Community photosynthesis of aquatic macrophytes

Abstract

We compared 190 photosynthesis-irradiance (P-E) experiments with single-and multispecies communities of macroalgae and vascular plants from freshwater and marine habitats. We found a typical hyperbolic P-E relation in all communities and no sign of photosaturation or photoinhibition of photosynthesis at the highest irradiances of about 2,000 mmol m 22 s 21 . Macrophyte communities displayed much higher maximum gross production (GP max ), respiration, and light compensation point than separate phytoelements because of the multilayered structure and extensive self-shading in the communities, whereas light use efficiency at low irradiance (a) was the same. Although GP max and a varied extensively among the 190 communities, their upper limits increased linearly and predictably with community absorption reaching 26.3 mmol m 22 s 21 O 2 and 0.090 mol mol 21 photon at 100% absorption. The upper limit of a is close to a realistic limit of O 2 of 0.10 mol mol 21 photon. The upper limit of GP max , however, is markedly below the theoretically attainable 180 mol m 22 s 21 O 2 , reflecting a suboptimal three-dimensional structure and light distribution. Indirect measures supported this explanation as GP max increased fourfold from communities with a very uneven to a more even light distribution. Photosynthetic characteristics of communities are strongly influenced by plant density, absorption, and distribution of light and cannot be interpreted from the photosynthetic behavior of phytoelements. Thus, many examples of carbon and nutrient limitation in experiments with separate phytoelements may not withstand at the relevant ecological scale of communities where light almost always constrains photosynthetic production.

Comparing Productivity and Biogeochemistry of Native and Transplanted Thalassia testudinum and Halodule beaudettei in Big Lagoon, Florida, USA

“…The dominant factor driving primary productivity in the seagrass beds was ambient light level (Fig. 3), with no indication of saturation of gross productivity, as has been observed with other aquatic macrophytes (Binzer et al, 2006;Rheuban et al, 2014). Productivity of seagrass beds in Big Lagoon was also comparable to that of prior studies that took place in Mississippi (Moncreiff et al, 1992), Virginia (Murray and Wetzel, 1987;Hume et al, 2011), south Florida (Nagel, 2007;Yarbro and Carlson, 2008), and Australia (Eyre and Ferguson, 2002;Eyre et al, 2011b), but much higher than rates measured in H. beaudettei patches in the region (Stutes et al, 2007) (Table 2).…”

Photosynthesis and growth in macroalgae: linking functional-form and power-scaling approaches

“…Prior to incubations in the darkness, the water was vigorously stirred to reach oxygen saturation levels, whereas water was bubbled with nitrogen to get lower , and it was enhanced by using highly reflective aluminium foil at the shelves of the incubation chamber. This value is close to the mean irradiance saturation point for thallus pieces or phytoelements (250 µmol photons m -2 s -1 ; Binzer & Sand-Jensen 2006). Thus, it was also assumed to be saturating for the majority of the assayed species.…”

Ecosystem metabolism along a colonization gradient of eelgrass (Zostera marina) measured by eddy correlation

“…When shoot density is high, although the upper parts of the leaves may be light-saturated, the understory may not be saturated even at high light levels because of self-shading (Short 1980;Binzer et al 2006). Thus, ecosystem-scale measurements may be very different from P-I relationships determined by isolating a part of the plant such as a leaf segment (Binzer et al 2006;Sand-Jensen et al 2007). This non-saturating response was not evident in the 5 yr site, which instead showed a hyperbolic tangent relationship between photosynthesis and irradiance (Fig.…”

“…Rheuban et al (2014) found a strong correlation between rates of GPP and R with shoot density and attributed this relationship to several co-varying factors. GPP is influenced by shoot density directly through increased aboveground biomass and indirectly through increased efficiency of light absorption and thus photosynthesis with higher leaf area (Binzer et al 2006;Sand-Jensen et al 2007). Shoot density also influences respiration through increased belowground biomass or through indirect processes such as increased exudation of labile dissolved organic matter (Penhale and Smith 1977;Wetzel and Penhale 1979) and O 2 (Frederiksen and Glud 2006) from leaves, roots, and rhizomes.…”

“…Seagrass meadows have higher production relative to unvegetated sediments (Barró n et al 2004;Eyre et al 2011;Hume et al 2011) in part because they have greater primary producer and heterotrophic consumer biomass (Hemminga and Duarte 2000). In addition, changes in ecosystem structure through the development of a seagrass canopy can increase efficiency of light absorption on an ecosystem scale relative to unvegetated sites (Binzer et al 2006;Ralph et al 2007). Finally, seagrass communities affect flow conditions and create a depositional environment (Gacia and Duarte 2001;Hansen and Reidenbach 2012), increase sediment organic content (Fourqurean et al 2012;McGlathery et al 2012;Greiner et al 2013), and change sediment oxygenation through release of oxygen from the roots and rhizomes (Frederiksen and Glud 2006).…”