ALGAL DISTRIBUTION IN A SMALL,INTERMITTENT STREAM RECEIVING ACID MINE-DRAINAGE1

Lynx Creek, a small intermittent creek in the Bradshaw Mountains of Arizona, is subjected to drainage from an abandoned copper mine. The mine-drainage decreases the pH of the Creek about three units and greatly increases sulfate and heavy metal concentrations. Chemical recovery of the Creek occurs downstream through precipitation of metal hydroxides and dilution by tributaries. Changes in Creek chemistry are accompanied by changes in algal flora. Above the mine and downstream after substantial recovery, the flora is dominated by Tribonema affine⁴, Achnanthes spp., and Synedra ulna and several zygnematacean species. In the mine seep entering the Creek and in the Creek just below the seep the flora is reduced in species richness and dominated in abundance by Microthamnion kuetzingianum and Eunotia tenella.     

ALGAL RESPONSE TO NUTRIENT ENRICHMENT IN FORESTED OLIGOTROPHIC STREAM1

Nutrient input in streams alters the density and species composition of attached algal communities in open systems. However, in forested streams, the light reaching the streambed (rather than the local nutrient levels) may limit the growth of these communities. A nutrient‐enrichment experiment in a forested oligotrophic stream was performed to test the hypothesis that nutrient addition has only minor effects on the community composition of attached algae and cyanobacteria under light limitation. Moderate nutrient addition consisted of increasing basal phosphorus (P) concentrations 3‐fold and basal nitrogen (N) concentrations 2‐fold. Two upstream control reaches were compared to a downstream reach before and after nutrient addition. Nutrients were added continuously to the downstream reach for 1 year. Algal biofilms growing on ceramic tiles were sampled and identified for more than a year before nutrient addition to 12 months after. Diatoms were the most abundant taxonomic group in the three stream reaches. Nutrient enrichment caused significant variations in the composition of the diatom community. While some taxa showed significant decreases (e.g., Achnanthes minutissima, Gomphonema angustum), increases for other taxa (such as Rhoicosphenia abbreviata and Amphora ovalis) were detected in the enriched reach (for taxonomic authors, see Table 2 ). Epiphytic and adnate taxa of large size were enhanced, particularly during periods of favorable growth conditions (spring). Nutrients also caused a change in the algal chl a, which increased from 0.5–5.8 to 2.1–10.7 μg chl · cm^?2. Our results indicate that in oligotrophic forested streams, long‐term nutrient addition has significant effects on the algal biomass and community composition, which are detectable despite the low light availability caused by the tree canopy. Low light availability moderates but does not detain the long‐term tendency toward a nutrient‐tolerant community. Furthermore, the effects of nutrient addition on the algal community occur in spite of seasonal variations in light, water flow, and water chemical characteristics, which may confound the observations.

Table 2. Percent abundances of the most frequent taxa in three reaches of the Fuirosos stream. U1 and U2 untreated; E, enriched both in the periods before (bef) and after (aft) the enrichment of the E reach. Acronyms identifying the taxa are indicated.

PHYSIOLOGICAL ECOLOGY OF NITROGEN FIXING BLUE-GREEN ALGAL CRUSTS IN THE UPPER-SUBALPINE LIFE ZONE1

Blue-green algal (cyanobacterial) crusts composed of nitrogen fixing Nostoc commune Voucher ex Born. et Flah. and Tolypothrix conglutinata var. colorata Ghose were studied in the upper-subalpine life zone, Mission Mountain Wilderness, Montana. Rates of ethylene production were highest in the submerged shoreline crusts, lower for exposed crusts pioneering rocky shorelines and lowest in the Carex meadow. Nitrogenase activity (acetylene reduction technique) was constant between 200–285% crust moisture content (wet/dry weight) and then rapidly declined to 0 between 200–140%. Optimal temperatures for ethylene production by illuminated cells was between 20–30° C for T. conglutinata, 20° C for N. commune and about 25° C in darkness for both species. Nitrogenase activity by T. conglutinata in culture was unaffected by repeated freeze-thaw treatments whereas N. commune was severely inhibited. In contrast, N₂-ase activity of these two species in an intact crust was unaffected by repeated freeze-thaw treatments. Application of nitrogen-free growth medium to intact crusts increased nitrogenase activity by 3.7 times implying that these were mineral deficient under field conditions. Photosynthesis was light saturated at 125 μmol-m^?2.s^?1 whereas nitrogenase activity was light independent for cells with carbohydrate reserves. When carbohydrate reserves were reduced by 8 h incubation in darkness, between 1–3 h of illumination were required to restore nitrogenase activity to 80% of the maximum rate. Biochemical pathway inhibitor studies employing DCMU, MFA, and CCCP showed that oxidative metabolism was the source of reductant for acetylene reduction. Tetrazolium precipitation in heterocysts paralleled acetylene reduction activity in the inhibitor treated cells.     

