G. Peralta

BIOMASS AND DYNAMICS OF GROWTH OF ULVA SPECIES IN PALMONES RIVER ESTUARY1

During the last decade, the Palmones River estuary has undergone severe eutrophication followed by a green tide episode; two species of Ulva, rotundata Blid. and Ulva curvata (Kütz.) De Toni, were the main macroalgae responsible for this bloom. From November 1993 to December 1994, we followed the biomass, the growth dynamics, and tissue elemental composition (C:N:P)of Ulva species, as well as some physicochemical variables in the estuary. Maximum biomass (up to 375 g dry wt·m−2 in some spots, corresponding to a thallus area index of nearly 17 m2Ulva·m−2 sediment) were observed in June and December. However, the biomass varied among the sampling stations. Water nitrate, ammonia, and phosphate showed high concentrations throughout the year, with extremely high transient pulses, sustaining the high growth rates observed. Growth rates were estimated directly in the field. The rates were generally higher in Ulva discs maintained in net cages than those estimated by changes in biomass standing stock between two consecutive samplings. The difference between both estimates was used to quantify the importance of the processes causing loss of biomass, which were attributable to grazing, exported biomass, and thallus decomposition under anaerobic conditions resulting from extreme self‐shading. Maximum chlorophyll content was found in winter, whereas the minimum was in spring. Atomic N:P ratios were generally higher in the algae than in the water. However, the absolute concentrations of tissue N and P were always higher than the critical levels for maximum growth, which suggests that growth was not limited by inorganic N or P availability. The results suggested that the increase in nutrient loading in the river may have triggered the massive development of green algae and that light limitation and temperature stress in summer seem to be the main factors controlling the abundance of Ulva in the estuary. In addition to light availability and thermal stress, the different loss processes may have a decisive role in the dynamics of Ulva biomass.

Effects of light availability on growth, architecture and nutrient content of the seagrass Zostera noltii Hornem

The growth vs. irradiance response of the seagrass Zostera noltii from Cadiz Bay Natural Park (southwestern Spain) was characterised. Plants were exposed along 14 days to different light treatments (1%, 7%, 42% and 100% surface irradiance, SI), using shade screens in an outdoor mesocosm. Growth at 100% SI (1.6 mg DW plant(-1) day(-1)) was lower than that at 42% SI (2.4 mg DW plant(-1) day(-1)), suggesting photo inhibition. The minimum light requirement estimated was 0.8 mol photons m(-2) day(-1) (2% SI). Light availability affected the pattern of plant development and the overall plant growth. The contribution of the apical shoots to the aboveground production was nearly constant (c.a. 1.13 cm plant(-1) day(-1)) regardless of the light level (except at 1% SI). In contrast, recruitment and growth of lateral shoots arising from the main rhizome axes accounted for the observed differences in aboveground growth, Rhizome branching was only observed at 42% SI. The possibility of a light threshold for rhizome branching could explain the seasonality of shoot recruitment, as well as the observed decrease in shoot density along depth (or light) gradients in seagrass meadows. Carbon demands at low irradiances (1% and 7% SI) were partially met by mobilization of carbohydrate reserves (sucrose in belowground and starch in aboveground parts). Plant nitrogen content decreased with increasing light, especially in belowground parts, reaching critical levels for growth. [KEYWORDS: branching, C/N ratio, growth rate, light, nonstructural carbohydrates, plant architecture, seagrass]

Morphological and physiological differences between two morphotypes of Zostera noltii Hornem. from the south-western Iberian Peninsula

