A Biooptical Model of Irradiance Distribution and Photosynthesis in Seagrass Canopies

Although extremely vulnerable to coastal eutrophication, seagrasses represent important structuring elements and sources of primary production in shallow waters. They also generate an optical signature that can be tracked remotely. Accurate knowledge of light absorption and scattering by submerged plant canopies permits the calculation of important plant- and ecosystem-level properties, including rates of photosynthesis, vegetation abundance, and distribution. The objectives of this study were to develop a realistic, yet simply parameterized two-flow model of plane irradiance distribution through a seagrass canopy submerged in an optically active water column, to evaluate its performance against in situ measurements, and to explore the impacts of variations in canopy architecture on irradiance distribution and photosynthesis within the canopy. Allometric functions derived from leaf length-frequency data enabled simple parameterization of canopy architecture. Model predictions of downwelling spectral irradiance distributions in seagrass canopies growing in both oligotrophic and eutrophic waters were within 15% of field measurements. Thus, the model provides a robust tool for investigating photosynthetic performance of seagrass canopies as functions of water quality, depth distribution, canopy architecture, and leaf orientation. Model predictions of upwelling irradiance were less reliable, particularly in the upper half of the canopies. The model was more sensitive to leaf orientation than leaf optical properties, seabed reflectance, or the average cosine of downwelling irradiance. Better knowledge of leaf orientation appears to be a fruitful avenue for improving our understanding of the interaction between seagrasses and the submarine light environment

Acceleration of Nutrient Uptake by Phytoplankton in a Coastal Upwelling Ecosystem: A Modeling Analysis

Studies of upwelling centers in the eastern Pacific suggest that maximum rates of nitrate uptake (light and nutrient saturated) increase, or shift-up, as newly upwelled water moves downstream. The rate of shift-up appears to be related to irradiance and the ambient concentration of limiting nutrient at the time of upwelling. A mathematical model was developed to evaluate effects of irradiance and initial nitrate concentration on temporal patterns of shift-up and subsequent time scales of nutrient utilization over a range of simulated upwelling conditions. When rates consistent with field studies were used, complete shift-up was possible only under certain conditions, and the time scale was on the order of 7-10 d. These results are consistent with field observations. Increased initial nitrate concentrations resulted in more rapid depletion of the nutrient supply. Making acceleration of V max constant and independent of the nitrate concentration reversed the qualitative pattern of nutrient utilization and predicted longer time scales in the region of optimal growth (12- 15 d) than have been observed in the field. Since changes in nitrogen-specific V max observed in situ may be due to downstream sinking of detrital nitrogen, a third hypothesis was evaluated, in which there was no shift-up in Vmax. This last scenario is untenable, predicting time scales of nutrient utilization two to three times longer than observed in the field

Modeling the Vertical Distributions of Downwelling Plane Irradiance and Diffuse Attenuation Coefficient in Optically Deep Waters

The diffuse attenuation coefficient Kᵈ is critical to understand the vertical distribution of underwater downwelling irradiance (Eᵈ). Theoretically Eᵈ is composed of the direct solar beam and the diffuse sky irradiance. Applying the statistical results from Hydrolight radiative transfer simulations, Kᵈ is expressed into a mathematical equation (named as PZ06) integrated from the contribution of direct solar beam and diffuse sky irradiance with the knowledge of sky and water conditions. The percent root mean square errors (RMSE) for the vertical distribution of Eᵈ(z) under various sky and water conditions between PZ06 and Hydrolight results are typically less than 4%. Field observations from the southern Middle Atlantic Bight (SMAB) and global in situ data set (NOMAD) also confirmed the validity of PZ06 in reproducing Kᵈ. PZ06 provides an alternative and improvement to the simpler models (e. g., Gordon, 1989; and Kirk, 1991) and an operational ocean color algorithm, while the latter two kinds of models are valid to limited sky and water conditions. PZ06 can be applied to study Kᵈ from satellite remotely sensed images and seems to improve Kᵈ derivation over current operational ocean color algorithm