DISTRIBUTION AND PHYSIOLOGICAL DETERMINANTS OF BLUE-GREEN ALGAL NITROGEN FIXATION ALONG A THERMOGRADIENT1

The capacity of thermal algal-bacterial mats to fix nitrogen (N₂) was examined in an alkaline thermal stream, Rabbit Creek, of Yellowstone National Park. Nitrogenase activity and nitrogen-fixation rates of mat cores placed in serum bottles and incubated in situ were estimated by the acetylene-reduction technique. Active nitrogenase was not detected at 60 or 65 C in either the blue-green algal or bacterial undermat components of the mats. Acetylene was reduced by all mats ≤55 C along the thermogradient; mean fixation estimates for the mats ranged from 7 to 5,028 nmoles N₂ fixed · mg Chl a^?1· hr^?1. Maximum fixation occurred at 35 C in the stream; statistical comparison of mean rates ordered the thermogradient mats according to estimated activities: 35 > 40 > 30 > 50 ≥ 55 ≥ 45 C. Mats (≤40 C) dominated by species of Calothrix accounted for ca. 97% of the total nitrogen fixation observed in the stream; the remaining activity was associated with mats containing Mastigocladus laminosus Cohn. Light intensity significantly affected fixation rates of the Calothrix mats which responded in a linear fashion from 9–100% full sunlight (ca. 1,900 μEin · m^?2· sec^?1). Calothrix mats from 30 and 40 C had maximum nitrogenase activity at their growth temperature suggesting that nitrogen fixation along the thermogradient was optimally adapted to in situ temperatures.     

LIPIDS IN BLUE-GREEN ALGAL MATS (MODERN STROMATOLITES) FROM ANTARCTIC OASIS LAKES1

Lipids comprising the stenols, stanols, polar lipid fatty acids, alkanes and alkenes of blue-green algal-(diatomaceous)-microbial mats and cores (modern cold water stromatolites) collected from three Antarctic lakes were identified and compared with those of other algae. The major stenols were: (cholesta-5, 22-dien-3β-ol, cholest-5-en-3β-ol, 24-methylcholesta-5, 22-dien-3β-ol, 24-methyl-cholest-5-en-3β-ol, 24-ethylcholesta-5, 22-dien-3β-ol, and 24-ethylcholest-5-en-3β-ol). The presence of C28 Δ^{3, 22} stenols, as well as other C28 stenols, was suggestive of diatom input. C29 stenols may have originated from blue-grern algae. However, the high concentrations of stenols present and the lack of Δ⁷ stenols was atypical for known stenol components of several blue-green algal species previously reported. The occurrence of these stenols and other lipid markers strongls implicate diatoms as well as blue-green algae as important biogenetic sources of lipids and has established the potential for studies of lipid diagenesis in these unique cold, freshwater stromatolites .     

NOSTOC (CYANOPHYTA) PRODUCTIVITY IN OREGON STREAM ECOSYSTEMS: INVERTEBRATE INFLUENCES AND DIFFERENCES BETWEEN MORPHOLOGICAL TYPES1

Nostoc parmelioides colonies housing dipteran larvae (Cricotopus) had higher rates of weight specific photosynthesis than colonies without the larvae. A change in colony shape, which allowed the alga to be exposed to higher light intensities, occurred, in the presence of the larvae. This change in morphology together with potential nutrient additions by the larvae and other effects may have caused the increase in photosynthetic rates. Nostoc colonies were typically found in open areas of small streams in western Oregon mountains where the ability to respond to high light would be advantageous in supporting the metabolically expensive process of nitrogen fixation.     