Abstract The morphological and physiological differences between two morphotypes of Z. noltii Hornem. were studied in the intertidal meadows on the south-western Iberian Peninsula (Palmones river estuary and Ria Formosa). A small-leaved morphotype (SM) grows mainly at high intertidal sites, meadow edges or in recently deposited sandbanks, whereas a large-leaved morphotype (LM) generally thrives in well-structured beds or in deeper places. This study deals with the morphological, biochemical and physiological differences between these morphotypes as well as the ecological implications of the occurrence of different morphotypes in the same meadow. Shoot length, leaf width, rhizome internode length, roots per node, root length, leaf nutrient and pigment contents, and photosynthetic rates of both morphotypes were compared. The below-ground architecture (root and rhizome complex) of both morphotypes was more developed in sites characterized by higher hydrodynamics and/or a lower nitrogen content in sediments. Both morphotypes showed similar values for photosynthetic efficiency, dark respiration rate and compensation irradiance. On the other hand, the net photosynthetic capacity was much greater (5-fold) for the SM. This difference could explain the greater growth rate and faster leaf turnover rate of the SM compared with the LM. The occurrence of the SM in newly settled areas (and in the meadow edges) could be explained on the basis of its higher growth rate, which would allow a faster spreading of the meadow and/or better recovery after burial resulting from stormy weathers.

SEASONAL VARIATION OF PHOTOSYNTHETIC PERFORMANCE AND LIGHT ATTENUATION IN ULVA CANOPIES FROM PALMONES RIVER ESTUARY1

The primary production of Ulva populations relies on their photosynthetic performance, which is dependent on the light availability under natural conditions. This study concerns the light attenuation characteristics in Ulva canopies and the seasonal photosynthetic performance of two different species (Ulva rotundata Blid., Ulva curvata (Kütz.) De Toni) blooming in the Palmones river estuary. Light within canopies differed from that reaching the surface. Light availability was reduced through the water column (at high tide) and Ulva canopies. In addition, light was spectrally filtered. As a result, the photosynthetically usable radiation (PUR) was further attenuated through Ulva canopies, increasing the photosynthetically active radiation/PUR ratio. The muddy sediment deposited on and between the Ulva thalli also drastically restricted the light availability. Thick Ulva mats are frequently found covering the intertidal mudflats, and therefore, thalli within these mats may be subjected to steep light gradients. As a consequence, individual Ulva growth rates cannot be extrapolated to estimate the primary production of Ulva canopies. Interspecific differences were observed for light‐saturated photosynthetic rates (Pmax) and light compensation points (LCP), with Ulva curvata generally displaying higher values than did U. rotundata. For both species, maxima were recorded in winter for Pmax, quantum yield, chlorophyll content, and absorptance, whereas minima were found in summer. Dark respiration (Rd) was not seasonally affected, and a maximum LCP was found in summer. To extrapolate these data to field situations, the temperature dependence of photosynthesis should be considered. The Q10 values were 2.44 for Rd and 1.79 for Pmax, whereas the photosynthesis rate at subsaturating light levels was unaffected. The Q10 values showed an enhanced respiratory rate in summer and a minimum in winter, whereas the seasonal differences on Pmax were damped.

The effect of photoacclimation on the photosynthetic physiology of Ulva curvata and Ulva rotundata (Ulvales, Chlorophyta)

The effects of photoacclimation on growth, photosynthesis, pigment content and elemental composition of Ulva curvata and Ulva rotundata, which grow together in eutrophic areas of southern Spain, were investigated. Cultures were grown for 6 days at different photon fluence rates (PFR) ranging from darkness up to 200 µmolm-2 s-1 under nutrient-sufficient conditions (artificial seawater supplemented with ammonium and phosphate). Growth rates were not light-saturated (up to 200 µmolm-2s-1), reaching a value close to 0·2d-1. Growth rates based on mass, area or C content were equivalent, except in darkness and very low light levels (2 µmolm-2s-1), where thallus expansion occurred by diluting internal biomass and C. Chlorophyll and absorptance showed a bell-shaped PFR-response curve, with maxima at 30–60 µmolm-2s-1 and lower values at light saturation and under light-limiting conditions. Although net NH4 + incorporation was not affected by growth-PFR, there was inefficient assimilation of N at low light levels, ...

