Soil and periphyton indicators of anthropogenic water-quality changes in a rainfall-driven wetland

Surface soils and periphyton communities were sampled across an oligotrophic, soft-water wetland to document changes associated with pulsed inputs of nutrient- and mineral-rich canal drainage waters. A gradient of canal-water influence was indicated by the surface-water specific conductance, which ranged between 743 and 963 μS cm⁻¹ in the canals to as low as 60 μS cm⁻¹ in the rainfall-driven wetland interior. Changes in soil chemistry and periphyton taxonomic composition across this gradient were described using piecewise regressions models. The greatest increase in soil phosphorus (P) concentration occurred at sites closest to the canal while soil mineral (sulfur, calcium) concentrations increased most rapidly at the lower end of the gradient. Multiple periphyton shifts occurred at the lower end of the gradient and included; (1) a decline in desmids and non-desmid filamentous chlorophytes, and their replacement by a diatom-dominated community; (2) the loss of soft-water diatom indicator species and their replacement by hard-water species. Increased dominance by cyanobacteria and eutrophic diatom indicators occurred closer to the canals. Soil and periphyton changes indicated four zones of increasing canal influence across the wetland: (1) a zone of increasing mineral concentrations where soft-water taxa remained dominant; (2) a transition towards hard-water, oligotrophic diatoms as mineral concentrations increased further; (3) a zone of dominance by these hard-water species; (4) a zone of rapidly increasing P concentrations and dominance by eutrophic taxa. In contrast to conclusions drawn from routine water-chemistry monitoring, measures of chemical and biological change presented here indicate that most of this rainfall-driven peatland receives some influence from canal discharges. These changes are multifaceted and induced by shifts in multiple chemical constituents.     

Structure, biomass, production and depth distribution of periphyton on artificial substratum in shallow lakes with contrasting nutrient concentrations

1. To examine how the vertical distribution of periphytic biomass and primary production in the upper 0–1 m of the water column changes along an inter‐lake eutrophication gradient, artificial substrata (plastic strips) were introduced into the littoral zones of 13 lakes covering a total phosphorus (TP) summer mean range from 11 to 536 μg L^?1. Periphyton was measured in July (after 8 weeks) and September (after 15 weeks) at three water depths (0.1, 0.5 and 0.9 m). 2. Periphyton chlorophyll a concentration and dry weight generally increased with time and the communities became more heterotrophic. Mean periphytic biomass was unimodally related to TP, reaching a peak between 60 and 200 μg L^?1. 3. The proportion of diatoms in the periphyton decreased from July to September. A taxonomic shift occurred from dominance (by biovolume) of diatoms and cyanobacteria at low TP to dominance of chlorophytes at intermediate TP and of diatoms (Epithemia sp.) in the two most TP‐rich lakes. 4. The grazer community in most lakes was dominated by chironomid larvae and the total biomass of grazers increased with periphyton biomass. 5. Community respiration (R), maximum light‐saturated photosynthetic rate (P_max), primary production and the biomass of macrograzers associated with periphyton were more closely related to periphyton biomass than to TP. Biomass‐specific rates of R, P_max and production declined with increasing biomass. 6. Mean net periphyton production (24 h) was positive in most lakes in July and negative in all lakes in September. Net production was not related to the TP gradient in July, but decreased in September with increasing TP. 7. The results indicate that nutrient concentrations alone are poor predictors of the standing biomass and production of periphyton in shallow lakes. However, because periphyton biomass reaches a peak in the range of phosphorus concentration in which alternative states occur in shallow lakes, recolonisation by submerged macrophytes after nutrient reduction may potentially be suppressed by periphyton growth.     

High nutrient availability leads to weaker top‐down control of stream periphyton: Compensatory feeding in Ancylus fluviatilis

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PRESENCE AND DIVERSITY OF ALGAL TOXINS IN SUBTROPICAL PEATLAND PERIPHYTON: THE FLORIDA EVERGLADES,USA1