LIPOPHILIC PIGMENTS FROM CYANOBACTERIAL (BLUE-GREEN ALGAL) AND DIATOM MATS IN HAMELIN POOL,SHARK BAY,WESTERN AUSTRALIA1

Lipophilic pigments were examined in microbial mat communities dominated by cyanobacteria in the intertidal zone and by diatoms in the subtidal and sublittoral zones of Hamelin Pool, Shark Bay, Western Australia. These microbial mats have evolutionary significance because of their similarity to lithified stromatolites from the Proterozoic and Early Paleozoic eras. Fucoxanthin, diatoxanthin, diadinoxanthin, β-carotene, and chlorophylls a and c characterized the diatom mats, whereas cyanobacterial mats contained myxoxanthophyll zeaxanthin, echinenone, β-carotene, chlorophyll a and, in some cases, sheath pigment. The presence of bacteriochlorophyll a with in the mats suggest a close association of photosynthetic bacteria with diatoms and cyanobacteria. The high carotenoids: chlorophyll a ratios (0.84–2.44 wt/wt) in the diatom mats suggest that carotenoids served a photoprotective function in this high light environment. By contrast, cyanobacterial sheath pigment may have largely supplanted the photoprotective role of carotenoids in the intertidal mats.     

ESTABLISHMENT,PERSISTENCE AND DOMINANCE OF CORALLINA (RHODOPHYTA) IN ALGAL TURF1

The dominance of Corallina thalli in an intertidal algal assemblage was examined by a series of algal removal experiments to test the hypothesis that other tam are competitively excluded from rock substrate. An alternate explanation, that environmental factors seasonally filter out taxa leaving Corallina as the only alga adapted for year to year survival, was considered. Development of vegetation on patches of naturally exposed bare rock was monitored and compared with manipulated surfaces. Thalli of several species were selectively removed from exposed surfaces and intact turf;. changes in tam occupying primary substrate were recorded over more than three years. No significant differences in mean percent cover for Corallina, Lithothrix, bare rock, or algal crusts were found among treatments. Except for initial growth of colonizing species, abundances of other species dad not increase in the absence of Corallina. Large amounts of rock remained bare or intermittently covered by transient populations of short-lived algae. Data from single quad-rats, where individual clumps of Lithothrix were followed from month to month, indicated that the continued presence of this co-occurring and often abundant species depended on turnover of short-lived thalli rather than persistence of the same clumps. No interactions were found among the several categories of species that appeared after rock substrate was cleared. Most species were the same ones that grew, epiphytically in intact turf at the same time. In control quadrats Corallina maintained 59-95% cover while slowly increasing on surfaces earlier exposed. I predict that Corallina species will regain their dominance in the absence of competitors for primary substrate if the slowly spreading basal crusts remain undisturbed. Morphological and life history characteristics are identified that adapt Corallina to its dominant role in this habitat.     

SEASONAL CHANGES IN STRUCTURE OF A SUBMERGED BLUEGREEN ALGAL/BACTERIAL COMMUNITY FROM A GEOTHERMAL HOTSPRING1

The species (cytospecies) structure and seasonal dynamics of a thermophilic bluegreen algal/bacterial community from Mimbres, New Mexico are described. Using stereological techniques, several common quantitative ecological parameters (biomass, density, distribution measurements, Species Diversity Indices, importance values) were described for the community growing ca. 1 m beneath the water surface on the inner walls of a concrete cistern. Temperature and pH were relatively constant while water flow rates, duration and intensity of light showed seasonal variation. The total volume of the mat occupied by living organisms did not exceed 2% during the entire year. The remaining volume was occupied by gas and water bubble chambers, gelatinous matrix, empty tubes, mineral deposits, and water. The total biomass remained constant except during a period of severe, seasonal light reduction (November). Species Diversity Indices, density and importance values for the bluegreen algal and bacterial populations showed seasonal changes. The three dominant bluegreen algal cytospecies which represented the greatest amount of biomass alternated in their abundance on a seasonal basis while the three dominant bacterial cytospecies remained the same throughout the period of study. Although considerable season change occurred in the species structure of this community, the total living volume remained relatively constant with the major changes in the bluegreen algal populations. Many of the seasonal changes observed appear to correlate with seasonal changes in light intensity and duration.     

