Phytoplankton community metrics based on absolute and relative abundance and biomass: implications for multivariate analyses

Phytoplankton and water samples were collected at 12 locations along the temperate lowland Rideau River, Ontario, Canada. The stations were visited twice a month from May to September 1998, 1999, and 2000. Phytoplankton communities were quantified based on cell abundance, entity abundance (colonies, filaments or free-living cells) and biomass (converted from biovolume estimates based on cell shape and biometry), and were expressed as absolute and relative values. The resulting phytoplankton dataset was composed of six different metrics. The general objective was to assess which metric best explained the spatial and temporal variability in the phytoplankton communities of the Rideau River in response to fluctuating environmental variables. Relationships between phytoplankton metrics and water quality variables were assessed using canonical correspondence analyses. The absolute cell abundance metric showed the best relationship with water quality, followed by the cell entity metric. The biomass metric showed the poorest relationship with water quality variables, indicating that accounting for cell size does not provide additional information. The data expressed as absolute values were consistently better predictors of water quality compared to relative values.     

Efficacy of algal metrics for assessing nutrient and organic enrichment in flowing waters

1. Algal-community metrics were calculated for periphyton samples collected from 976 streams and rivers by the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) Programme during 1993–2001 to evaluate national and regional relations with water chemistry and to compare whether algal-metric values differ significantly among undeveloped and developed land-use classifications.
2. Algal metrics with significant positive correlations with nutrient concentrations included indicators of trophic condition, organic enrichment, salinity, motility and taxa richness. The relative abundance of nitrogen-fixing algae was negatively correlated with nitrogen concentrations, and the abundance of diatom species associated with high dissolved oxygen concentrations was negatively correlated with both nitrogen and phosphorus concentrations. Median algal-metric values and nutrient concentrations were significantly lower at undeveloped sites than those draining agricultural or urban catchments.
3. Total algal biovolume did not differ significantly among major river catchments or land-use classifications, and was only weakly correlated with nitrate (positive) and suspended-sediment (negative) concentrations. Estimates of periphyton chlorophyll a indicated an oligotrophic–mesotrophic boundary of about 21 mg m⁻² and a mesotrophic–eutrophic boundary of about 55 mg m⁻² based on upper and lower quartiles of the biovolume data distribution.
4. Although algal species tolerance to nutrient and organic enrichment is well documented, additional taxonomic and autecological research on sensitive, endemic algal species would further enhance water-quality assessments.     

Diatoms and biomonitoring: should cell size be accounted for?

Despite the fact that biovolume calculation is a common procedure in most phytoplankton and periphyton studies, diatom community analyses are usually based on relative abundance data. In a biomonitoring context, a community metric that accounts for cell size could be of interest due to the potential differences that might exist in nutrient uptake between large and small-sized species. This paper addresses the question of whether diatom community analysis should be based on relative abundance, biovolume or cell surface. The results show that although community structure expressed as relative proportion of taxa varied according to the metric used, the ordinations conducted with each metric were similar. The explained percentage of species variance was slightly higher with the relative abundance metric compared to the metrics based on relative biovolume or cell surface area. Partial CCAs showed that each water chemistry variable generally explained a higher portion of species variance when the relative abundance was used. The analyses conducted with two size groups (small and large taxa) expressed as relative abundance and relative biovolume showed similar results. Moreover, our data showed that there is no significant relationship between diatom size and total phosphorus. According to these results, it seems that relative abundance would be the most appropriate metric to use for biomonitoring purposes. The biovolume and cell surface area calculations added substantially to the total analysis time due to the numerous measurements required, but did not improve the variance explained in community structure, and site ordinations were not significantly different. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Interactive multiple‐stressor effects of the antibiotic monensin,cattle effluent and light on stream periphyton

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Periphyton‐based transfer functions to assess ecological imbalance and management of a subtropical ombrotrophic peatland

