No abstract

The observation that the relative importance of picophytoplankton is greatest in warm and nutrient-poor waters was tested here based on a comprehensive review of the data available in the literature from oceanic and coastal estuarine areas. Results show that picophytoplankton dominate (Ն50%) the biomass and production in oligotrophic (chlorophyll a [Chl a] Ͻ 0.3 mg m Ϫ3 ), nutrient poor (NO 3 ϩ NO 2 Ͻ 1 M), and warm (Ͼ26ЊC) waters, but represent Ͻ10% of autotrophic biomass and production in rich (Chl a Ͼ 5 mg m Ϫ3 ) and cold (Ͻ3ЊC) waters. There is, however, a strong covariation between temperature and nutrient concentration (r ϭ Ϫ0.95, P Ͻ 0.001), but the number of observations where both temperature and nutrient concentrations are available is too small to allow attempts to statistically separate their effects. The results of mesocosm nutrient addition experiments during summer in the Mediterranean Sea allowed the dissociation of the effects of temperature from those of nutrients on picophytoplankton production and biomass and validated the magnitude at which picoplankton dominates (Ն50%) autotrophic biomass and production obtained in the comparative analysis. The fraction contributed by picoplankton significantly declined (r 2 ϭ 0.76 and 0.90, respectively, P Ͻ 0.001) as total autotrophic production and biomass increased. These results support the increasing importance of picophytoplankton in warm, oligotrophic waters. The reduced contribution of picophytoplankton in warm productive waters is hypothesized here to be due to increased loss rates, whereas the dominance of picophytoplankton in warm, oligotrophic waters is attributable to the differential capacity to use nutrients as a function of differences in size and capacity of intrinsic growth of picophytoplankton and larger phytoplankton cells.

In this study, we confirm the relationship between temperature and Synechococcus sp. experimental growth rates (r = 0.87, p < 0.005) and provide evidence of the existence of a general relationship. This link leads to a strong seasonality of abundance and biomass of Synechococcus sp. in the Bay of Blanes (NW Mediterranean), which was followed for 2 yr (1995, 1996), with high values in summer months (6 X 10' cells I-') and low values in winter (5 X 105 cells I-'). The growth rate achieved in summer months (1.5 d-') IS close to or at the maximum possible at the in situ water temperature. As a result, Synechococcus growth may exceed the grazing capacity of its predators in summer, and this explains its significant contribution of >30%, of the total gross autotrophic production and >20% of the total autotroph~c blomass in summer Thus, Sj~nechococcus is an important source of organic C and nutrients for the coastal Mediterranean food web in the summer.

All seagrasses are rhizomatous plants that grow by reiteration of a limited set of modules. Their past growth history can therefore be reconstructed from the scars left by abscised leaves and flowers on the long-lived rhizomes or the seasonal slgnals Imprinted in the frequency and size of their modules. We provide here the basic foundations and assumptions of these reconstruction techniques and the calculations involved in their application. We then show their reliability and potential to quantlfy an array of ecological processes, such as plant demography, leaf and rhlzome production, flowering ~ntensity, and seagrass responses to anthropogenic perturbations, based on our recent studies of Mediterranean, Caribbean and Indo-Pacific seagrass species. Reconstruction techniques have also proven useful in demonstrating the role of seagrasses as tracers of sedlment movement over seagrass beds and the rates of colonisation and expansion of seagrass patches. These reconstruction techniques should provide a powerful tool to improve our knowledge of seagrass species and populations from remote areas based on a single or just a few visits This should, therefore, allow us to sample many seagrass meadows using limited resources, thus generating a strong foundation for the study of comparatlve seagrass ecology and testing of theories previously applied to terrestrial plant populations.

Leaf product~on, shoot demography and rhlzome growth and branchlng were quantified for the common seagrass specles In a muted seagrass bed on the Bollnao leef flat (Luzon The Ph~llp-plnes) to assess the contribution of these s p e c~e s to canopy maintenance meadow biornass and productlvlty We tested the hypothesis that seagrass growth rates correlated negatively with shoot size and a g e when compared across specles, and found that shoot recruitment leaf turnover and honzontal rhizome elongation and branchlng rates were lower for s p e c~e s wlth older and larger shoots ~Vedian shoot ages for the short-llved species were generally less than a year, those for the longer-llved Enhalus acoroides ( L f ) Royle and Thalassla hempnchii (Ehrenb ) Aschers were sllghtly more than 1 5 yr The oldest E acoroldes had almost reached 10 yr Generally, shoot mortality and ~e c r u l t m e n t balanced each other falrly well The rhizomes of longer-lived E acoroldes and T hempnchli elongated at rates of 5 and 21 cm yr-l, respectively, and those of the short-llved Synngodi~lm isoetifobum (Aschers ) Dandy and Halophila ovalls (R Br ) Hook f at rates of 135 and 141 cm yr-' Vertical shoot elongat~on ranged from 2 to 13 cm shoot ' yr ' and was not correlated wlth size or a g e The meadow had a total bion~ass of 624 g dry wt (DW) m-' (roots excluded), to whlch the larger and longer-llved specles 7 hempnchil and E acoro~des contnbuted substantially (52 and 37%, respectively) Leaf product~on dominated total annual ploductlvity, constituting 91 % of 2143 g DW m ' yr ' (roots excluded) thls productlvlty was malnly due to T hempnchli (74 %), and not to the oldest and slowest-growing E acoroides ( 1 0 % ) KEY WORDS Troplcal seagrasses Shoot life spans Recruitment and mortal~ty P~o n e e r vs cllmax specles Allon~etry