COMMUNITY STRUCTURE DYNAMICS OF EPILITHIC AND EPIPHYTIC DIATOMS IN A MISSISSIPPI STREAM1

Epilithic and epiphytic diatoms were collected monthly from 18 June 1979 through 19 May 1980 from both shaded and unshaded sites of Sessums Creek, a shallow, oligotrophic and strongly alkaline stream in northeastern Mississippi. The dominant macroalgae were filamentous green algae belonging to the non-mucilage producing genera Cladophora and Rhizoclonium. Ninety taxa were identified with Navicula minima Grun. and Nitzschia dissipata (Kütz.) Grun. accounting for 50% of all valves counted. Other important taxa were Navicula menisculus var. upsaliensis (Grun.) Grun., N. cryptocephala var. veneta (Kütz.) Rabh., Achnanthes minutissima Kütz. and Cymbella turgida (Greg.) Cl. With few exceptions, the more dominant taxa were equally abundant in the shaded and unshaded sites and also in the epilithon and epiphyton. Species diversity (H') and the number of taxa in a sample in all four habitats showed an identical pattern, being lowest in winter and highest in fall. No one pair of habitats was more structurally similar than any-other pair when compared by a similarity index (SIMI). Apparently, differences in light intensity and substrate type had little effect on diatom distribution in Sessums Creek and it was concluded that the locality sampled supported a single, nearly homogeneous diatom community.     

INFECTION,GROWTH, AND COMMUNITY-LEVEL CONSEQUENCES OF A DIATOM PATHOGEN IN A SONORAN DESERT STREAM1

We describe effect of a pathogen that spread through a dense, rapidly growing, benthic diatom community during two infection periods (February and mid-April 1991) in Sycamore Creek, Arizona. Infected areas appeared as gray rings within a matrix of healthy diatom growth and spread rapidly, eventually covering all benthic substrata and causing algal sloughing (within 2 wk in February and 1 wk in April). Examination of algal material with transmission electron microscopy revealed the presence of invasive bacteria within diatom cells from infected areas, suggesting a pathogenic bacterium as the most probable cause of this phenomenon. Infected area supported lower chlorophyll a concentrations and contained higher percentages of diatom cells with fragmented or reduced chloroplasts than uninfected areas. Spread of the pathogen appeared to be linked most strongly with diatom densities. The infection spread most rapidly in April, when cell densities were highest, and decimated all diatom species populations. The February infection was more species-specific in its action, affecting large motile and rosetteforming taxa more strongly than small, adnate diatoms. This latter group likely resided at the base of communities and may have been buffered from pathogen transfer by mucilage and/or dentrital particles. Consequently, relative abundance of small, adnate diatom taxa increased in algal communities as a result of the February infection. Pathogen-induced alteration of diatom species composition and abundance should influence primary production in this ecosystem and affect the dynamics of organisms that exploit algae as a resource.     

PHOTO‐ACCLIMATION RESPONSE OF BENTHIC STREAM ALGAE ACROSS EXPERIMENTALLY MANIPULATED LIGHT GRADIENTS: A COMPARISON OF GROWTH RATES AND NET PRIMARY PRODUCTIVITY1

The objective of this study was to measure the photo‐acclimation response of stream algae inhabiting thin biofilms across a range of light treatments comparing both growth rates and net primary productivity (NPP). Algae were grown on clay tiles incubated in artificial stream channels where light levels were manipulated by layering a neutral‐density shade cloth over each channel. We measured NPP and algal growth rates in the early stages of community development and compared assemblages that were either acclimated or unacclimated to a given light treatment. Algal growth rates did acclimate to light treatment, with saturation occurring at light levels that were substantially lower for the acclimated communities. Growth efficiency calculated from algal biovolume increased by a factor of 3. However, algal NPP showed a weaker photo‐acclimation response, with only a 30% increase in photosynthetic efficiency. Our results indicate that diatom‐dominated periphyton in thin biofilms are probably not light limited in many shaded streams with respect to growth rates, but are light limited with respect to NPP.     

