Benthic Irradiance Research Articles

A benthic light index of water quality in the Great Barrier Reef, Australia

Good water quality is essential to the health of marine ecosystems, yet current metrics used to track water quality in the Great Barrier Reef are not strongly tied to ecological outcomes. There is a need for a better water quality index (WQI). Benthic irradiance, the amount of light reaching the seafloor, is critical for coral and seagrass health and is strongly affected by water quality. It therefore represents a strong candidate for use as a water quality indicator. Here, we introduce a new index based on remote sensing benthic light (bPAR) from ocean color. Resulting bPAR index timeseries, based on the extent to which the observed bPAR fell short of the locally- and seasonally-specific optimum, showed strong spatial and temporal variability, which was consistent with the dynamics that govern changes in water clarity in the Great Barrier Reef. Our new index is ecologically relevant, responsive to changes in light availability and provides a robust metric that may complement current Great Barrier Reef water quality metrics.

Model for deriving benthic irradiance in the Great Barrier Reef from MODIS satellite imagery: erratum.

Corrections for equations in our recently published paper [Opt. Express27, A1350 (2019)] are presented.

Global distribution of photosynthetically available radiation on the seafloor

Abstract. A 21 year (1998–2018) continuous monthly data set of the global distribution of light (photosynthetically available radiation, PAR, or irradiance) reaching the seabed is presented. This product uses ocean color and bathymetric data to estimate benthic irradiance, offering critical improvements on a previous data set. The time series is 4 times longer (21 versus 5 years), the spatial resolution is better (pixel size of 4.6 versus 9.3 km at the Equator), and the bathymetric resolution is also better (pixel size of 0.46 versus 3.7 km at the Equator). The paper describes the theoretical and methodological bases and data processing. This new product is used to estimate the surface area of the seafloor where (1) light does not limit the distribution of photosynthetic benthic organisms and (2) net community production is positive. The complete data set is provided as 14 netCDF files available on PANGAEA (Gentili and Gattuso, 2020a, https://doi.org/10.1594/PANGAEA.910898). The R package CoastalLight, available on GitHub (https://github.com/jpgattuso/CoastalLight.git, last access: 29 July 2020), allows us (1) to download geographical and optical data from PANGAEA and (2) to calculate the surface area that receives more than a given threshold of irradiance in three regions (nonpolar, Arctic, and Antarctic). Such surface areas can also be calculated for any subregion after downloading data from a remotely and freely accessible server.

Nutrient enrichment offsets the effects of low light on growth of the kelp Ecklonia radiata

AbstractIncreased input of terrigenous matter and pollution into coastal marine ecosystems results in both a decline in benthic irradiance and an influx of nutrients. Though low light conditions have been demonstrated to have adverse effects on benthic primary producers, associated increases in nutrients may alleviate some of this stress by enhancing growth and productivity. We examined the interactive effect of low light and increased nitrogen (N) availability on the growth and physiological response of the kelp Ecklonia radiata using field and mesocosm experiments. Nutrient enrichment had positive effects on the growth of E. radiata in both experiments regardless of light treatment, though increases were c. 50% higher at low light. In contrast, the ability of E. radiata to acclimate to low light and net primary production (NPP) was not affected by nutrient addition, but NPP was over 2 times greater in high light treatments than low light treatments. In low light conditions, E. radiata allocated a higher proportion of assimilated C to growth compared to those in high light conditions and this was accentuated with N‐enrichment. These results indicate that growth, rather than photosynthesis, in E. radiata is limited by N availability and may also indicate a low light coping strategy whereby E. radiata invest more energy into the development of photosynthetically active tissues in order to increase photosynthetic potential. Thus, nutrient enrichment associated with increased turbidity may alleviate some of the stresses associated with living under low light and facilitate the persistence of E. radiata in turbid environments.