1. To assess the biological status and response of aquatic resources to management actions, managers and decision‐makers require accurate and precise metrics. This is especially true for some parts of the Florida Everglades where multiple stressors (e.g. hydrologic alterations and eutrophication) have resulted in a highly degraded and fragmented ecosystem. Biological assessments are required that directly allow for the evaluation of historical and current status and responses to implantation of large‐scale restoration projects. 2. Utilising periphyton composition and water‐quality data obtained from long‐term (15 years) monitoring programmes, we developed calibrated and verified periphyton‐based numerical models (transfer functions) that could be used to simultaneously assess multiple stressors affecting the Everglades peatland (e.g. salinity, nitrogen and phosphorus). Periphyton is an ideal indicator because responses to stressors are rapid and predictable and possess valued ecological attributes. 3. Weighted averaging partial least squares regression was used to develop models to infer water‐quality concentrations from 456 samples comprising 319 periphyton taxa. Measured versus periphyton‐inferred concentrations were strongly related for log‐transformed salinity ( = 0.81; RMSEP 0.15 mg L^?1) and log‐transformed total phosphorus (TP; = 0.70; RMSEP 0.18 mg L^?1), but weakly related for total Kjeldahl nitrogen (TKN) ( = 0.46; RMSEP 0.12 mg L^?1). Validation results using an independent 455 sample data set were similar (log(salinity) r² = 0.78, log(TP) r² = 0.65 and log(TKN) r² = 0.38). 4. Water Conservation Area 1 (WCA‐1), a large ombrotrophic subtropical peatland impacted by multiple water‐quality stressors that has undergone major changes in water management, was used as a case study. The models were applied to a long‐term periphyton data set to reconstruct water‐quality trends in relation to restoration efforts to reduce nutrient loading to the Everglades. The combination of biologically inferred TP and salinity was used to identify the ecological status of periphyton assemblages. Periphyton assemblages were ecologically imbalanced with respect to salinity and TP. Salinity imbalance varied spatially and temporally, whereas TP was spatially restricted. Imbalances caused by water management were owing to salinity more so than to TP. 5. The transfer functions developed for the Everglades are trait‐based quantitative numerical methods and are ideal because the abundances of species are modelled numerically in relation to a stressor. The resulting inferred value is a numerical representation of the stressor’s effect on biological condition that can be compared against the management of the stressor independent of other factors. The benefits are that biological lags or hysteresis events can easily be identified and environmental conditions can be estimated when measurements are lacking. Reporting biological assessments in terms of well‐defined water‐quality metrics (e.g. numeric criterion) increased the communicative ability of the assessment. The use of multiple metrics to assess ecological imbalance increased the ability to identify probable causes.     

COMPARISON OF SIMPLE AND MULTIMETRIC DIATOM‐BASED INDICES FOR GREAT LAKES COASTLINE DISTURBANCE1

Because diatom communities are subject to the prevailing water quality in the Great Lakes coastal environment, diatom‐based indices can be used to support coastal‐monitoring programs and paleoecological studies. Diatom samples were collected from Great Lakes coastal wetlands, embayments, and high‐energy sites (155 sites), and assemblages were characterized to the species level. We defined 42 metrics on the basis of autecological and functional properties of species assemblages, including species diversity, motile species, planktonic species, proportion dominant taxon, taxonomic metrics (e.g., proportion Stephanodiscoid taxa), and diatom‐inferred (DI) water quality (e.g., DI chloride [Cl]). Redundant metrics were eliminated, and a diatom‐based multimetric index (MMDI) to infer coastline disturbance was developed. Anthropogenic stresses in adjacent coastal watersheds were characterized using geographic information system (GIS) data related to agricultural and urban land cover and atmospheric deposition. Fourteen independent diatom metrics had significant regressions with watershed stressor data; these metrics were selected for inclusion in the MMDI. The final MMDI was developed as the weighted sum of the selected metric scores with weights based on a metric’s ability to reflect anthropogenic stressors in the adjacent watersheds. Despite careful development of the multimetric approach, verification using a test set of sites indicated that the MMDI was not able to predict watershed stressors better than some of the component metrics. From this investigation, it was determined that simpler, more traditional diatom‐based metrics (e.g., DI Cl, proportion Cl‐tolerant species, and DI total phosphorus [TP]) provide superior prediction of overall stressor influence at coastal locales.     