Production of toxic secondary metabolites by cyanobacteria, collectively referred to as cyanotoxins, has been well described for eutrophied water bodies around the world. However, cohesive cyanobacterial mats also comprise a significant amount of biomass in subtropical oligotrophic wetlands. As these habitats generally do not support much secondary production, cyanotoxins, coupled with other physiological attributes of cyanobacteria, may be contributing to the minimized consumer biomass. Periphyton from the Florida Everglades has a diverse and abundant cyanobacterial assemblage whose species produce toxic metabolites; therefore, by screening periphyton representative of the greater Everglades ecosystem, six different cyanotoxins and one toxin (domoic acid) produced by diatoms were identified, ranging in content from 3 × 10^?9 to 1.3 × 10^?6 (g · g^?1), with saxitoxin, microcystin, and anatoxin‐a being the most common. While content of toxins were generally low, when coupled with the tremendous periphyton biomass (3–3,000 g · m^?2), a significant amount of cyanotoxins may be present. While the direct effects of the toxins identified here on the local grazing community need to be determined, the screening process utilized proved effective in showing the broad potential of periphyton to produce a variety of toxins.     

Spatial and seasonal patterns of periphyton biomass and productivity in the northern Everglades,Florida, U.S.A.

We sampled periphyton in dominant habitats at oligotrophic and eutrophic sites in the northern Everglades during the wet and the dryseasons to determine the effects of nutrient enrichment on periphytonbiomass, taxonomic composition, productivity, and phosphorus storage. Arealbiomass was high (100–1600 g ash-free dry mass [AFDM]m⁻²) in oligotrophic sloughs and in stands of the emergentmacrophyte Eleocharis cellulosa, but was low in adjacent stands of sawgrass,Cladium jamaicense (7–52 g AFDM m⁻²). Epipelon biomasswas high throughout the year at oligotrophic sites whereas epiphyton andmetaphyton biomass varied seasonally and peaked during the wet season.Periphyton biomass was low (3–68 g AFDM m⁻²) and limitedto epiphyton and metaphyton in open-water habitats at eutrophic sites andwas undetectable in cattail stands (Typha domingensis) that covered morethan 90% of the marsh in these areas. Oligotrophic periphytonassemblages exhibited strong seasonal shifts in species composition and weredominated by cyanobacteria (e.g., Chroococcus turgidus, Scytonema hofmannii)during the wet season and diatoms (e.g. Amphora lineolata, Mastogloiasmithii) during the dry season. Eutrophic assemblages were dominated byCyanobacteria (e.g., Oscillatoria princeps) and green algae (e.g., Spirogyraspp.) and exhibited comparatively little seasonality. Biomass-specific grossprimary productivity (GPP) of periphyton assemblages in eutrophic openwaters was higher than for comparable slough assemblages, but areal GPP wassimilar in these eutrophic (0.9–9.1 g C m⁻²d⁻¹) and oligotrophic (1.75–11.49 g C m⁻²d⁻¹) habitats. On a habitat-weighted basis, areal periphytonGPP was 6- to 30-fold lower in eutrophic areas of the marsh due to extensiveTypha stands that were devoid of periphyton. Periphyton at eutrophic siteshad higher P content and uptake rates than the oligotrophic assemblage, butstored only 5% as much P because of the lower areal biomass.Eutrophication in the Everglades has resulted in a decrease in periphytonbiomass and its contribution to marsh primary productivity. These changesmay have important implications for efforts to manage this wetland in asustainable manner. This revised version was published online in July 2006 with corrections to the Cover Date.     

NONPHOSPHORUS LIPIDS IN PERIPHYTON REFLECT AVAILABLE NUTRIENTS IN THE FLORIDA EVERGLADES,USA1

Algal and plant production of nonphosphorus lipids in place of phospholipids is a physiological response to low phosphorus (P) availability. This response has been shown in culture and in marine plankton studies, but examples from freshwater algae remain minimal. Herein, we analyzed the nutrient contents and lipid composition of periphyton communities across the Florida Everglades ecosystem. We hypothesized that in phosphate‐poor areas, periphyton in high‐ and low‐sulfate waters would vary the proportion of sulfolipids (SLs) and betaine lipids (BLs), respectively. In phosphate‐enriched areas, periphyton would produce more phospholipids (PLs). We observed that at low‐P sites, PLs were a minor lipid component. In cyanobacteria‐dominated periphyton where sulfate was abundant, BLs were only slightly more abundant than SLs. However, in the low‐P, low‐sulfate area, periphyton were comprised to a greater degree green algae and diatoms, and BLs represented the majority of the total lipids. Even in a P‐rich area, PLs were a small component of periphyton lipid profiles. Despite the phosphorus limitations of the Everglades, periphyton can develop tremendous biomass. Our results suggest a physiological response by periphyton to oligotrophic conditions whereby periphyton increase abundances of nonphosphorus lipids and have reduced proportions of PLs.     