Annual benthic irradiance levels at the Northwestern Cuban shelf

This study, aimed at determining the average annual benthic light irradiance in the northwestern shelf to later calculate its macrophytobenthos total primary production, is the first and only one ever conducted on this subject in Cuban waters to date. Organic mud constitutes 39% of all bottom types, sand is 37%, and the remaining 24% is made up of carbonate mud, hard bottom, rubble, coral, and their combinations. The water average depth is 7.9 m (2 to 28 m range), with an average color 6 (2 to 14 range) in the Forel-Ule scale. Average water temperatures are 28.1 ºC at bottom, and 28.4 ºC at 0.5 m depth, indicating a well-mixed water column; the average bottom water currents speed of 0.116 m s−1 is unremarkable. Visible down to bottom in 73% of all stations, the average Secchi disk depth is 7.1 m (90% transparency) at the remaining 27% stations. The value of the diffuse attenuation coefficient of down welling irradiance “k” is affected by the hour of day, seasons, water color, runoffs, distance from the coast, and depth where it is determined. With an average surface-to-bottom “k” (k0−bottom) value of 0.13 m−1, this shelf water is 34% less transparent than nearby open ocean water. The bottom light illumination is 49% of that at surface, allowing an instantaneous irradiance energy at bottom of 0.31 kW m−2, which is almost twice the average energy most abundant macrophytobenthos species require to reach maximum photosynthesis rates.

Model for deriving benthic irradiance in the Great Barrier Reef from MODIS satellite imagery.

We demonstrate a simple, spectrally resolved ocean color remote sensing model to estimate benthic photosynthetically active radiation (bPAR) for the waters of the Great Barrier Reef (GBR), Australia. For coastal marine environments and coral reefs, the underwater light field is critical to ecosystem health, but data on bPAR rarely exist at ecologically relevant spatio-temporal scales. The bPAR model presented here is based on Lambert-Beer's Law and uses: (i) sea surface values of the downwelling solar irradiance, Es(λ); (ii) high-resolution seafloor bathymetry data; and (iii) spectral estimates of the diffuse attenuation coefficient, Kd(λ), calculated from GBR-specific spectral inherent optical properties (IOPs). We first derive estimates of instantaneous bPAR. Assuming clear skies, these instantaneous values were then used to obtain daily integrated benthic PAR values. Matchup comparisons between concurrent satellite-derived bPAR and in situ values recorded at four optically varying test sites indicated strong agreement, small bias, and low mean absolute error. Overall, the matchup results suggest that our benthic irradiance model was robust to spatial variation in optical properties, typical of complex shallow coastal waters such as the GBR. We demonstrated the bPAR model for a small test region in the central GBR, with the results revealing strong patterns of temporal variability. The model will provide baseline datasets to assess changes in bPAR and its external drivers and may form the basis for a future GBR water-quality index. This model may also be applicable to other coastal waters for which spectral IOP and high-resolution bathymetry data exist.

Morphology and photosynthetic response of the kelp Ecklonia radiata across a turbidity gradient

AbstractIncreasing turbidity in coastal waters reduces benthic irradiance levels and consequently poses a fundamental threat to benthic primary producers and the food webs they support. It is therefore necessary to better understand how important primary producers, such as kelp, can adapt both physiologically and morphologically to these declining light conditions. This study examines how variation in turbidity influences the morphology of the kelp Ecklonia radiata, and how this relates to photosynthetic performance, by measuring multiple morphological characteristics, photosynthesis‐irradiance response, and photosynthetic pigments in adult kelp across a large‐scale turbidity gradient. Kelp morphology was strongly related to variation in benthic irradiance among sites, but this was not consistent with morphological acclimation. Kelp at the most turbid sites had short stipes (< 0.4 m) and lower surface area to volume ratios (SA:V) than kelp at moderate to high‐light sites, which had long stipes (~ 1 m) and high SA:V. Photosynthetic parameters and pigment content also varied considerably in relation to benthic irradiance, with increased photosynthetic efficiency and pigment content at low‐light sites. However, maximum rates of photosynthesis were highest at high‐light sites and strongly positively related to higher SA:V of kelp at these sites. These results demonstrate that while physiological adaptation may increase photosynthetic efficiency at low‐light sites, maximum photosynthetic output is strongly related to thallus morphology rather than cellular acclimation. This suggests that the large‐scale variation in kelp morphology observed across the turbidity gradient likely has important consequences for overall primary production as well as other important ecosystem functions.