Low algal carbon content and its effect on the C : P stoichiometry of periphyton

1. We examined the contribution of algal cells to periphytic organic carbon and assessed the effects of variable biomass composition on the carbon : phosphorus (C : P) ratio of periphyton. We compiled more than 5000 published and unpublished observations of periphytic carbon : chlorophyll a (C : Chl) ratios, an index of algal prevalence, from a variety of substrata collected from lake and low‐salinity coastal habitats. In addition, we converted estimates of algal biovolume into algal C to obtain an independent measure of cellular algal carbon in periphyton. This information was used in a model relating periphyton C : P ratio to algal cellular carbon, the algal C : P ratio, and the C : P ratio of non‐algal organic matter in periphyton. 2. The mean C : Chl ratio of periphyton (405) was relatively high with values in >25% of the samples exceeding 500. On average, 8.4% of total periphyton C was accounted for by C in algal cells. Only 15% of samples were found to have more than 15% periphyton C in cellular algal carbon. Our model showed a nonlinear relationship between periphytic C : P ratios and the C : P ratio of algal cells in the periphyton when non‐algal organic matter was present. However, even at relatively low cellular algal C (<10% of total C), algal C : P ratios can strongly affect the C : P ratio of periphyton as a whole (i.e. algal cells plus other organic matter). 3. The high C : Chl ratios and the low biovolume‐derived algal C of periphyton samples in our data set indicate that algal cells are typically a minor component of organic carbon in periphyton, However, this minor contribution would not preclude algal cellular stoichiometry from notably influencing periphyton C : P ratios.     

Structure of the Phytoplankton Community and Its Relationship to Water Quality in Donghu Lake, Wuhan, China

The phytoplankton community structure, in terms of species composition, total standing crop,and abundance of the dominant algal species, at four stations in Donghu Lake, Wuhan, China, was investigated monthly from January 1994 to December 1996. A total of 260 taxa was observed, of which Chlorophyta (106 taxa) contributed the highest portion of the total number of taxa, followed by Bacillariophyta (82 taxa) and Cyanophyta (32 taxa). The total standing crop measured by means of chlorophyll a content, cell density,and cell biovolume, as well as the abundance of the dominant species, declined in the order of Station I to Station IV. Seasonal changes of the standing crop varied greatly among the four stations. Although the cell density at the four stations showed a single peak within a year, the peak density varied from July to November, dependent on the sampling year and the station. For chlorophyll a content and cell biovolume,multiple peaks were observed at Stations I and II, but a single peak was found at Stations III and IV. The phytoplankton community structure indicated that the trophic status was the highest at Station I (most eutrophic), followed by Station II; Stations III and IV were the least trophic areas. The long-term changes in phytoplankton community structure further suggested that changes in phytoplankton community structure were correlated with water quality, and eutrophication of Donghu Lake had been aggravated since the 1950s.     

Patterns of benthic algae and cyanobacteria along twin‐stressor gradients of nutrients and fine sediment: a stream mesocosm experiment

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Structure of the Phytoplankton Community and Its Relationship to Water Quality in Donghu Lake, Wuhan, China

The phytoplankton community structure, in terms of species composition, total standing crop,and abundance of the dominant algal species, at four stations in Donghu Lake, Wuhan, China, was investigated monthly from January 1994 to December 1996. A total of 260 taxa was observed, of which Chlorophyta (106 taxa) contributed the highest portion of the total number of taxa, followed by Bacillariophyta (82 taxa)and Cyanophyta (32 taxa). The total standing crop measured by means of chlorophyll a content, cell density,and cell biovolume, as well as the abundance of the dominant species, declined in the order of Station I to Station Ⅳ. Seasonal changes of the standing crop varied greatly among the four stations. Although the cell density at the four stations showed a single peak within a year, the peak density varied from July to November, dependent on the sampling year and the station. For chlorophyll a content and cell biovolume,multiple peaks were observed at Stations Ⅰ and Ⅱ, but a single peak was found at Stations Ⅲ and Ⅳ. The phytoplankton community structure indicated that the trophic status was the highest at Station Ⅰ (most eutrophic), followed by Station Ⅱ; Stations Ⅲ and Ⅳ were the least trophic areas. The long-term changes in phytoplankton community structure further suggested that changes in phytoplankton community structure were correlated with water quality, and eutrophication of Donghu Lake had been aggravated since the 1950s.     