ALGAL COMMUNITIES OF SPRINGS AND STREAMS IN THE MT. ST. HELENS REGION,WASHINGTON, U.S.A. FOLLOWING THE MAY 1980 ERUPTION1

Mt. St. Helens, a volcanic peak in the Cascade Range in southern Washington erupted violently on May 18, 1980, causing enormous damage to both terrestrial and aquatic ecosystems. The initial explosion evaporated, scoured or buried all springs and streams in the blast impact area. Ash fall and erosion from defoliated hillsides subsequently filled most of the lotic habitats with organic debris and volcanic ash. Recolonization of springs and streams by algae occurred quickly in areas where erosion through the ash progressed down to bedrock. Within 15 months or less of the eruption, algal communities were established throughout the blast impact area. However, as a result of the initial and continued disturbance these communities remained in an early successional stage. Floral assemblages were highly variable except that they were composed mostly of diatoms, with Achnanthes minutissima dominating most lotic sites. Springs showed the most rapid development toward stable floras.     

COMPARISON OF IN SITU PHOTOSYNTHETIC ACTIVITY OF EPIPHYTIC,EPIPELIC AND PLANKTONIC ALGAL COMMUNITIES IN AN OLIGOTROPHIC LAKE,SOUTHERN FINLAND1

The photosynthetic activity of different algal communities at the outer edge of an Equisetum fluviatile L. stand in an oligotrophic lake (Pääjärvi, in southern Finland) was investigated. Production by the algal communities was measured simultaneously using a modified ¹⁴C-method, and the results were related to the volume of algae and the available irradiance. The relative production rate (P/B quotient) of phytoplankton was ca. 3 × that of epiphyton and ca. 20 × that of epipelon. Epiphyton productivity remained almost constant although the algal volume varied greatly, suggesting that the surface layer of the algal community was mainly responsible for the photosynthetic activity. In the littoral area (at 1 m depth) primary production/m² of lake surface by phytoplankton, epiphyton and epipelon was similar but in the littoriprofundal area (2–4 m) phytoplankton production was twice that of epipelon. Primary productivity of epiphyton and epipelon/m² of substratum was about equal to phytoplankton productivity/m³ of water at the same irradiance. This relation provided a means of estimating the relative contributions of the different algal communities to the total algal production in the lake.     

LOTIC ALGAL COMMUNITIES IN THE MT. ST. HELENS REGION SIX YEARS FOLLOWING THE ERUPTION1

A survey of 21 lotic habitats from the Mt. St. Helens region, Washington, was conducted during late summer 1986, six years after the eruption. A total of 152 taxa was observed (94 diatoms, 36 chlorophytes, 19 cyanophytes, 2 xanthophytes, and I rhodophyte). Sampling sites were classified by TWINSPAN (two-way indicator species analysis) and ordinated by DECORANA (detrended correspondence analysis). In general, both techniques corresponded well with each other and grouped sites according to the intensity of disturbance that they experienced. Species richness and diversity values tended to be lowest at the most heavily disturbed sites. Chlorophyll a values ranged over two orders of magnitude and tended to be higher at sites sampled from small, low-discharge tributaries. Loowit Creek, a thermally influenced stream that begins within the volcanic crater, was dominated by thick mats of the cyanophyte Mastigocladus laminosus. A general comparison of the present study with one conducted just after Mt. St. Helens erupted suggests that successional processes are operating in these lotic algal communities.     

