Coastal North Carolina has a humid subtropical climate with hot humid summers and cool to mild winters.
Despite the proximity and influence of the Gulf Stream, this seasonality in air temperatures is evident in
the seasonal fluctuations in coastal water temperatures. Water temperatures recorded at the Duke
University Marine Lab near Beaufort Inlet and in the SAV meadows in Back Sound average between 24-
30° C in summer while winters range between 13-17° C (Bartenfelder et al. 2022). Likewise, mean monthly
temperatures in low salinity regions of Albemarle Sound and Pamlico River Estuary range between 5° C in
January and 28°C in July and August (Copeland et al. 1983a, Copeland et al. 1983b). The growth and
reproduction of all SAV is closely coupled to the seasonal cycle of temperature, with optimal temperatures
experienced between March and October and less optimal temperatures for SAV between November and
February (Penhale 1977, Thayer et al. 1984). These temperatures and time periods coincide with the
longest daily photoperiods and the largest quantity of incident solar radiation, both of which are critical
for sustaining primary production at optimal temperatures. Consequently, light depravation during these
periods of time will have the most significant impact on SAV. While some perennial SAV biomass persists
through winter, the most important period for SAV growth and reproduction in NC occurs between March
and October.

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c. Why is a median an appropriate measure?
Two of the optical water quality constituents that affect water clarity measurements (Chlorophyll &
Turbidity) can display a skewed distribution (log normal) over time (Biber et al. 2008); in which case a
mean value can either inflate or deflate an estimate of the central tendency of the data. For example,
periodic windstorm events can elevate suspended sediment concentrations that yield high turbidity
values and temporarily reduce water clarity. Likewise, short-term plankton blooms can temporarily
elevate chlorophyll concentrations, which reduce water clarity. These data appear as outliers in the
seasonal data distribution and can inflate the mean value for water clarity. To avoid this potential
problem, the median is the preferred measure of central tendency for water clarity.

5. Use of a “not greater than” frequency (rate of exceedance)
a. Why is the “not greater than” approach appropriate for protecting SAV in NC waters?
b. What scientific support is there for not allowing the growing season median to exceed
the extinction coefficient?

Setting a frequency for a water clarity standard based on SAV light requirements needs to take into
consideration natural variability in light over multiple scales, differences in SAV species responses to low
light stressors (Staehr and Borum, 2011), and interactions between multiple stressors and light that
impact SAV growth and survival (Koch 2001). When spatially explicit continuous water clarity data are
not available, a frequency of “not greater than” has been used in SAV based water clarity standards to
reduce the impacts of missed low clarity events on the resource (Yates et al. 2011; Chartrand et al. 2016).
Reducing the frequency in exceedance from once over multiple growing seasons to once a growing
season minimizes the uncertainty of capturing low water clarity events. Using “not greater than” at a
higher frequency is essential, as low clarity events as short as 2 weeks can result in SAV declines
continuing up to 8 weeks post stressor (Chartrand et al. 2016). In NC, declines in SAV due to low clarity
events may be exacerbated by the high reliance on successful flowering and seed production for yearly
high salinity SAV meadow maintenance and recovery (Jarvis et al. 2012). Reduction of flowering or seed
output in one growing season will severely limit the ability of the meadows to reestablish and limit
meadow resilience to additional stressors (Jarvis et al. 2012; Combs et al. 2020). Multiple years of
reduced seed production are predicted to result in significant SAV declines (Jarvis et al. 2014). Therefore,
implementing a frequency of “not more than one exceedance in three years” may not sufficiently protect
NC SAV resources. In addition, by using median and not mean values calculated across an entire growing
season, concerns about pulse events (e.g., hurricanes) triggering exceedance are reduced, as outlying
values will have a smaller effect on median seasonal light attenuation. The approach of a frequency of
“not greater than” using median values calculated across the SAV growing season accounts for potential
missed low clarity events and SAV life history strategies expressed by NC SAV, while also limiting the
influence of short term (< 2 weeks) low clarity events on triggering criteria exceedance. Ultimately,
protecting the spatial presence of SAV consistent with historical distributions is the intent of the water
clarity standard. Consideration of a frequency component different than the proposed “not greater
than” approach should be based on data to demonstrate the alternative is protective of SAV
communities.

