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

Grouping species into functional-form groups and measuring directly their surface area to volume ratio are 2 common approaches to forecast primary production of marine macroalgae. A link between the functional-form model (FFM) and the power-scaling approach (PSA) for a wide variety of marine macroalgae has been attempted for the first time in the present work. To test both approaches, thalli of 44 species of marine benthic macroalgae were collected from intertidal zones adjacent to Cádiz Bay. Metabolic rates, tissue nutrient content, surface area to biomass ratio (SA/B) and specific growth rates were measured for these species. PSA slopes were close to 2/3 power for growth rate, while metabolic rates scaled very close to, or matched, 3/4 power. The FFM descriptive model provided similar results to the PSA when it was transformed to a numerical model through the SA/B ratio. Even though both models appear to be valid, the problems derived from species allocation into morphological groups, and other previous criticisms, make the direct use of SA/B ratios more suitable for representing primary production in macroalgal functional groups in numerical models of coastal ecosystems.info:eu-repo/semantics/publishedVersio

Photosynthetic and morphological photoacclimation of the seagrass Cymodocea nodosa to season, depth and leaf position

The photoacclimation capacity of the seagrass Cymodocea nodosa was evaluated considering temporal (i.e. seasonal) and spatial (i.e. depth and within-leaf position) factors of variation. Changes along the leaf were measured in a population growing along a depth gradient (from intertidal to subtidal) in Cadiz Bay (Southern Spain) from 2004 to 2005. Photoacclimation was evaluated by photosynthesis (P–E curves), pigment content and leaf morphology. Plants of Cymodocea nodosa showed large physiological and morphological plasticity (mean %CV = 35.8 ± 3.4) according to the three factors considered. Seasonal patterns appeared for photosynthesis, respiration, pigment content and morphology. Nevertheless, seasonal patterns were not consistent with depth or leaf portions. The resulting data set offered different information depending on the analysis conducted; when only one factor (season, depth or leaf portion) was considered, some tendencies observed in the 3-way full design were masked. Accordingly, considering spatio–temporal variability is crucial when describing photoacclimation and estimating productivity in seagrass meadows

Clonal extent, apical dominance and networking features in the phalanx angiosperm Zostera noltii Hornem.

Disaggregating seagrass meadows and studying its components separately (clones, ramets, shoots) can provide us insights on meadow dynamics and growth patterns. The clonal growth, dependent upon clonal rules may regulate and impose constraints to plant architecture and, therefore, determine how individual clones evolve into the environment. In order to investigate the relationship between clonal growth rules and clone architecture, the belowground network architecture of single-clones of the seagrass Zostera noltii was studied. Networks were traced in situ after washing out the overlying sediment, and network characteristics were measured using digital analysis: area covered by clone, total rhizome length, type of rhizomatic axes (main, secondary, tertiary, quaternary), number and length of the internodes, branching angles and branching frequencies. This approach revealed that Z. noltii is able to develop into large clones integrating up to 300 internodes, 676 cm of rhizome, 208 shoots and 4,300 cm2 of plant area. Internodal length depended on both, the distance to the apical shoot (time effect) and the axes type (apical dominance effect). However, average branching angle was independent of axis type (average 58.3 ± 0.75), but varied significantly depending on the distance from the apical shoot. This average branching angle allows Z. noltii maximize the rate of centrifugal expansion, maintaining a high density in colonized areas to produce close stands but also minimizing the investment in belowground biomass and ramets overlapping. The clonal architecture of Z. noltii seems to be regulated by the interaction of both, apical dominance strength and clonal integration distance. Moreover, clonal growth rules and growth pattern seem to constrain clonality through (clonal) plant architecture regulations (i.e. branching is restricted in secondary axes, similar average branching angles regardless the axes, the higher the distance to the apex the higher the number of internodes in secondary axes, shorter internodes in secondary and tertiary axes). Future research efforts should focus on how these complex relationships between apical dominance and clonal integration interact to elucidate the temporal (seasonal) and spatial scales of both processes and the outcome at the plant architectural level.