The role of hydrodynamics in structuring in situ ammonium uptake within a submerged macrophyte community

In low-nutrient, macrophyte-dominated coastal zones, benthic ammonium (NH4+) uptake may be influenced by the structural properties of plant canopies via their effect on near-bed hydrodynamics. Using a dual-tracer (uranine and 15NH4+) method that does not require enclosures, we examined how this process affects nutrient uptake rates within a tidally dominated, patchy Caulerpa prolifera–Cymodocea nodosa landscape. NH4+ uptake was determined by calculating tissue 15N excesses and correcting for 15N enrichment as derived from uranine concentration. Vertical hydrodynamic profiles were measured in the downstream flow direction from outside to inside of the C. nodosa bed by using an array of acoustic Doppler velocimeters. The transition from a C. prolifera to a C. nodosa bed included a change in both benthic canopy properties (short and dense to tall and sparse) and sediment topography (0.2-m increase in water column depth) that resulted in an increase in longitudinal advection and turbulent diffusivity within the C. nodosa canopy between 0.5 and 1.5 m from the leading edge. Vertical differences in canopy water exchange appeared to explain variations in uptake between biotic functional groups; however, no clear differences in longitudinal uptake were found. Using in situ labeling, this study demonstrated for the first time the role of hydrodynamics in structuring NH4+ uptake within an undisturbed, patchy macrophyte landscape.

Effects of intertidal seagrass habitat fragmentation on turbulent diffusion and retention time of solutes.

An in-depth knowledge of solutes advection and turbulent diffusion is crucial to estimate dispersion area and retention time (t(R)) of pollutants within seagrass habitats. However, there is little knowledge on the influence of seagrass habitat fragmentation on such mechanisms. A set of dye tracer experiments and acoustic Doppler velocimeter measurements (ADV) were conducted. Solute transport conditions were compared in between fragmented (FM) vs homogeneous (HM) intertidal meadows, and in vertical gradients (canopy vs overlaying flow). Results showed the highest horizontal diffusion coefficient (K(y), c.a. 10(-3)m(2)s(-1)) on FM and at the canopy-water column interface, whereas t(R) (2.6-5.6 min) was not affected by fragmentation. It suggests that (1) FM are more vulnerable to pollution events in terms of dispersion area and (2) at low tide, advection rather than turbulent diffusion determines t(R). Furthermore, Taylors theorem is revealed as a powerful tool to analyze vertical gradients on K(y) within seagrass canopies.

Latitudinal Patterns in European Seagrass Carbon Reserves: Influence of Seasonal Fluctuations versus Short-Term Stress and Disturbance Events

Seagrass meadows form highly productive and valuable ecosystems in the marine environment. Throughout the year, seagrass meadows are exposed to abiotic and biotic variations linked to (i) seasonal fluctuations, (ii) short-term stress events such as, e.g., local nutrient enrichment, and (iii) small-scale disturbances such as, e.g., biomass removal by grazing. We hypothesized that short-term stress events and small-scale disturbances may affect seagrass chance for survival in temperate latitudes. To test this hypothesis we focused on seagrass carbon reserves in the form of starch stored seasonally in rhizomes, as these have been defined as a good indicator for winter survival. Twelve Zostera noltei meadows were monitored along a latitudinal gradient in Western Europe to firstly assess the seasonal change of their rhizomal starch content. Secondly, we tested the effects of nutrient enrichment and/or biomass removal on the corresponding starch content by using a short-term manipulative field experiment at a single latitude in the Netherlands. At the end of the growing season, we observed a weak but significant linear increase of starch content along the latitudinal gradient from south to north. This agrees with the contention that such reserves are essential for regrowth after winter, which is more severe in the north. In addition, we also observed a weak but significant positive relationship between starch content at the beginning of the growing season and past winter temperatures. This implies a lower regrowth potential after severe winters, due to diminished starch content at the beginning of the growing season. Short-term stress and disturbances may intensify these patterns, because our manipulative experiments show that when nutrient enrichment and biomass loss co-occurred at the end of the growing season, Z. noltei starch content declined. In temperate zones, the capacity of seagrasses to accumulate carbon reserves is expected to determine carbon-based regrowth after winter. Therefore, processes affecting those reserves might affect seagrass resilience. With increasing human pressure on coastal systems, short- and small-scale stress events are expected to become more frequent, threatening the resilience of seagrass ecosystems, particularly at higher latitudes, where populations tend to have an annual cycle highly dependent on their storage capacity.