A Mini-Review of the Contribution of Benthic Microalgae to the Ecology of the Continental Shelf in the South Atlantic Bight

Benthic microalgae (BMA) inhabit the upper few centimeters of shelf sediments. This review summarizes the current information on BMA communities in the South Atlantic Bight (SAB) region of the Southeastern US continental shelf to provide insights into the potential role of these communities in the trophodynamics and biogeochemical cycling in shelf waters. Benthic irradiance is generally 2–6% of surface irradiance in the SAB region, providing sufficient light to support BMA primary production over 80–90% of the shelf width. BMA biomass greatly exceeds that of integrated phytoplankton biomass in the overlying water column on an areal basis. The SAB appears to have lower BMA biomass, but higher production than most temperate continental shelves. Annual production estimates average 101 and 89 g C m−2 year−1 for 5–20 and > 20 depth intervals, respectively. However, high variation in rates and biomass in time and space make comparisons between studies difficult. Submarine groundwater discharge (SGD) rather than the water column or in situ N regeneration from organic matter maybe the major “new” N source for BMA. The estimated supply of N (1.2 mmol N m−2 day−1) by SGD closely approximates the rates needed to support BMA primary production (3.1 to 1.6 mmol N m−2 day−1) in the sediments of the SAB. Identifying the source(s) of fixed N supporting the BMA community is essential for understanding the carbon dynamics and net ecosystem metabolism within the large area (76,000 km2) of the continental shelf in the SAB as well other temperate shelves worldwide.

Long-term patterns of benthic irradiance and kelp production in the central Beaufort sea reveal implications of warming for Arctic inner shelves

This study synthesizes a multidecadal dataset of annual growth of the Arctic endemic kelp Laminaria solidungula and corresponding measurements of in situ benthic irradiance from Stefansson Sound in the central Beaufort Sea. We incorporate long-term data on sea ice concentration (National Sea Ice Data Center) and wind (National Weather Service) to assess how ice extent and summer wind dynamics affect the benthic light environment and annual kelp production. We find evidence of significant changes in sea ice extent in Stefansson Sound, with an extension of the ice-free season by approximately 17 days since 1979. Although kelp elongation at 5–7 m depths varies significantly among sites and years (3.8–49.8 cm yr−1), there is no evidence for increased production with either earlier ice break-up or a longer summer ice-free period. This is explained by very low light transmittance to the benthos during the summer season (mean daily percent surface irradiance ± SD: 1.7 ± 3.6 to 4.5 ± 6.6, depending on depth, with light attenuation values ranging from 0.5 to 0.8 m−1), resulting in minimal potential for kelp production on most days. Additionally, on month-long timescales (35 days) in the ice-free summer, benthic light levels are negatively related to wind speed. The frequent, wind-driven resuspension of sediments following ice break-up significantly reduce light to the seabed, effectively nullifying the benefits of an increased ice-free season on annual kelp growth. Instead, benthic light and primary production may depend substantially on the 1–3 week period surrounding ice break-up when intermediate sea ice concentrations reduce wind-driven sediment resuspension. These results suggest that both benthic and water column primary production along the inner shelf of Arctic marginal seas may decrease, not increase, with reductions in sea ice extent.

Stimulation of Saginaw Bay charophyte photosynthesis by phosphorus

Abstract Like many other benthic algae, charophytes can attain high abundance in shallow waters, thereby affecting benthic ecology and nutrient cycling, and their subsequent death, detachment, and shoreline deposition contribute to beach fouling within the Laurentian Great Lakes. In-situ nutrient enrichment experiments and stoichiometric analyses have shown that other types of Great Lakes benthic algae are frequently phosphorus (P)-limited, but comparable information is lacking for charophytes. This study coupled short-term P enrichment of charophytes collected from inner Saginaw Bay with fluorometric estimates of algal photosynthesis to assess potential nutrient limitation. Benthic irradiance at the experimental sites was frequently sufficient to saturate charophyte photosynthesis, and charophyte photosynthesis was stimulated by experimental P enrichment, suggesting that these algae were P-limited under natural conditions. Reduction of P loading may be an effective charophyte control measure, even in relatively light-poor and nutrient-rich areas of the Great Lakes.