The invasive New Zealand mudsnail, <Emphasis Type="Italic">Potamopyrgus antipodarum</Emphasis>, is an effective grazer of algae and altered the assemblage of diatoms more than native grazers

Exploitation of shared resources often mediates the impacts of invasive species on native species. In a field experiment, we compared the ability to graze periphyton and the genera of diatoms removed by the invasive New Zealand mudsnail, Potamopyrgus antipodarum, a native caddisfly larva (Brachycentrus sp.), and a native mayfly nymph (Ephemerella sp.) over 1 week. P. antipodarum removed as much or slightly more periphyton than the native grazers, depending on whether chlorophyll a or ash-free dry mass was used to measure periphyton biomass. When we examined the diatoms in the periphyton, P. antipodarum altered the diatom assemblage more than the native grazers. Effective grazing of periphyton by P. antipodarum may impact native grazers by consuming shared algal resources. In particular, because Ephemerella sp. were also effective grazers, these mayflies may compete for periphyton with P. antipodarum in the western United States. Taken together, these results suggest that ability to procure food resources may contribute to the invasion success of P. antipodarum.     

Seasonal variation in effects of urea and phosphorus on phytoplankton abundance and community composition in a hypereutrophic hardwater lake

1. Urea accounts for half of global agricultural fertiliser applications, yet little is known of its role in eutrophication of freshwater ecosystems, nor how it interacts with phosphorus (P) in regulating phytoplankton composition, especially during spring and autumn.
2. To identify when and how urea and P inputs interact across the ice-free period, we conducted seven monthly fertilisation experiments in 3,240-L mesocosms from ice-off to ice-formation in a hypereutrophic lake. In addition, we ran bioassays with ammonium (NH₄⁺) to compare the effects of urea with those of NH₄⁺, the immediate product of chemical decomposition of urea.
3. Analysis of water-column chlorophyll a and biomarker pigments by high-performance liquid chromatography revealed that addition of inorganic P alone (100 µg P L^–1 week^–1) had no significant impact on either algal abundance or community composition in hypereutrophic Wascana Lake. Instead, fertilisation with urea (4 mg N L⁻¹ week^–1) alone, or in concert with P, significantly (p < 0.05) increased algal abundance in spring and much of summer, but not prior to ice formation in October. In particular, urea amendment enhanced abundance of cryptophytes, chlorophytes, and non-diazotrophic cyanobacteria during April and May, while fertilisation in summer and early autumn (September) increased only chlorophytes and non-diazotrophic cyanobacteria.
4. Comparison of urea mesocosms with NH₄⁺ bioassays demonstrated that urea lacked the inherent toxicity of NH₄⁺ in cool waters, but that both compounds stimulated production during summer experiments.
5. This study showed that urea pollution can degrade water quality in P-rich lakes across a variety of seasonal conditions, including spring, and underscores the importance of quantifying the timing and form of N inputs when managing P-rich freshwaters.

     

Periphyton as a potential phosphorus sink in the Everglades Nutrient Removal Project

Phosphorus uptake and release by periphyton mats were quantified in the Everglades Nutrient Removal Project (ENRP) to evaluate the potential for periphyton P removal. Short-term P uptake rates were determined by incubating cyanobacteria (Oscillatoria princeps and Shizothrix calcicola) and Chlorophycean (primarily Rhizoclonium spp.) algal mat samples for 0.5–2 h under ambient conditions in BOD bottles spiked with soluble reactive P (SRP). Cyanobacterial mats removed P more than twice as fast (80–164 μg P h⁻¹ g⁻¹ AFDM) as Chlorophycean mats (33–61 μg P h⁻¹ g⁻¹ AFDM) during these incubations. In a longer term study, fiberglass cylinders were used to enclose 1.8 m² plots within the wetland and were dosed weekly for 7 weeks with: (1) no nutrients; (2) SRP (0.25 g P m⁻² week⁻¹); or (3) SRP plus nitrate (0.42 g N m⁻² week⁻¹) and ammonium (0.83 g N m⁻² week⁻¹). Phosphorus uptake rates by this periphyton assemblage, which was dominated by the chlorophytes Stigeoclonium spp. and Oedogonium spp., were measured weekly and were similar among nutrient treatments on most dates, indicating that the algal storage compartment for P was not saturated despite repeated P additions. Decomposition rates and P loss by cyanobacteria and Chlorophycean mats were determined by measuring biomass loss and SRP release in darkened BOD bottles over 28–42 day periods under anaerobic and aerobic conditions. First-order aerobic and anaerobic decomposition rates for cyanobacterial mats (k = 0.1095 and 0.1408 day⁻¹, respectively) were 4–20-fold higher than rates for Chlorophycean mats (k = 0.0066 and 0.0250 day⁻¹, respectively) and cyanobacteria released considerably more P back to the water column. Our findings suggest that periphyton can be an important short-term sink for P in treatment wetlands and that retention is strongly affected by the taxonomic composition of the periphyton assemblage.     