Episodic Nutrient Supply to a Kelp Forest Ecosystem in Southern California

Temporal patterns of nutrient input into a Southern California kelp forest were measured using traditional hydrocast sampling coupled with high frequency temperature profiling. Patterns of nutrient input were related to growth rates of Macrocystis pyrifera located in an adjacent kelp forest. There were 2 distinct components to the pattern of nutrient availability. The long term, or seasonal, component was consistent with large-scale storm-induced mixing and horizontal advection during winter months. In addition, vertical motions of the thermocline, bringing nutrients into the kelp forest, occurred throughout the year with a frequency of about 2 per day and were strongest during the summer months. Weekly hydrocast sampling methods were inadequate for measuring these episodic events, and high frequency sampling was required to resolve the pattern of nutrient input accurately. Although measurable, nutrient input from vertical thermocline motion was inadequate to sustain maximum growth of Macrocystis pyrifera at 10 m depth during the summer months. Thus, the major component of nutrient input came during the winter. These results indicate that nitrate limitation of M. pyrifera is a likely cause of reduced summer growth. Further, high frequency sampling is necessary to predict nutrient availability in nearshore ecosystems dominated by benthic macrophytes where the pattern of nutrient input is dominated by episodic events of short duration

Effects of El Nino on Local Hydrography and Growth of the Giant Kelp, Macrocystis Pyrifera, at Santa Catalina Island, California

Deepened isotherms associated with El Niño resulted in severe nutrient limitation and very low kelp productivity during the last half of 1983. Frond growth rates were so low that terminal blades formed before reaching the surface, eliminating the canopy. Frond initiation rates were also extremely low, resulting in significant reductions in mean plant size. Plants growing above 10m were more severely affected than plants at 20m. Nutrient pulses associated with internal waves are thus critical for survival of Macrocystis pyrifera in nutritionally marginal habitats in Southern California

Response of Eelgrass Zostera marina to CO2 Enrichment: Possible Impacts of Climate Change and Potential for Remediation of Coastal Habitats

Projected increases in dissolved aqueous concentrations of carbon dioxide [CO2(aq)] may have significant impacts on photosynthesis Of CO2-limited organisms such as seagrasses. Short-term CO2(aq) enrichment increases photosynthetic rates and reduces light requirements for growth and survival of individual eelgrass Zostera marina L. shoots growing in the laboratory under artificial light regimes for at least 45 d. This study examined the effects of long-term CO2(aq) enrichment on the performance of eelgrass growing under natural light-replete (33% surface irradiance) and light-limited (5% surface irradiance) conditions for a period of 1 yr. Eelgrass shoots were grown at 4 CO2(aq) concentrations in outdoor flow-through seawater aquaria bubbled with industrial flue gas containing approximately 11% CO2. Enrichment with CO2(aq) did not alter biomass-specific growth rates, leaf size, or leaf sugar content of above-ground shoots in either light treatment. CO2(aq) enrichment, however, led to significantly higher reproductive output, below-ground biomass and vegetative proliferation of new shoots in light-replete treatments. This suggests that increasing the CO2 content of the atmosphere and ocean surface will increase the area-specific productivity of seagrass meadows. CO2(aq) enrichment did not affect the performance of shoots grown under light limitation, suggesting that the transition from carbon- to light-limited growth followed Liebig\u27s Law. This study also demonstrated that direct injection of industrial flue gas could significantly increase eelgrass productivity; this might prove useful for restoration efforts in degraded environments. The broader effects Of CO2(aq) enrichment on the function of natural seagrass meadows, however, require further study before deliberate CO2 injection could be considered as an engineering solution to the problem of seagrass habitat degradation

Impact of Sea Grass Density on Carbonate Dissolution in Bahamian Sediments

Carbonate dissolution has been widely observed in shallow water tropical sediments. However, sediment budgets C have generally not been closed with respect to the amount of acid required to produce the observed carbonate dissolution. Recently it has been suggested that enhanced oxygen transport into sediments through the roots and rhizomes of sea grasses might play a role in resolving this mass balance problem. We conducted studies of sea grass-carbonate sediment interactions around Lee Stocking Island, Exuma Islands, Bahamas to further examine this problem. Our studies showed that alkalinity, total dissolved inorganic carbon (ΣCO2) and Ca2+ increased with depth in the pore waters, while pH and calculated carbonate ion concentration decreased with depth. These observations are consistent with the occurrence of carbonate dissolution in these sediments. The magnitude of pore water alkalinity, ΣCO2, and Ca2+ changes was also related to sea grass density, with the largest gradients seen in the sediments of dense sea grass beds. Calculations suggested that less than similar to 50% of the O2 needed to drive aerobic respiration (and ultimately carbonate dissolution via CO2 production) could be supplied by transport processes such as diffusion, bioturbation, and physical pore water advection. Furthermore, the O2 needed to balance the carbonate dissolution budget could be provided by the transport of \u3c15% of the photosynthetically derived O2 to the sediments through sea grass roots and rhizomes without enhancing the removal of carbonate dissolution end products. Thus sea grasses play an important role in controlling the rates of carbonate dissolution in shallow water tropical marine sediments