Stressor tolerance values for benthic macroinvertebrates in Mississippi

Conceptually, tolerance values represent the relative capacity of aquatic organisms to survive and reproduce in the presence of known levels of stressors. Operationally, they represent the relative abundance and co-location of organisms and stressors. These numeric values are then used for calculating tolerance metrics. Defensibility of biological assessments using tolerance metrics is compromised if the origins of the tolerance values or technical foundations of metrics are unknown. To minimize circularity and maximize objectivity, we define stressed conditions using physical and chemical factors. Also, since single, isolated stressors in stream systems are rare, we used an approach that combines multiple physical and chemical characteristics into a single general stressor gradient. In this paper, we describe development of tolerance values for benthic macroinvertebrate taxa collected from 455 wadeable stream sites throughout Mississippi, USA, except the Alluvial Plain. Principal components analysis (PCA) was used to develop a gradient that incorporated direct (instream physical and chemical) and indirect (land use) stressors, which was then scaled from 0 to 10. Weighted averaging of the relative abundance of each taxon was used to assign tolerance values based on the point of greatest relative abundance along the stressor gradient. Tolerance values were derived for 324 of the 567 taxa collected from the study sites, and primarily represented sensitivity to agricultural influences including degradation of physical habitat and nutrient enrichment, the dominant stressors within the state. We suggest that this approach could be used in other areas of the country to develop new tolerance values, refine existing ones, and may be a useful approach for other taxonomic groups.     

Climate warming and agricultural stressors interact to determine stream periphyton community composition

Lack of knowledge about how the various drivers of global climate change will interact with multiple stressors already affecting ecosystems is the basis for great uncertainty in projections of future biological change. Despite concerns about the impacts of changes in land use, eutrophication and climate warming in running waters, the interactive effects of these stressors on stream periphyton are largely unknown. We manipulated nutrients (simulating agricultural runoff), deposited fine sediment (simulating agricultural erosion) (two levels each) and water temperature (eight levels, 0–6 °C above ambient) simultaneously in 128 streamside mesocosms. Our aim was to determine the individual and combined effects of the three stressors on the algal and bacterial constituents of the periphyton. All three stressors had pervasive individual effects, but in combination frequently produced synergisms at the population level and antagonisms at the community level. Depending on sediment and nutrient conditions, the effect of raised temperature frequently produced contrasting response patterns, with stronger or opposing effects when one or both stressors were augmented. Thus, warming tended to interact negatively with nutrients or sediment by weakening or reversing positive temperature effects or strengthening negative ones. Five classes of algal growth morphology were all affected in complex ways by raised temperature, suggesting that these measures may prove unreliable in biomonitoring programs in a warming climate. The evenness and diversity of the most abundant bacterial taxa increased with temperature at ambient but not with enriched nutrient levels, indicating that warming coupled with nutrient limitation may lead to a more evenly distributed bacterial community as temperatures rise. Freshwater management decisions that seek to avoid or mitigate the negative effects of agricultural land use on stream periphyton should be informed by knowledge of the interactive effects of multiple stressors in a warming climate.     

空间层次及人工基质对着生藻类建群特性的影响

为全面了解着生藻类在建群中群落变化的生态学特性,揭示着生藻类的建群规律,在以丝状藻类为优势藻的生态塘中,采用花岗岩和瓷砖为附着材料,设置水体底部和中部为附着位点,进行频次为10d的采样分析。结果表明,生态塘中共检出8门73属117种着生藻类,其中以硅藻、蓝藻、绿藻为优势类群。同时不同人工基质和不同空间层次条件下着生藻类的建群特征较一致,早期以单细胞硅藻如舟形藻(Navicula sp.)、脆杆藻(Fragilaria sp.)、曲壳藻(Achnanthes sp.)等为优势,后期以丝状藻类如鞘丝藻(Lyngbya sp.)、颤藻(Oscillatoria sp.)、伪鱼腥藻(Pseudanabaena sp.)等为优势;研究结果发现不同人工基质(花岗岩和瓷砖)对着生藻类的种类组成、细胞密度、生物量和藻类多样性无显著影响,花岗岩和瓷砖上附着的着生藻类具有较高的相似性;但不同的空间层次对着生藻类建群特征影响明显,水体底部具有更多的硅藻种类数,中部具有更多的绿藻,随着建群时间的发展,蓝藻比例不断增加;就生物量而言,底部的着生藻类叶绿素a显著高于水体中部,但两者的细胞密度无显著性差异;随着建群过程的发展,水体底部的着生藻类生物量达峰值所需的时间比中部更长。通过相关性分析,生态塘中着生藻类的生长主要受总磷的影响。     