Alterations in the biomass-specific productivity of periphyton assemblages mediated by fish grazing

1. In an experimental flume, we examined the effects of a biomass reduction and alteration of taxonomic composition, because of grazing by the fish Plecoglossus altivelis, on the net biomass accumulation of periphyton. 2. Grazed and ungrazed assemblages with different biomass and taxonomic composition were first prepared in fish enclosures and exclosures, respectively. These assemblages were then set out in the flume and incubated for 2 days under grazing‐free conditions to examine (i) the relationship between biomass and biomass accumulation rate and (ii) the effect of taxonomic composition on the relationship between these two. 3. The grazed and ungrazed assemblages were dominated by upright filamentous cyanobacteria and diatoms, respectively. The rate of biomass accumulation decreased with increasing periphyton biomass in both the grazed and ungrazed assemblages, and was lower in the grazed than the ungrazed assemblages at any biomass level. 4. The results showed that the reduction in biomass and the alteration of taxonomic composition due to fish grazing have opposite effects on biomass‐specific productivity. Biomass accumulation rate increased in response to biomass reduction, although a shift in dominance from diatoms to upright filamentous cyanobacteria decreased the overall productivity of the periphyton.     

Does light intensity modify the effect mayfly grazers have on periphyton?

1. A factorial experiment was conducted in artificial outdoor streams to quantify the effects of irradiance (two levels) and two mayfly grazers (four densities of each) on periphytic community structure. The mayflies were Ecdyonurus venosus (Heptageniidae), a grazer using brushing mouthparts, and Baetis spp. (Baetidae) a grazer which uses mandibles and maxilla to scrape and gather periphyton. The experiment ran for 16 days. 2. Grazer densities in channels approximated those existing in a shoreline habitat in the River Sihl, Switzerland. Light treatments were natural (daily mean = 810 μmol m^–2 s^–1) and shaded (daily mean = 286 μmol m^–2 s^–1). 3. Higher irradiance increased total algal abundance by a factor of 4. Algae most affected were prostrate/motile and erect diatoms, filamentous chlorophytes and Hydrurus foetidus. 4. Both species of mayfly reduced periphytic and algal biomass. Mayfly–mayfly interactions, however, were associated with statistical increases in algal biovolume and chlorophyll-a content, indicating that the two grazers may have interfered with one another as their densities increased. The mayfly–mayfly interaction did not influence periphytic ash-free dry mass (AFDM). Light modified the influence of Ecdyonurus such that this mayfly produced greater reductions in algal biovolume under high irradiance. 5. Despite efforts to exclude other grazers, chironomids colonized experimental channels. Chironomid biomass was approximately eight times less than mayflies across treatments and was positively correlated with all measures of periphytic abundance, suggesting that these grazers were responding to periphyton rather than controlling it. Chironomids were also associated with an increase in the abundance of diatoms having a prostrate/motile physiognomy. The only physiognomy to show a negative relationship with chironomid biomass was the thallus type, a form which comprised less than 1% of the algal biovolume across channels. 6. Ecdyonurus and Baetis had distinct influences on algal physiognomy. Ecdyonurus, for example, reduced adnate, stalked and Achnanthes-type physiognomies, but was associated with a significant increase in the abundance of filamentous chlorophytes (primarily Ulothrix sp.). Baetis reduced erect, Achnanthes-type and thallus physiognomies. Neither mayfly influenced the abundance of prostrate/motile diatoms; a physiognomy that comprised 21% of the algae in channels. 7. Light and mayfly interactions affected algal community structure. The interaction of Ecdyonurus with light had a negative effect on erect diatoms, filamentous chlorophytes and the thallus physiognomy, but a positive effect on stalked and Achnanthes-type physiognomies. Baetis interacting with light had a positive effect on adnate diatoms. 8. Although both mayfly taxa influenced periphytic community structure, physiognomy was not a good predictor of algal susceptibility to grazing. The type of substratum to which an alga is attached (detritus or algal filaments vs hard surfaces) and location within the periphytic matrix may be better indicators of vulnerability to grazing than physiognomy.     