Effect of Light/Dark Transition on Carbon Translocation in Eelgrass Zostera marina Seedlings

Carbon translocation in the marine macrophyte Zostera marina L. (eelgrass) was investigated to elucidate the impact of light/dark transitions on sucrose partitioning between roots and shoots. After exposure of leaves to C-14-bicarbonate, the level of C-14-labelled photoassimilates increased monotonically in both leaves and fully aerobic roots of plants maintained in the light. Accumulation of C-14 in roots and leaves ceased abruptly when plants were transferred to darkness that induced root anaerobiosis even though C-14 levels remained high in the dark-exposed leaves. Thus, translocation of C-14 photoassimilates from shoots to roots was inhibited when roots became anoxic. Anoxia induced by light limitation of photosynthesis, whether due to day/night transitions or periods of extreme light attenuation in the water column, can have an impact on carbon availability in subterranean tissues of eelgrass. As a consequence, light availability is likely to control the productivity and distribution of eelgrass in highly variable and light-limited coastal environments through its effects on carbon partitioning between shoots and roots, in addition to whole-plant carbon balance

Dynamics of Carbon Allocation in a Deep-Water Population of the Deciduous Kelp Pleurophycus gardeneri (Laminariales)

Pleurophycus gardneri (Laminariales) is common in the low intertidal of the Northeast Pacific, but dominates many deep (30 to 40 m) rocky reefs in central California. Seasonal dynamics of productivity and resource allocation of a deep-water population of this deciduous, stipitate kelp were studied to understand how blade abscission affects the annual carbon budget. Patterns of growth, metabolism, and carbon storage and mobilization were measured monthly for 1 yr relative to in situ light and temperature, and used to model the annual carbon budget. The resulting carbon budget was used to determine if blade abscission effectively reduced respiratory demand during the winter period of low light availability. Metabolic properties (photosynthesis, photoacclimation, and respiration) were seasonally constant and showed evidence of photoacclimation to this deep, low-light environment. Blades grew between February and July, followed by senescence and sloughing from August to December. Concentrations of laminaran and mannitol increased in the blades from the onset of sloughing in August until just prior to blade abscission in mid-December, suggesting translocation of these carbohydrates may have occurred from the blade to the stipe and holdfast. Carbon budget estimates revealed that scalar irradiance measures overestimated the light available for photosynthesis of these paddle-shaped kelp blades by 50 to 75%. The calculations also revealed that blade retention allowed for the maintenance of positive carbon balance throughout the year. Thus, conservation of the internal carbon reserve for metabolic survival during the low-light period does not appear to be a viable explanation for the deciduous life history of P. gardneri. Abscission may reduce hydrodynamic drag, thus minimizing the probability of dislodgment of entire plants during winter storm events, or promote spore dispersal as abscised blades and sori drift away from the parent holdfast

In Situ Growth and Chemical Composition of the Giant Kelp, Macrocystis pyrifera: Response to Temporal Changes in Ambient Nutrient Availability

Temporal variations in growth of the giant kelp Macrocystis pyrifera were examined in relation to ambient nutrient availability and chemical composition of mature blades, the primary site of nutrient and carbohydrate storage in M. pyrifera. The effect of nutrient availability on growth was well approximated by a Monod rectangular hyperbola, with growth saturating at ambient nitrate concentra- tions between 1 and 2 FM. M. pyrlfera was unable to generate nutrient reserves that would last beyond 30 d. Nitrogen reserves were stored as free amino acids, and generally constituted about 10 % of total tissue nitrogen. Total nitrogen content was never more than 2.5% of dry weight. There was no significant correlation between growth and tissue nitrogen. In contrast, carbohydrate levels were negatively correlated with growth rates, tissue nitrogen content, and ambient nutrient availability. Although concentrations of nitrogen and carbohydrate reserves showed familiar variations described for other kelps, the physical environment in southern California is probably not amenable to M. pyrifera making strategic use of these reserves. Nutrient availability appears to be too low to permit accumulation of more than 30 d reserve of nitrogen, and light levels are probably never low enough to make stored carbohydrate reserves necessary for survival