Acclimation of seagrass Zostera noltii to co-occurring hydrodynamic and light stresses

Seagrasses may frequently experience a combination of velocity and light stresses, as elevated hydrodynamics often enhances turbidity and the subsequent light reduction. The objective of this study is to investigate the effects that these stressors induce on morphometric and dynamic seagrass features depending on the initial biomass partitioning. For that purpose, a factorial mesocosm experiment was conducted on plants of Zostera noltii subjected to combinations of two contrasting light levels (2.5 ± 0.6 and 15.6 ± 2.5 mol photons m-2 d-1) and three unidirectional flow velocities (0.35, 0.10 and 0.1 m s-1). No interactive effects of both variables were recorded except for plant survival and leaf length, and generally, light effects prevailed over hydrodynamic ones. Plants responded to light reduction regardless the flow velocity treatments, showing low survival rates (which improved at high velocity), high aboveground/belowground biomass ratios (AG/BG) and a poorly developed root-rhizome system compared to plants under saturating light conditions. Plant morphometry only responded to hydrodynamic stress under saturating light: at high current velocity, plants allocated preferentially biomass into BG structures, bearing short leaves and high internode and root appearance rates. Overall, light reduction promoted similar responses in plants with different AG/BG biomass ratio, but dissimilarities were recorded for current velocity. Thus, it can be concluded that, under simultaneous light and hydrodynamic stresses, light effects prevailed to hydrodynamic ones in Z. noltii, while acclimation to hydrodynamics only occurred under saturating light.

Light-dependent uptake, translocation and foliar release of phosphorus by the intertidal seagrass Zostera noltii Hornem

Light-dependent P uptake by root-rhizomes, acropetal translocation and subsequent foliar release by Zostera noltii Hornem. was studied under laboratory conditions in two-compartment chambers using 32P. The uptake by underground parts was unaffected by light conditions but the acropetal translocation proceeded more rapidly in light than in dark, indicating a coupling to the metabolic activity of the plants. The translocated P was mainly accumulated in the youngest leaves (30%), i.e., the most actively growing parts. Foliar release of P might be considered negligible (2–4% of the P taken up by root-rhizomes), indicating that the role of Z. noltii as a “P pump” is of minor importance in the cycling of P between sediment and water. This was calculated for part of the Oosterschelde estuary, Zeeland, The Netherlands

Clonal building, simple growth rules and phylloclimate as key steps to develop functional-structural seagrass models

The repetitive clonal growth of the seagrasses Zostera noltii Hornem. and Cymodocea nodosa Aschers at the module level was used to implement a deterministic, individual-based, numerical model using the simplest growth rules. Inter- and intraspecific variability in plant morphology and meadow attributes was simulated, and the results yielded by the model were compared with an existing large data set recorded for both species. The model outputs showed that intra- and interspecific morphological variability can be accurately described (r = 0.99, p < 0.0001, n = 19) by a restricted number of parameters (plastochrone interval [PI] and elongation rates for rhizome [RER] and leaf [LER], which are species-specific parameters). Interspecific differences in meadow properties were recorded; however, simulated values were double those observed. This result was mainly attributable to a lack of density-dependence phenomena in the model assumptions, revealing the importance of such phenomena in structuring seagrass populations. In general, species with high PIs displayed longer modules (leaves and internodes) and lower shoot densities, whereas species with lower PI values developed shorter modules and crowded stands. This result corresponds with the relationship indicated by the self-thinning law and by previous studies. The model also showed that plant morphology arises as an emergent property of a simple set of growth rules acting at the module level, and that plant dynamic parameters can be tuned by seagrasses in response to their local environmental conditions. Thus, the whole-plant response to the environment can be determined by the sum of all the modular responses. This model, together with a better knowledge of the regulation of plant dynamic parameters by control variables (light, temperature, nutrients, etc.), provides a conceptual framework that allows the incorporation of module, plant morphology and meadow properties into functional–structural seagrass models, in which feedbacks among plant morphology, plant development and phylloclimate (i.e. the physical environment actually perceived by each individual organ or plant population) can be included. [KEYWORDS: Cymodocea nodosa · Zostera noltii ; Clonal growth ; Light reduction ; Nutrient enrichment ; Plant dynamic parameters ; Plastochrone interval ; Rhizome-elongation rate ; Leaf-elongation rate]

On the use of sediment fertilization for seagrass restoration: a mesocosm study on Zostera marina L.