Aquatic ecosystem health and trophic status classification of the Bitter Lakes along the main connecting link between the Red Sea and the Mediterranean

The Bitter Lakes are the most significant water bodies of the Suez Canal, comprising 85% of the water volume, but spreading over only 24% of the length of the canal. The present study aims at investigation of the trophic status of the Bitter Lakes employing various trophic state indices, biotic and abiotic parameters, thus reporting the health of the Lake ecosystem according to the internationally accepted classification criteria’s. The composition and abundance of phytoplankton with a dominance of diatoms and a decreased population density of 4315–7376?ind. l^?1 reflect the oligotrophic nature of this water body. The intense growth of diatoms in the Bitter Lakes depends on silicate availability, in addition to nitrate and phosphate. If the trophic state index (TSI) is applied to the lakes under study it records that the Bitter Lakes have an index under 40. Moreover, in the total chlorophyll-a measurements of 0.35–0.96?µg?l^?1 there are more indicative of little algal biomass and lower biological productivity. At 0.76–2.3?µg?l^?1, meanwhile, the low quantity of Phosphorus is a further measure of low biological productivity. In the Bitter Lakes, TN/TP ratios are high and recorded 147.4, and 184.7 for minimum and maximum ratios, respectively. These values indicate that in Bitter lakes, the limiting nutrient is phosphorus and confirm the oligotrophic status of the Bitter Lakes. The latter conclusion is supported by Secchi disc water clarity measurements, showing that light can penetrate, and thus algae can photosynthesize, as deep as >13?m. This study, therefore, showed that the Bitter Lakes of the Suez Canal exhibit oligotrophic conditions with clear water, low productivity and with no algal blooming.     

Periphyton biomass,potential production and respiration in a shallow lake during winter and spring

Abundance, depth distribution, potential productivity and respiration of periphyton on short-time (1 month) and long-time incubated strips were followed monthly during the winter–spring (January–May) transition in a shallow eutrophic lake. A taxonomic shift occurred from dominance of diatoms under ice to chlorophyte dominance in spring communities on the long-time incubated strips, while diatoms dominated until May on the short-time incubated strips. Periphyton biomass accrual was low during the ice-covered winter months (November–January: 4 mg chl a m⁻² month⁻¹), but increased to a maximum of 112 mg chl a m⁻² month⁻¹ immediately after ice-out in February. During February–April, the biomass remained constant before declining in May. Periphyton on long-time incubated strips was equally distributed in the water column in winter (January–February), but was higher near the water surface in spring (March–May). Periphyton did not change with depth on the short-time incubated strips. The potential production to respiration ratio (P/R) was negatively correlated with periphyton biomass. Throughout the study, P/R was <1 for the short-time incubated periphyton, while this was only the case in March–April for the long-time incubations. This study showed a high productive capacity of winter periphyton, resulting in accumulation of a relatively high periphytic biomass early in the season. A massive periphyton density in eutrophic lakes already in winter–spring may potentially delay or prevent the establishment and re-occurrence of submerged macrophytes in the early oligotrophication phase following a reduction of the external nutrient loading. Handling editor: Luigi Naselli-Flores     

Phosphorus removal from eutrophic lakes using periphyton on submerged artificial substrata

The potential of periphyton for phosphorus removal from lakes has been investigated using a novel method involving polypropylene (PP) substrate carriers submerged in the pelagial. The study area Lake 'Fühlinger See' in Cologne (Germany) is a complex of mesoeutrophic gravel pit lakes. The whole site is intensively used as a recreation area. Visitors are thought to be the most important single contributors to lake eutrophication. Carriers were exposed at different depths (2, 3.5, 5 m), for different time intervals (1–8 months) and from March to November PP-sheets were readily colonised by periphyton and a biofilm consisting mainly of benthic diatoms developed. Seasonal variability of periphyton on substrates was observed since filamentous green algae colonised the artificial substrates mainly between July and November. Chlorophyll a content of periphyton on the PP-fleece was up to 240-fold higher than chlorophyll a concentrations in the same volume in the epilimnion. Up to around 100 mg of total phosphorus per m² PP-fleece was bound and can be eliminated from the lake by removal of the substrate carriers together with the periphyton after four months of exposure. Though large-scale validations are needed, this method may be applicable as a technique to harvest phosphorus from the water column in larger-scale settings.     