PHOTOSYNTHESIS OF THE RED ALGA GASTROCLONIUM COULTERI (RHODOPHYTA) IN RESPONSE TO CHANGES IN TEMPERATURE,LIGHT INTENSITY,AND DESICCATION1

Previously reported transplantation experiments in the field showed that Gastroclonium coulteri (Harvey) Kylin could survive above its normal intertidal range (0.0–0.5 m above MLLW), except during periods of daytime low tides in spring. Net photosynthetic rate measurements in the laboratory were performed to determine which physical factors might determine the upper boundary for this species in the intertidal zone. Maximum net photosynthesis occurred between 15 and 20° C, but remained positive between 4 and 35° C. The air temperature extremes observed in the field were 2° C (only seen once) and 26° C. Net photosynthesis increased as expected with light intensity to the highest value obtainable in the laboratory, 1400 μEin m^?2 s^?1. Plants collected from the field under higher light intensity (up to 2000 μEin m^?2 s^?2) also showed high rates of photosynthesis. Neither the temperature nor light levels observed in the field were directly damaging to photosynthesis. Desiccation, however, resulted in a sharp decrease in both photosynthesis and respiration. G. coulteri fully recovered from successive daily treatments of about 35% desiccation, but not from successive treatments of 50% desiccation. One exposure to 70% desiccation allowed no recovery of photosynthetic capacity.     

OSCILLATORIA (TRICHODESMIUM) THIEBAUTII (CYANOPHYTA) IN THE CENTRAL NORTH ATLANTIC OCEAN12

Physiological rate measurements were made with Oscillatoria thiebautii (Gom.) Geitler in the subtropical north Atlantic Ocean between Spain and Bermuda during May and June of 1975. The near surface C:N fixation ratios averaged 6.5, and the cellular composition ratio was 6.2, suggesting that N₂ fixation is the major path of nitrogenous nutrition for this alga. Compared to other oceanic phytoplankters, it has a low affinity for orthophosphate at oceanic concentrations (k_s= 9.0); however, it has a high potential for utilizing phosphomonoesters (170–300 ng atoms P ·μg chl a^?1· h^?1). Maximal photosynthesis occurred at 450–700 μ Einstein · m^?2· s^?1, and was inhibited by full sunlight. Calculated cell division rates (ca. 180 days) suggest that relative to other phytoplankters in this oceanic region, O. thiebautii must be subjected to negligible grazing pressure. No major differences in C, N, chl a or ATP were observed between the tuft (fusiform) and puff (spherical) colonies. ATP concentrations relative to other cellular constituents varied greatly between colonies, suggesting a general inter-colony physiological variability in the open Atlantic. With increasing depth in the euphotic zone, there was no evidence for chromatic adaption. The observations that O. thiebautii represents only a small fraction of total phytoplankton biomass and that its growth rate is 10–100 times slower than that of the other indigenous phytoplankton, strongly suggest that N₂ fixation by this alga is a virtually insignificant component of the nitrogenous nutrition for the phytoplankton of the North Atlantic central gyre in late Spring.     

BENTHIC DIATOM DISTRIBUTION IN A PENNSYLVANIA STREAM: ROLE OF pH AND NUTRIENTS1

Buffalo Creek is in a forested watershed in eastern Pennsylvania and is relatively acid in upstream reaches (pH～6), becoming alkaline downstream (pH～8). Temperature, nitrogen (NO₃-N) and phosphorus (O-PO₄) increase significantly downstream whereas N/P declines. Nutrient-diffusing substrata were deployed in triplicate at an upstream and downstream site. Six treatments included two concentrations of nitrate, two concentrations of phosphate, nitrogen + phosphate, and a control. Substrata were collected after 18 days, scraped and analyzed for accrual of chlorophyll a and algal community structure. Chlorophyll a and algal biovolume were greatest downstream across all nutrient treatments. At the community level, accrual appeared to be limited by phosphorus at upstream sites. Downstream accrual also may have been phosphorus-limited, but the results were equivocal. Benthic algae on all treatments at both sites were ～96% diatoms. Minimal overlap in species composition was observed between upstream and downstream sites. Of the 75 species of diatoms encountered in the study, 58 species did not occur at the upstream site and 10 species did not occur at the downstream site. The upstream site was depauperate in species and dominated by Eunotia exigua (Bréb. ex Kütz.) Rabh., which showed a positive response to phosphorus and accounted for over 50% of the biomass across treatments. The downstream site showed a four-fold increase in species richness. Communities at this site contained some species that appeared to be phosphorus-limited, e.g. Melosira varians Ag., and others that seemed to be nitrogen-limited, e.g. Diatoma vulgare Bory and Navicula seminulum Grun. We conclude that extreme conditions upstream (low pH, high N/P) result in a species-poor community dominated by acidophilous phosphorus-limited diatoms. Increases in downstream nutrients and pH result in a relatively rich and diverse community.     