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c. Discussion of differences between this clarity standard and the clarity standard approach used
in the Chesapeake Bay area.
i. Why is the “not greater than” approach more appropriate than the Chesapeake Bay
program frequency of “not more than one exceedance in three years”?

In data-rich environments, such as the Chesapeake Bay, the uncertainty in setting SAV water clarity
standards is addressed by using frequency values of “not more than one exceedance in three years”
(Tango and Batiuk 2013). However, to calculate exceedance of water quality standards a combination of
data from high frequency (every 15 min) continuous water quality monitoring stations, high throughput
spatial water quality sampling (e.g., dataflow) conducted monthly throughout the SAV growing season,
and yearly SAV aerial surveys are collected for all water quality segments (USEPA 2007). These data are
then used to calculate SAV acreage and water quality acreage values for each segment, which are then
used separately or in combination to determine if individual segments meet water quality criteria. The
data necessary to determine water quality exceedance values in a similar procedure to the Chesapeake
Bay do not currently exist for North Carolina estuarine waters. Therefore, low water clarity events that
would be clearly documented as part of the high frequency temporal data sets collected over broad
spatial scales in the Chesapeake Bay, may be missed by the existing NC program which has access to data
collected in monthly sampling at fixed stations with a comparatively limited spatial distribution. The
approach taken for frequency standards in data limited systems like NC should be designed to account
for this uncertainty.

6. Use of the current & historical extent of SAV vegetated areas as the basis for where (spatially) the
clarity standard will apply.
a. Why limit the standard to only those areas that currently have SAV or have a historic
presence of SAV?
As described in Chapter 4 of the CHPP (NCDEQ 2021), the known historical extent of SAV in NC was
compiled from miscellaneous observations of SAV presence originating from many different sources
dating back to 1981 and includes more recent quantitative mapping efforts by the Albemarle Pamlico
National Estuary Partnership (APNEP) (Field et al. 2021). This composite delineation represents our best
estimate of potential SAV habitat in both high and low salinity estuarine environments. Analyses of
change in SAV historical extent in high and low salinity regions indicate that both have experienced
declines; 33% in low salinity and 5.6% in high salinity (NCDEQ 2021). Assuming there have not been
significant physical changes in these regions (e.g., water depths), it can be argued that the known historical
extent of SAV is a plausible SAV restoration goal. Furthermore, healthy SAV meadows are ecosystem
engineers that trap and stabilize sediments and recycle and sequester nutrients, such that the mere
presence of SAV improves local water quality and clarity (Ward et al. 1984, van der Heide et al. 2007,
Moore 2009). If both conservation and restoration of SAV are agreed upon priorities, a water clarity
standard should be applied in areas of both historical and current SAV extent.
b. Why not apply the standard in all parts of a waterbody?
Portions of some waterbodies will have depths that exceed the known maximum depth of growth for SAV.
Also, per the discussion above in section 5a, there will likely be differences in water quality if SAV is present
in shallow water. Studies in the Chesapeake Bay have shown that water quality monitoring stations
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located offshore of SAV meadows may not accurately characterize the conditions near or within the
meadows (Moore et al 1995, Moore et al. 1996). “Temporally intensive water quality studies (e.g., Moore
et al. 1995, 1996) in vegetated and un-vegetated shallows and adjacent channel areas in the Bay have
demonstrated that differences in water quality between the two can be significant, and predictions of SAV
transplant growth and survival using the closest available mid-channel, water quality monitoring data,
have had poor success” (Water Quality Monitoring Strategy, Office of Ecology Water Monitoring and
Assessment Program, Virginia Dept. of Environmental Quality, Revision 5, February 2022,
https://www.deq.virginia.gov/water/water-quality/monitoring). This is one of the main reasons why the
Virginia Chesapeake Bay water quality monitoring program stratifies its sampling program into multiple
but distinct categories, e.g., the main stem channel, shallow water habitats, and tributaries. Using the
map delineated locations of the combined historical and current SAV extent from Chapter 4 of the CHPP
will ensure that the appropriate locations are monitored and that those beyond the delineation would
not be covered by the clarity criteria. If any new extent of coverage is discovered, the maps can be updated
through the triennial review process to reflect such changes.
c. Is the species list in the language appropriate for NC waters?
To be determined and finalized.
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