The long-term effect of artificial destratification on phytoplankton species composition in a subtropical reservoir

1. The response of phytoplankton to the installation of an artificial destratification system in North Pine Dam, Brisbane (Australia) was investigated over an 18 year period (1984–2002); 11 years before and 7 years after installation. 2. An overall increase in phytoplankton abundance was revealed for some groups (in particular, diatoms, cyanobacteria and chlorophytes), but not for others (chlorophytes). Changes in the abundance of chlorophyte functional groups was attributed to eutrophication. 3. A strong spatial gradient in phytoplankton abundance and chlorophyll a was observed, with low abundance in the downstream regions affected by the destratification system which was likely because of light limitation induced by vertical mixing. The upstream region acted as a surrogate for the unaltered state of the reservoir, particularly as an indicator of eutrophication without direct influence from the destratification system. Despite the continuous trend in eutrophication of the reservoir, there has been a definite decrease in the rate of eutrophication (approximately 30%) since the installation of the destratification system at the downstream locations. 4. Correlations of the dominant cyanobacteria Cylindrospermopsis raciborskii with other genera changed after destratification, indicating that prior to destratification the dominance of Cylindrospermopsis was because of its ability to compete for phosphorus, whereas after destratification its dominance was because of its ability to compete for light.     

Catabolic diversity of periphyton and detritus microbial communities in a subtropical wetland

The catabolic diversity of wetland microbial communities may be a sensitive indicator of nutrient loading or changes in environmental conditions. The objectives of this study were to assess the response of periphyton and microbial communities in water conservation area-2a (WCA-2a) of the Everglades to additions of C-substrates and inorganic nutrients. Carbon dioxide and CH₄ production rates were measured using 14 days incubation for periphyton, which typifies oligotrophic areas, and detritus, which is prevalent at P-impacted areas of WCA-2a. The wetland was characterized by decreasing P levels from peripheral to interior, oligotrophic areas. Microbial biomass and N mineralization rates were higher for oligotrophic periphyton than detritus. Methane production rates were also higher for unamended periphyton (80 mg CH₄-C kg⁻¹ d⁻¹) than detritus (22 mg CH₄-C kg⁻¹ d⁻¹), even though the organic matter content was higher for detritus (80%) than periphyton (69%). Carbon dioxide production for unamended periphyton (222 mg CO₂-C kg⁻¹ d⁻¹) was significantly greater than unamended detritus (84 mg CO₂-C kg⁻¹ d⁻¹). The response of the heterotrophic microbial community to added C-substrates was related to the nutrient status of the wetland, as substrate-induced respiration (SIR) was higher for detritus than periphyton. Amides and polysaccharides stimulated SIR more than other C-substrates, and methanogenesis was greater contributor to SIR for periphyton than detritus. Inorganic P addition stimulated CO₂ and CH₄ production for periphyton but not detritus, indicating a P limitation in the interior areas of WCA-2a. Continued nutrient loading into oligotrophic areas of WCA-2a or enhanced internal nutrient cycling may stimulate organic matter decomposition and further contribute to undesirable changes to the Everglades ecosystem caused by nutrient enrichment.     

Removal of settled sediments and periphyton from macrophytes by grazing invertebrates in the littoral zone of a large oligotrophic lake

• 1 The resistance and resilience of littoral zone communities to sedimentation will depend both on the extent to which sediment deposition affects productivity, and on interactions within the communities. A series of hypotheses were set up and tested to examine interactions and feedback mechanisms among deposited sediments, periphyton, macrophytes and grazers in a large oligotrophic lake subject to fluctuating sediment loadings.
• 2 Although sediments incorporated into periphyton reduced light availability to macrophytes, periphytic algae were generally the dominant light absorbing component under natural conditions. When grazers were absent, both sediments incorporated in the periphyton and periphytic algal densities increased, and both were then important in reducing light available to macrophytes.
• 3 Grazing rate and assimilation efficiency for the dominant grazer, the prosobranch gastropod Potamopyrgus antipodarum, increased with increasing sediment content under natural lake conditions to reach a maximum at 10 mg sediment cm^?2.
• 4 An increase in sediment incorporation into periphyton films resulted in an increased grazing rate and hence grooming of sediments from macrophytes.
• 5 Grazing invertebrates can play a major role in maintenance of littoral communities by continuously grooming macrophyte hosts of periphytic algae and settled sediments.