The use of nitrogen and phosphorus sediment fertilization for seagrass restoration is explored. Special attention was given to the effects of nitrogen sediment fertilization. The sediment fertilization treatment combined different levels of nitrogen (0, 30, 500 mg N g DW-1) with sufficient phosphorus to avoid P-limitation (fertilizer N:P ratio<0.25). Using indoor mesocosms, we studied the effects of sediment fertilization, and its interactions with light availability (55, 200 mol m-2 s-1) and sediment redox conditions (300, -100 mV), on Zostera marina L. We assessed (1) treatment effects on growth and plant biomass distribution, (2) the capacity of Z. marina roots to meet the plant nutrient demand, (3) plant tolerance to high nutrient porewater concentration, and (4) pro's and con's of use NH4NO3 as the N source in sediment fertilization for seagrass restoration. Plant biomass, growth and leaf turnover rate were stimulated by light and sediment fertilization. Biomass partitioning was not affected by light availability, whereas the relative root production was decreased in fertilized sediments. Root uptake following fertilization met nutrient plant demand. After high sediment fertilization, ammonium porewater concentration was high (30 mM) regardless of redox conditions. On the other hand, nitrate availability was also high, but 80% lower in reduced sediments (0.7–4 mM) compared to non-reduced ones (20 mM). Plants of Z. marina exhibited a remarkable tolerance to high N+P sediment fertilization. However, plant inhibition (reduction in plant weight, leaf growth and leaf turnover rate) was detected when porewater N concentrations exceeded 30 mM. The effects of phosphorus and ammonium toxicity were discarded because availability was similar for both inhibited and non-inhibited plants. We attributed the Z. marina inhibition to the extra porewater nitrogen available as nitrate (20 mM). Experimental treatments did not inhibit the photosynthetic apparatus of Z. marina. The mechanisms of inhibition might be related with deficiencies in energy or C-skeletons, since inhibitory effects were buffered when saturating irradiance and/or nitrate levels decreased in reduced sediments. In conclusion, we consider that the combined N+P sediment fertilization, with NH4NO3 as N source and high P supply, is highly beneficial for Z. marina restoration. This species has positive response to N+P sediment fertilization, high tolerance to the extensive porewater enrichment, and bacterial metabolism may reduce the porewater nitrate availability in anoxic seagrass sediments. However, for adequate sediment fertilization for restoration purposes, several precautions are suggested. [KEYWORDS: Biomass partitioning; Leaf growth; Light; Nitrogen; Redox potential; Sediment fertilization; Zostera marina]

A comprehensive analysis of mechanical and morphological traits in temperate and tropical seagrass species

Knowledge of plant mechanical traits is important in understanding how plants resist abiotic and biotic forces and in explaining ecological strategies such as leaf lifespan. To date, these traits have not been systematically evaluated in seagrasses. We analysed mechanical (breaking force and tensile strength) and associated traits (thickness, width, length, fibre content, mass area, and lifespan) of leaves in 22 seagrass species (around one-third of all known seagrass species) to examine (1) the inter-specific variation of these traits in relation to growth form and bioregions, (2) the contribution of morphology to leaf breaking force, (3) how breaking force scales to leaf dimensions, and (4) how mechanical and structural traits correlate to leaf longevity. We also compared our seagrass dataset with terrestrial plant databases to examine similarities between them. Large variation in leaf breaking force was found among seagrass species but, on average, temperate species resisted higher forces than tropical species. Variation in leaf breaking force was largely explained by differences in leaf width rather than thickness, likely due to the benefits in leaf reconfiguration and light interception. Species of large dimensions (long leaves) typically had high leaf breaking force, plausibly to tolerate the drag forces they may experience, which are proportional to the leaf area. Leaves of long-lived species typically had high mass per leaf area and fibre content and they supported high breaking forces. Compared to terrestrial plants, seagrasses are short-lived species with moderately strong fibre-reinforced leaves, which probably evolved to withstand the hydrodynamic forces occurring in the sea, and in response to other environmental factors. Overall, our analysis provides new insights into the physical performance of seagrasses in the marine environment.info:eu-repo/semantics/publishedVersio