Notes: The tropical rainforest mesocosm within the Biosphere 2 Laboratory, a model system of some 110 species developed over 12 years under controlled environmental conditions, has been subjected to a series of comparable drought experiments during 2000–2002. In each study, the mesocosm was subjected to a 4–6 week drought, with well-defined rainfall events before and after the treatment. Ecosystem CO2 uptake rate (Aeco) declined 32% in response to the drought, with changes occurring within days and being reversible within weeks, even though the deeper soil layers did not become significantly drier and leaf-level water status of most large trees was not greatly affected. The reduced Aeco during the drought reflected both morphological and physiological responses. It is estimated that the drought-induced 32% reduction of Aeco has three principal components: (1) leaf fall increased two-fold whereas leaf expansion growth of some canopy dominants declined to 60%, leading to a 10% decrease in foliage coverage of the canopy. This might be the main reason for the persistent reduction of Aeco after rewatering. (2) The maximum photosynthetic electron transport rate at high light intensities in remaining leaves was reduced to 71% for three of the four species measured, even though no chronic photo-inhibition occurred. (3) Stomata closed, leading to a reduced ecosystem water conductance to water vapour (33% of pre-drought values), which not only reduced ecosystem carbon uptake rate, but may also have implications for water and energy budgets of tropical ecosystems. Additionally, individual rainforest trees responded differently, expressing different levels of stress and stress avoiding mechanisms. This functional diversity renders the individual response heterogeneous and has fundamental implications to scale leaf level responses to ecosystem dynamics.

Notes: Abstract  Effects of nutrient treatments on photoacclimation of the hermatypic coral Stylophora pistillata (Esper) were studied. Studies on photoacclimation of colonies from different light regimes in the field were evaluated and used to design laboratory experiments. Coral colonies were collected in the Gulf of Eilat (Israel) from January to March 1993. Exterior branches of colonies from different depths (1 to 40 m) displayed different trends in production characteristics at reduced and very low levels of illumination. From 24 ± 3% to 12 ± 2% of incident surface photosynthetic active radiation (PARo), zooxanthella population density and chlorophyll a+c per 106 zooxanthellae increased, a trend seen in the range of light levels optimal for coral growth (90 to 30% PARo). The P max of CO2 per 106 zooxanthellae decreased, while P max of CO2 per 103 polyps increased, indicating an increase in zooxanthella population density at low light levels. Proliferous zooxanthella frequency (PZF, a measure of zooxanthella division) declined significantly at light levels 〈18 ± 3% PARo. At the lowest levels of illumination (〈5% PARo), zooxanthella population density decreased, as did the PZF; chl a+c per 106 zooxanthellae was unchanged. In 28-d experiments, exterior coral branches from the upper surfaces of colonies from 3 m depth (65 ± 4% PARo) were incubated in aquaria under bright (80 to 90% PARo), reduced (20 to 30% PARo), and extremely low (2 to 4% PARo) light intensities. At each light intensity, the corals were maintained in three feeding treatments: sea water (SW); ammonium enriched SW (SW + N); SW with Artemia salina nauplii (SW + A). An increase in P max of CO2 per 103 polyps was found in corals acclimated to reduced light (20 to 30% PARo) in nutrient-enriched SW, while in SW, where the increase in zooxanthella population density was smaller, it did not occur. Nutrient enrichments (SW + N at 2 to 4% PARo and SW + A at 20 to 30% PARo) increased zooxanthella population density, but had no effect on chl a+c per 106 zooxanthellae. Acclimation for 14 d to reduced (10 to 20% PARo) and extremely low (1 to 3% PARo) light intensities shifted 14C photoassimilation into glycerol and other compounds (probably glycerides), rather than sugars. Both ammonium addition and feeding with Artemia salina nauplii resulted in an increase in photosynthetic assimilation of 14C into amino acids. We conclude that acclimation to reduced light consists of two processes: an increase in photosynthetic pigments and in zooxanthella population density. Both processes require nitrogen, the increase in zooxanthella population density needing more; this adaptation is therefore limited in nitrogen-poor sea water.