     

Periphyton removal related to phosphorus and grazer biomass level

1. The proliferation of nuisance periphyton in enriched streams may be dependent on the biomass of the grazing macroinvertebrates present. In the present study, the effectiveness of grazer size and biomass in controlling periphyton and the extent to which grazing effectiveness was affected by enrichment level were determined. 2. Two sets of experiments with two caddisfly grazers were conducted in laboratory channels during spring and summer 1995 and 1996. The first set tested the combined effect of phosphorus enrichment and grazing, while the second set tested the effect of variable grazer biomass on periphyton biomass. 3. Grazing reduced periphyton biomass in excess of 80%, compared to ungrazed controls. Grazers were equally effective in controlling filamentous green algae, Stigeoclonium, diatoms and small colonial greens. Near complete removal of periphyton biomass by grazing occurred at even at the lowest grazer biomass level (750 mg m^??2, i.e. approximately one-third of natural levels). 4. Grazing controlled periphyton biomass more than did enrichment with soluble reactive phosphorus (SRP). 5. Grazing rates in the phosphorus-grazing interaction experiments averaged about 6 mg chl a g invertebrate^??1 day^??1, which was similar to past work in these channels and elsewhere, while rates were about five-fold higher in the variable grazer biomass experiments. 6. Simulating effects of SRP and grazing with a calibrated model suggests that higher SRP levels would be necessary to exceed a nuisance periphyton biomass level if grazers were present. However, if grazer biomass was more than 1500 mg m^??2, a nuisance level would probably not be exceeded at any SRP.     

Light,nutrients and the fatty acid composition of stream periphyton

1. While the balance of light and nutrients is known to influence the food quality of herbivores by altering algal phosphorus and nitrogen content, the combined effects of light and nutrients on fatty acid synthesis in freshwater periphyton are relatively unknown. In this study, we manipulated light and phosphorus concentration in large, flow‐through experimental streams to examine their effects on both elemental stoichiometry and fatty acid content in periphyton. 2. Two levels of phosphorus (4 and 80 μg L^?1) and three of light (17, 40, 110 μmol photons m^?2 s^?1) were applied in a factorial design in two separate experiments. Diatoms dominated periphyton communities in both experiments, comprising >95% of algal biovolume. Periphyton growth in the streams was simultaneously affected by both resources, even at low rates of supply. 3. Periphyton C/P and C/N ratios increased with light augmentation and decreased with phosphorus enrichment, and consistent with the light : nutrient hypothesis (LNH). Light effects were strongest in streams with low phosphorus concentrations. 4. Periphyton fatty acids reflected the dominance of diatoms : palmitic (16 : 0), palmitoleic (16 : 1ω7) and eicosapentanoic (20 : 5ω3) were the principal saturated (SAFA), monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA), respectively. Linoleic (18 : 2ω6) and linolenic (18 : 3ω3) acids, characteristic of chlorophytes and cyanophytes, were rare, comprising <2% of total fatty acids. 5. Periphyton fatty acid profiles were highly sensitive to light and phosphorus. The proportion of fatty acids comprised by SAFA and MUFA increased with light augmentation and decreased with phosphorus enrichment, whereas PUFA decreased with light and increased with phosphorus. Light effects on fatty acid composition were strongest in phosphorus‐poor streams. PUFA declined with increasing light/phosphorus ratios in the streams, whereas ‘energy’ fatty acids (16 : 0 and 16 : 1) increased. The ratio of SAFA/PUFA was strongly and positively correlated with C/P and C/N ratios. SAFA and MUFA, normalised to dry mass, increased two‐ to threefold with increasing light, while PUFA normalised to dry mass was not significantly affected by light. 6. Similarities in the responses of fatty acids and elemental stoichiometry to light and phosphorus treatments suggested that they were influenced by a common mechanism. Both components of food quality appeared to be sensitive to light‐regulated rates of carbon fixation which, when coupled with insufficient supplies of phosphorus, caused diatom cells to store surplus carbon in SAFA, MUFA and other carbon‐rich compounds that diluted both essential fatty acids and mineral nutrients.