Macroalgal species richness and assemblage composition of the Great Barrier Reef seabed

Understanding the drivers of broad-scale patterns of biodiversity is an overarching goal in ecology. We analysed environmental drivers of macroalgal species richness and composition on the continental shelf seabed of Australia's Great Barrier Reef (GBR), and mapped these patterns to show phycologically diverse and depauperate areas. Although shelf seabed habitats constitute similar to 61% of the GBR Marine Park area, previous floristic studies have been largely confined to intertidal and coral reef areas. Recognising the lack of knowledge of this habitat, the GBR Seabed Biodiversity Project (SBP) surveyed environmental variables and associated biodiversity across the shelf. We used SBP data for 1195 epibenthic sled sites, of which 639 sites recorded 370 macroalgal taxa, including 250 taxa not previously described in the GBR. Regression Random Forests were used to identify the environmental variables that most influence algal richness. Patterns of species composition, or assemblages, were investigated using partitioning around medoids (pam) clustering, and classification Random Forests identified the environmental variables most influential, and shapes of responses, for each assemblage. The 5 assemblages were distinguished based on taxonomy, dominant species, functional form or abundance and species richness. Overall, sediment grain size composition and light availability had the greatest influence on species richness and assemblages, with strong thresholds at 20% mud and at relative benthic irradiance of similar to 0.06 (equivalent to PAR approximate to 120 mu mol m(-2) s(-1)). This study is the first systematic analysis of the macroalgal communities of the GBR shelf seabed, providing valuable information to stimulate future research on taxonomy, productivity and ecosystem services of this habitat.

Coastal retreat and improved water quality mitigate losses of seagrass from sea level rise

The distribution and abundance of seagrass ecosystems could change significantly over the coming century due to sea level rise (SLR). Coastal managers require mechanistic understanding of the processes affecting seagrass response to SLR to maximize their conservation and associated provision of ecosystem services. In Moreton Bay, Queensland, Australia, vast seagrass meadows supporting populations of sea turtles and dugongs are juxtaposed with the multiple stressors associated with a large and rapidly expanding human population. Here, the interactive effects of predicted SLR, changes in water clarity, and land use on future distributions of seagrass in Moreton Bay were quantified. A habitat distribution model of present day seagrass in relation to benthic irradiance and wave height was developed which correctly classified habitats in 83% of cases. Spatial predictions of seagrass and presence derived from the model and bathymetric data were used to initiate a SLR inundation model. Bathymetry was iteratively modified based on SLR and sedimentary accretion in seagrass to simulate potential seagrass habitat at 10 year time steps until 2100. The area of seagrass habitat was predicted to decline by 17% by 2100 under a scenario of SLR of 1.1m. A scenario including the removal of impervious surfaces, such as roads and houses, from newly inundated regions, demonstrated that managed retreat of the shoreline could potentially reduce the overall decline in seagrass habitat to just 5%. The predicted reduction in area of seagrass habitat could be offset by an improvement in water clarity of 30%. Greater improvements in water clarity would be necessary for larger magnitudes of SLR. Management to improve water quality will provide present and future benefits to seagrasses under climate change and should be a priority for managers seeking to compensate for the effects of global change on these valuable habitats.

Lyngbya wollei in western Lake Erie

We report on the emergence of the potentially toxic filamentous cyanobacterium, Lyngbya wollei as a nuisance species in western Lake Erie. The first indication of heavy L. wollei growth along the lake bottom occurred in September 2006, when a storm deposited large mats of L. wollei in coves along the south shore of Maumee Bay. These mats remained intact over winter and new growth was observed along the margins in April 2007. Mats ranged in thickness from 0.2 to 1.2 m and we estimated that one 100-m stretch of shoreline along the southern shore of Maumee Bay was covered with approximately 200 metric tons of L. wollei. Nearshore surveys conducted in July 2008 revealed greatest benthic L. wollei biomass (591 g/m 2 ± 361 g/m 2 fresh weight) in Maumee Bay at depth contours between 1.5 and 3.5 m corresponding to benthic irradiance of approximately 4.0–0.05% of surface irradiance and sand/crushed dreissenid mussel shell-type substrate. A shoreline survey indicated a generally decreasing prevalence of shoreline L. wollei mats with distance from Maumee Bay. Surveys of nearshore benthic areas outside of Maumee Bay revealed substantial L. wollei beds north along the Michigan shoreline, but very little L wollei growth to the east along the Ohio shoreline.