SUBSIDY AND STRESS RESPONSES OF STREAM PERIPHYTON TO GRADIENTS IN WATER VELOCITY AS A FUNCTION OF COMMUNITY GROWTH FORM

Previous studies have shown major differences in the way biomass of stream periphyton is controlled by spatial variations in velocity. We hypothesize that these differences may be the result of different growth forms within the community. Some dense and coherent growth forms (e.g. mucilaginous diatom/cyanobacterial mats) may be resistant to diffusion and also resistant to dislodgment by shear stress. Higher velocities applied to such communities could therefore be expected to enhance biomass accrual by increasing rates of mass transfer, but without greatly increasing losses through sloughing. Conversely, other growth forms (e.g. long filamentous green algae) have an open matrix, and high rates of diffusion into the mats can potentially occur even at low velocity. However, as velocities increase, high skin friction and form drag should lead to higher rates of sloughing. The overall result of these processes should be that maximum biomass occurs at low velocities. This “subsidy-stress” hypothesis was tested twice with each of three different periphytal growth forms: a coherent, mucilaginous, diatom community; a moderately coherent, stalked/ short, filamentous diatom community; and an open-weave, long, filamentous green algal community. A monotonic increase in chl a biomass occurred as a function of near-bed velocities for the first of the two mucilaginous diatom communities investigated. No biomass-velocity relationship was found, however, with the second mucilaginous community, probably because the waters were highly enriched and mass transfer driven by molecular diffusion was probably high throughout the velocity gradient. Biomass was moderate at low velocities, peaked at near-bed velocities from 0.18 to 0.2 m·s^?1 (～0.40–0.45 m·s^?1 mean column velocity), and then decreased at higher velocities in both of the stalked/ short filament communities of diatoms analyzed. With the long filamentous green algal communities, a monotonic reduction in biomass occurred as a function of increases in velocity. Proliferations greater than 100 mg·m^?2 chl a occurred at low near-bed velocities (i.e. <0.2 m·s^?1), after which biomass declined nearly exponentially as a function of increasing velocity to less than 10 mg·m^?2 chl a at velocities greater than 0.4 m·s^?1. These biomass-velocity trends support our hypothesis that community growth form determines periphytal responses to spatial variations in velocity within stream reaches.     

PRIMARY PRODUCTION OF EDAPHIC ALGAL COMMUNITIES IN A MISSISSIPPI SALT MARSH1

Primary production rates of edaphic algae associated with the sediments beneath four monospecific canopies of vascular plants were determined over an annual cycle in a Mississippi salt marsh. The edaphic algal flora was dominated by small, motile pennate diatoms. Algal production (as measured by ¹⁴C uptake) was generally highest in spring-early summer and lowest in fall. Hourly rates ranged from a low of 1.4 mg C/m² in Juncus roemerianus Scheele to a high of 163 mg C/m² beneath the Scirpus olneyi gray canopy. Stepwise multiple regressions identified a soil moisture index and chlorophyll a as the best environmental predictors of hourly production; light energy reaching the marsh surface and sediment and air temperature proved of little value. Adding the relative abundances of 33 diatom taxa to the set of independent variables only slightly increased R²; however, virtually all variables selected were diatom taxa. R² was only 0.38 for the Spartina alterniflora Loisel. habitat but ranged from 0.70 to 0.87 for the remaining three vascular plant zones. Annual rates of algal production (g C/m²) were estimated as follows: Juncus (28), Spartina (57), Distichlis spicata (L.) Greene (88), and Scirpus (151). The ratio of annual edaphic algal production to vascular plant net aerial production (EAP / VPP) was 10–12% for the first three habitats and 61% for Scirpus. Chlorophyll a concentrations, annual algal production rates, and EAP / VPP values were comparable to those determined in Texas, Delaware, and Massachusetts salt marshes but lower than those reported for Georgia and particularly California marshes.