RESPONSE OF A LENTIC PERIPHYTON COMMUNITY TO NUTRIENT ENRICHMENT AT LOW N:P RATIOS1

We examined the effect of nitrogen:phosphorus (N:P) ratios and nutrient concentrations on periphyton when nutrients (N and P) are provided in excess. A gradient of seven N:P ratios ranging from 7.5:1 to 1:7.5 and each at three absolute concentrations, was established using nutrient‐releasing substrata placed in a meso‐oligotrophic lake. Differences in total algal biovolume among nutrient ratios were significant (analysis of covariance [ANCOVA]) when P concentration was entered as the co‐variate. In addition, total algal biovolume was significantly correlated with N concentration but not P. To further evaluate the relationship between nutrient ratios and biovolume, we analyzed (using four 1‐way analysis of variances [ANOVAs]) four subsets of data defined as a series of treatments where one nutrient concentration remained relatively constant as the other changed creating different N:P ratios. Ratios of data subsets ranged from 1:1 to 7.5:1 and 1:1 to 1:7.5 with low and high concentrations of both series. Only diatom biovolume varied with ratio but these differences are most likely related to increased green algal abundance. Species richness and diversity differed among N:P ratios (ANCOVA) when P concentration was used as the co‐variate. Stigeoclonium tenue (Ag.) Gomont, which generally accounted for the increase in green algal abundance, varied with nutrient ratio (ANCOVA) when P was the co‐variate. Based on the ANCOVAs, correlations, and one‐way ANOVAs, periphyton in this system appears to be affected by N concentration but not by N:P treatment ratios under nutrient‐rich conditions. When compared with previous studies, these data also suggest that the response of periphyton to in situ treatments constructed with nutrient‐releasing substrata vary between years.     

The Effects of Ultraviolet Radiation and Nutrient Additions on Periphyton Biomass and Composition in a Sub‐Alpine Lake (Castle Lake,USA)

Rising levels of ultraviolet radiation (UVR) striking the Earth's surface have led to numerous studies assessing its inhibitory effects on phytoplankton and periphyton in aquatic systems. Mineral nutrients such as nitrogen (N) and phosphorus (P) have been shown to increase aspects of algal metabolism and compensate for UVR inhibition. An in situ substratum enrichment technique and UV shielding was used to assess the effects of nutrient additions on periphyton exposed to different levels of UVR in Castle Lake, California during July‐August, 1997. UV shielding had no effect on total periphyton biomass, but caused shifts in species composition. The dominant periphyton species, Anabaena circinalis RAB., demonstrated sensitivity to ambient levels of UV radiation possibly due to UV inhibition of N2 ‐fixation. Total diatom biovolume decreased when shielded from UVR. Phosphorus additions continually elicited an increase in periphyton biovolume at all levels of analysis. These results suggest an interaction between nutrient status/availability and UV sensitivity.     

Phosphorus scarcity and desiccation stress increase the occurrence of dominant taxa in wetland benthic primary producer communities

A few dominant species of plants often disproportionately contribute to primary production; however, dominance has an underappreciated influence on ecosystem processes and functioning. Cascading impacts of dominant species have been documented in ecosystems undergoing eutrophication, but competitive exclusion may also influence dominance structures when limiting nutrients become scarce (i.e., in lakes experiencing oligotrophication) or with exposure to stressors to which few species are adapted (i.e., desiccation stress in wetlands). To predict impacts of widespread changes in nutrients and hydrology on dominance structures in aquatic ecosystems, we need quantitative assessments of dominance of important primary producers, including algae and cyanobacteria, which can regulate other structural and functional properties of ecosystems. We used a highly spatiotemporally resolved (7 years, 165 sites) dataset from the abundant microbial mats of the Florida Everglades to assess how and why the degree of dominance and the identity of dominant taxa vary across nutrient and desiccation gradients. Using algal counts and the dimensions of algal units (cells, coenobia, colonies, and filaments), we measured dominance as relative biovolume. As hypothesized, the relative biovolume of dominant taxa increased and the number of taxa comprising 95% of the biovolume decreased with lower concentrations of limiting nutrient in the mats (phosphorus; P) and higher desiccation stress. Algal taxa that regulate the structural integrity of mats, such as the filamentous, calcium carbonate precipitating cyanobacterium Scytonema sp., strongly influenced these patterns through their tolerance of P scarcity and desiccation. Our indicators and approach can be used to test whether dominance of microscopic primary producers, and other organisms, increases with nutrient scarcity and desiccation stress in other aquatic ecosystems.     

