LONG-TERM EFFECTS OF MANIPULATING LIGHT INTENSITY AND NUTRIENT ENRICHMENT ON THE STRUCTURE OF A SALT MARSH DIATOM COMMUNITY1,2

The long-term effects of manipulating light intensity and nutrient enrichment on the structural characteristics of a diatom community inhabiting the sediments beneath a pure stand of dwarf Spartina alterniflora Loisel. were investigated over a yearly cycle. Clipping or shading the cord grass cover, or phosphorus enrichment caused significant decreases in both species diversity (H') and the number of diatom species, whereas nitrogen enrichment only significantly decreased the latter parameter. Of the 105 diatom taxa identified, only 10 were restricted to certain of the 12 study areas; and of these, 8 occurred exclusively in the clipped habitats. An analysis of variance (light × nutrient × collection date) involving 19 of the most abundant taxa revealed that certain experimental treatments had significant effects on the relative abundances of each and every taxon. However, attempts to group taxa with similar response patterns proved unsuccessful because of the frequent significance of the 3-way interaction term. Synthesis of these results with earlier work by the author showed that differences in structure of diatom communities inhabiting the sediments beneath the 3 dominant marsh grasses were not primarily caused by differences in reduction of light intensity by their grass canopies, and that clipping of the cord grass produced a shift in community structure towards that characteristic of a salt panne algal mat.     

SHORT-TERM EFFECTS OF UVB RADIATION ON CHLOROPHYLL FLUORESCENCE, BIOMASS, PIGMENTS, AND CARBOHYDRATE FRACTIONS IN A BENTHIC DIATOM MAT

The effects on UVB radiation on a subtidal, cohesive-sediment biofilm dominated by the diatom Gyrosigma balticum (Ehrenberg) Rabenhorst were investigated. Chlorophyll fluorescence parameters ( F _v/ F _m, φ_PSII), pigment concentrations, cell densities, and carbohydrate fractions were measured in four treatments (no UVBR, ambient UVBR, +7%, and +15% enhancement with UVBR). Enhanced UVBR was provided by a computer-controlled system directly linked to natural diel UVBR levels. Increases in φ_PSII values in the UVBR-enhanced treatments and a decrease in the steady-state fluorescence yield ( F _s) from the surface of the biofilms during the middle and latter part of daily exposure periods suggested that G. balticum responded to enhanced UVBR by migrating down into the sediment. Diatoms in the +15% UVBR treatment also had significantly higher concentrations of β-carotene after 5 days of treatment. Although G. balticum responded to enhanced UVBR by migration and increased β-carotene concentrations, significant reduction in maximum quantum yield of PSII ( F _v/ F _m) and in minimal fluorescence ( F _o) and decreases in cell densities occurred after 5 days. Concentrations of different carbohydrate fractions (colloidal carbohydrate, glucan, exopolymers [EPS]) associated with diatom biomass and motility also decreased in the UVBR-enhanced treatments. Short-term responses (migration) to avoid UVBR appear insufficient to prevent longer-term decreases in photosynthetic potential and biofilm carbohydrate concentration and biomass.     

DENSITY-DEPENDENT GROWTH,ECOLOGICAL STRATEGIES,AND EFFECTS OF NUTRIENTS AND SHADING ON BENTHIC DIATOM SUCCESSION IN STREAMS1

The importance of immigration, growth, and competition for nutrients and light in benthic diatom succession was studied in experimental channels in a low-nutrient stream. Diatom accumulation was greater in channels enriched with nitrate and phosphate (NP) than in control channels, reaching about 5 × 10⁶ and 2 × 10⁶ cells-cm^?2, respectively, after 30 d. Shading during late stages of community development reduced algal standing crop. Synedra ulna (Nitz.) Ehr. and Achnanthes minutissima Kütz. were codominant during early stages of community development in both habitats, but succession to an A. minutissima-dominated community was much faster in NP-enriched than in control conditions. Species dominating early stages tended to immigrate quickly, whereas species that increased in relative abundance during community development had either fast growth rates or fast immigration and average growth rates. Decreases in growth rates indicated resource supply became limiting during community development in control and enriched channels. Density-dependent competition was indicated because nutrient concentrations in the water column and light did not decrease during the 30-d study. Species autecologies were defined by effects of nutrient enrichment, shading, and community development on species growth rates. Differing autecologies of early and late succession species indicated that competition for nutrients was more important than competition for light. Species autecologies also indicated ecological strategies. The species most stimulated by nutrient enrichment were least able to maintain growth rates as algal abundances on substrata increased. In addition, these species that best sustained their growth rates during succession tended to have the highest immigration rates, indicating that drift and immigration may have been an important mechanism of persistence for some populations when resources become limiting within thick benthic mats.     

STUDIES ON THE AUTECOLOGY OF THE MARINE DIATOM THALASSIOSIRA NORDENSKIOELDII. II. THE INFLUENCE OF CELL SIZE ON GROWTH RATE,AND CARBON,NITROGEN, CHLOROPHYLL a AND SILICA CONTENT1

The effect of cell size on growth rates and some cellular contents of Thalassiosira nordenskioeldii Cleve has been measured at 0 and 10 C. At 0 C the growth rate did not vary with cell size. The 2 smallest clones at this temperature had reduced growth rates because of the induction of sexuality in that size range. The clones grown at 10 C showed a significant negative relationship between growth rate and valve diameter with the cell surface area/volume ratio positively related to growth rate. At both temperatures the smaller cells had proportionately more carbon and nitrogen/unit cell volume. The amount of chlorophyll a and silica/unit cell surface area increased with increasing cell surface area at both 0 and 10 C. Both the C/N and C/chl a ratios showed no significant change with cell size at either temperature but there was a significant increase in the C/chl a ratio at 0 C. The C/Si ratio decreased with increasing cell size at both 0 and 10 C.