Temporal dynamics in coral bioindicators for water quality on coastal coral reefs of the Great Barrier Reef

There is a need to identify effective coral bioindicators that provide quantifiable links between changes in water quality and the condition of coastal coral reefs. Temporal variation in a range of coral bioindicators including symbiont density, concentration of chlorophyll a, skeletal density and colony brightness of Pocillopora damicornis, as well as colony brightness and density of macro-bioeroders of massive Porites spp. was examined for 2 years on a coastal coral reef of the Great Barrier Reef. The specificity to changes in water quality varied among bioindicators. For example, a 2.5-fold variation in symbiont density of P. damicornis was related strongly to mean 14-day sea surface temperature and seasonal changes in water quality, suggesting medium specificity to changes in water quality. In contrast, the density of macro-bioeroders in Porites did not vary seasonally but there were consistently more macro-bioeroders at the coastal than mid-shelf reference locations, suggesting high specificity of spatial differences in water quality. In situ measurements of benthic irradiance and turbidity allowed the quantification of potential stress thresholds for coastal corals. Our data suggest long-term turbidity >3 NTU leads to sublethal stress, whereas long-term turbidity >5 NTU corresponds to severe stress effects on corals at shallow depths.

Light availability in the coastal ocean: impact on the distribution of benthic photosynthetic organisms and their contribution to primary production

Abstract. One of the major features of the coastal zone is that part of its sea floor receives a significant amount of sunlight and can therefore sustain benthic primary production by seagrasses, macroalgae, microphytobenthos and corals. However, the contribution of benthic communities to the primary production of the global coastal ocean is not known, partly because the surface area where benthic primary production can proceed is poorly quantified. Here, we use a new analysis of satellite (SeaWiFS) data collected between 1998 and 2003 to estimate, for the first time at a nearly global scale, the irradiance reaching the bottom of the coastal ocean. The following cumulative functions provide the percentage of the surface (S) of the coastal zone receiving an irradiance greater than Ez (in mol photons m−2 d−1): SNon-polar = 29.61 − 17.92 log10(Ez) + 0.72 log102(Ez) + 0.90 log103(Ez) SArctic = 15.99 − 13.56 log10(Ez) + 1.49 log102(Ez) + 0.70 log103(Ez) Data on the constraint of light availability on the major benthic primary producers and net community production are reviewed. Some photosynthetic organisms can grow deeper than the nominal bottom limit of the coastal ocean (200 m). The minimum irradiance required varies from 0.4 to 5.1 mol photons m−2 d−1 depending on the group considered. The daily compensation irradiance of benthic communities ranges from 0.24 to 4.4 mol photons m−2 d−1. Data on benthic irradiance and light requirements are combined to estimate the surface area of the coastal ocean where (1) light does not limit the distribution of primary producers and (2) net community production (NCP, the balance between gross primary production and community respiration) is positive. Positive benthic NCP can occur over 33% of the global shelf area. The limitations of this approach, related to the spatial resolution of the satellite data, the parameterization used to convert reflectance data to irradiance, the lack of global information on the benthic nepheloid layer, and the relatively limited biological information available, are discussed.

Temporal variation of light availability in coastal benthic habitats: Effects of clouds, turbidity, and tides

We analyzed the contributions of clouds, turbidity, and tides to variations in irradiance and predicted benthic primary productivity on a coastal coral reef over a period of 2 yr (2001–2002). At 1.5 m below lowest astronomical tide (3.8–m tidal range), attenuation by suspended solids (turbidity) accounted for 74–79% of the total annual variation in irradiance, clouds for 14–17%, and tides for 7–10%. With increasing depth, the contribution from turbidity to irradiance variation increased asymptotically toward 95%. Fourier (spectral) analyses indicated that the benthic irradiance regime followed strong 8‐week periodicities and weaker 2–4‐week periodicities. The 8‐week cycle was driven primarily by turbidity and secondarily by clouds and matches the periodicity of the intraseasonal Madden‐Julian atmospheric oscillation. The weaker 3–4‐week irradiance cycle was driven by turbidity; the 2‐week cycle was driven by tides and, to a lesser extent, clouds. Comparisons of the benthic irradiance pattern with predictions of physiologically optimal irradiance levels (parameter Ek) for the coral Turbinaria mesenterina suggested that corals at the site alternate between states of potential light limitation and light stress, with a 2–8‐week periodicity caused mainly by variations in turbidity. The effect of external sources of light reduction, such as episodic runoff events, on the energetics of benthic primary producers is likely to vary critically with the timing of such events.