PercentSI

Percentage of surface irradiance by site and dept

DailyDose

Daily dose of light by site and dept

Light Limitation within Southern New Zealand Kelp Forest Communities

<div><p>Light is the fundamental driver of primary productivity in the marine environment. Reduced light availability has the potential to alter the distribution, community composition, and productivity of key benthic primary producers, potentially reducing habitat and energy provision to coastal food webs. We compared the underwater light environment of macroalgal dominated shallow subtidal rocky reef habitats on a coastline modified by human activities with a coastline of forested catchments. Key metrics describing the availability of photosynthetically active radiation (PAR) were determined over 295 days and were related to macroalgal depth distribution, community composition, and standing biomass patterns, which were recorded seasonally. Light attenuation was more than twice as high in shallow subtidal zones along the modified coast. Macroalgal biomass was 2–5 times greater within forested sites, and even in shallow water (2m) a significant difference in biomass was observed. Long-term light dose provided the best explanation for differences in observed biomass between modified and forested coasts, with light availability over the study period differing by 60 and 90 mol photons m⁻² at 2 and 10 metres, respectively. Higher biomass on the forested coast was driven by the presence of larger individuals rather than species diversity or density. This study suggests that commonly used metrics such as species diversity and density are not as sensitive as direct measures of biomass when detecting the effects of light limitation within macroalgal communities.</p></div

Seasonal macroalgal dry biomass per square metre at (A) two metres and (B) 10 metre depth within the modified coast (white bars) and forested coast (grey bars), New Zealand.

<p>Error bars represent mean ± SE for modified <i>n</i> = 12, forested <i>n</i> = 18.</p

Location of study sites.

<p>Top left, New Zealand with the modified (East Otago) and forested (Stewart Island) coastlines highlighted. Top right, modified with Karitāne and Aramoana sites marked by black dot. Bottom, forested with West Head, Horseshoe Bay and Cooper Bay sites marked by black dot.</p

All Sites for calibration

Calibration data from four calibration campaigns

Percentage of surface irradiance pooled by coast at two metre (top) and 10 metre (bottom) depths from December 2012 to October 2013.

<p>Lines represent a weighted least-squares regression smoother (loess, see text for details). Shaded area represents 1.96-times the standard error (approximate 95% C.I.) of the loess smoother. Modified (green, solid line) <i>n</i> = 2, forested (blue, dashed line) <i>n</i> = 3.</p

Seasonal individual macroalgal dry weight at (A) two metres and (B) 10 metre depths within the modified coast (white bars) and forested coast (grey bars), New Zealand.

<p>Error bars represent mean ± SE for modified <i>n</i> = 12, forested <i>n</i> = 18.</p

Maximum and minimum depth distribution of dominant macroalgal species over all four seasons within the modified coast (white bars) and forested coast (grey bars), New Zealand.

<p>Modified <i>n</i> = 6, forested <i>n</i> = 9.</p

Percentage of surface irradiance, predicted from attenuation coefficients between two and 10 metre depths, pooled by coast between December 2012 to October 2013.

<p>Shaded areas represent empirical 90% confidence intervals for each region. Modified (green, solid line) <i>n</i> = 2, forested (blue, dashed line) <i>n</i> = 3.</p