Mesocosm experiments on nutrient and fish effects on shallow lake food webs in a Mediterranean climate

1. Nutrient and fish manipulations in mesocosms were carried out on food‐web interactions in a Mediterranean shallow lake in south‐east Spain. Nutrients controlled biomass of phytoplankton and periphyton, while zooplankton, regulated by planktivorous fish, influenced the relative percentages of the dominant phytoplankton species. 2. Phytoplankton species diversity decreased with increasing nutrient concentration and planktivorous fish density. Cyanobacteria grew well in both turbid and clear‐water states. 3. Planktivorous fish increased concentrations of soluble reactive phosphorus (SRP). Larger zooplankters (mostly Ceriodaphnia and copepods) were significantly reduced when fish were present, whereas rotifers increased, after fish removal of cyclopoid predators and other filter feeders (cladocerans, nauplii). The greatest biomass and diversity of zooplankton was found at intermediate nutrient levels, in mesocosms without fish and in the presence of macrophytes. 4. Water level decrease improved underwater light conditions and favoured macrophyte persistence. Submerged macrophytes (Chara spp.) outcompeted algae up to an experimental nutrient loading equivalent to added concentrations of 0.06 mg L^?1 PO₄‐P and 0.6 mg L^?1 NO₃‐N, above which an exponential increase in periphyton biomass and algal turbidity caused characean biomass to decline. 5. Declining water levels during summer favoured plant‐associated rotifer species and chroococcal cyanobacteria. High densities of chroococcal cyanobacteria were related to intermediate nutrient enrichment and the presence of small zooplankton taxa, while filamentous cyanobacteria were relatively more abundant in fishless mesocosms, in which Crustacea were more abundant, and favoured by dim underwater light. 6. Benthic macroinvertebrates increased significantly at intermediate nutrient levels but there was no relationship with planktivorous fish density. 7. The thresholds of nutrient loading and in‐lake P required to avoid a turbid state and maintain submerged macrophytes were lower than those reported from temperate shallow lakes. Mediterranean shallow lakes may remain turbid with little control of zooplankton on algal biomass, as observed in tropical and subtropical lakes. Nutrient loading control and macrophyte conservation appear to be especially important in these systems to maintain high water quality.     

Phytoperiphyton of the Samarga River basin (Primorskii Krai)

In the periphyton of the Samarga River basin, 313 species of algae (337, including infraspecific taxa) from eight divisions have been found. The groups of algae from the river main flow, creeks, streams, and the mouth are described. Data on the density and biomass of periphyton algae are presented. A decrease in these parameters after a flood is shown. In algal flora composition, benthic oligohalobic species of algae, preferring a slightly alkaline environment, dominate. At the river mouth a significant amount of mesohalobic species has been observed. A sanitary and biological evaluation of water quality has shown that the mass development of xenoand oligosaprobic algae defines the excellent quality of the river water. All parts of the main flow correspond to the oligosaprobic purification zone, class II of water purity (saprobic index of 0.95–1.48). The good quality of the Samarga River water is ensured by the absence of pollution sources and the satisfactory condition of its catchment area forests.     

Comparative study of periphyton community structure in long and short-hydroperiod Everglades marshes

The Florida Everglades is a mosaic of short and long-hydroperiod marshes that differ in the depth, duration, and timing of inundation. Algae are important primary producers in widespread Everglades’ periphyton mats, but relationships of algal production and community structure to hydrologic variability are poorly understood. We quantified differences in algal biomass and community structure between periphyton mats in 5 short and 6 long-hydroperiod marshes in Everglades National Park (ENP) in October 2000. We related differences to water depth and total phosphorus (TP) concentration in the water, periphyton and soils. Long and short-hydroperiod marshes differed in water depth (73 cm vs. 13 cm), periphyton TP concentrations (172μg g⁻¹ vs. 107 μg g⁻¹, respectively) and soil TP (284 μg g⁻¹ vs. 145 μg g⁻¹). Periphyton was abundant in both marshes, with short-hydroperiod sites having greater biomass than long-hydroperiod sites (2936 vs. 575 grams ash-free dry mass m⁻²). A total of 156 algal taxa were identified and separated into diatom (68 species from 21 genera) and “soft algae” (88 non-diatom species from 47 genera) categories for further analyses. Although diatom total abundance was greater in long-hydroperiod mats, diatom species richness was significantly greater in short- hydroperiod periphyton mats (62 vs. 47 diatom taxa). Soft algal species richness was greater in long-hydroperiod sites (81 vs. 67 soft algae taxa). Relative abundances of individual taxa were significantly different among the two site types, with soft algal distributions being driven by water depth, and diatom distributions by water depth and TP concentration in the water and periphyton. Periphyton communities differ between short and long-hydroperiod marshes, but because they share many taxa, alterations in hydroperiod could rapidly promote the alternate community. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.