Benthic Algal Growth Research Articles

Short-Term Impact of Decomposing Crown-of-Thorn Starfish Blooms on Reef-Building Corals and Benthic Algae: A Laboratory Study

Outbreaks of crown-of-thorn starfish (COTS) have caused dramatic declines in reefs through predation on corals, but the post-bloom effects of COTS may still potentially threaten the environment and living organisms due to massive organic decomposition. This stimulation experiment showed that the decomposition of COTS debris triggered an extra mineralization process and resulted in acidifying, hypoxic, and eutrophic seawater. Consequently, the photosynthetic efficiency of coral symbionts decreased by 83%, and coral bleached after removing the stress within two days, then the coral skeleton dissolved at rates of 0.02–0.05 mg cm−2 day−1. Within two weeks, the photosynthesis and growth of benthic algae were suppressed by 27–86% and 1.5–16%, respectively. The mortality of turf algae and coralline algae indicated compromised primary productivity and limited coral recruitment, respectively. However, macroalgae, as coral competitors, became the only survivors, with increasing chlorophyll content. This study suggests a continuing decline of reefs during the collapse phase of COTS outbreaks and highlights the need for improving control strategies for the COTS population.

Application of inorganic fertilizer combination with various kinds of organic fertilizer in the nursery phase of white shrimp (Litopenaeus vannamei)

The purpose of this research is to determine the effect of natural feed growth on the growth and survival rate of white shrimp, specifically in relation to the usage of inorganic fertilizers in conjunction with various organic fertilizers. The experimental design consisted of four treatments, each replicated three times. The treatments included a combination of urea, SP-36, and organic fertilizer derived from superintensive shrimp pond waste (Treatment A), a combination of urea, SP-36, and petroganic (Treatment B), a combination of urea, SP-36, and chicken manure (Treatment C), and a control treatment consisting of only urea and SP-36 without any organic fertilizer (Treatment D). Urea was administered at a dosage of 5.2 grams per container, SP-36 at a dosage of 2.6 grams per container, and organic fertilizer at a dosage of 52 grams per container. The factors that were monitored included the composition of plankton, the growth of benthic algae, the growth and survival of white shrimp, and the quality of pond water. The study’s findings revealed no statistically significant difference in the growth of live feed when inorganic and organic fertilizers were combined (P>0.05). However, the survival rate of white shrimp exhibited a significant difference (P<0.05). Using a mixture of inorganic fertilizer (namely urea and sp-36) and organic fertilizer, within the context of a super-intensive shrimp pond, yielded the most favorable outcome regarding white shrimp survival rate, with a recorded percentage of 94.67%.

The invasion of tilapia in South China freshwater systems: A review

AbstractTilapia invasion has become a significant concern to ecologists and conservationists, posing severe threat to freshwater biodiversity. Many tilapia species have been introduced to China since 1957, with some having invaded freshwater ecosystems in South China. Accordingly, the present review examined the driving factors for successful tilapia invasion and their ecological impacts, information essential for practical management purposes. This review identified aquaculture as the main pathway spreading tilapias into the wild freshwater systems through their escape and release. Their biological and life history traits, including flexible habitat requirements, environmental tolerance, and generalist feeding are essential factors facilitating their ability to inhabit varying habitats. Biodiversity loss and habitat degradation further promote tilapia invasions by creating exploitable habitats for them, noting their considerable tolerance to degraded ecosystems. Regarding ecological effects, tilapias stimulate phytoplankton growth by increasing nutrient concentrations in the water column through excretion and bioturbation. Tilapias also depress benthic algal growth by promoting sediment resuspension, deteriorating water quality, and enhancing eutrophication. Moreover, tilapias cause the decline of native fish species biomass through competition and may threaten fish biodiversity. The findings in the present review suggest direct removal of tilapias can be a useful practice for controlling them. Furthermore, imposing regulations on tilapias introductions, efficient aquaculture management, and biodiversity conservation should also be considered in controlling and preventing the invasion and spread of tilapias.

Influence of Lateglacial meltwater influx from alpine glaciers on the aquatic ecosystem of Kanas Lake in the Altai Mountains, China

Glacier retreat in high altitude areas triggered by anthropogenic warming exerts a profound impact on the ecology of glacier-fed lakes. In this paper, we report on the results of a multi-proxy analysis of a Lateglacial (13.3–11.3 cal kyr B.P.) core obtained from the glacier-fed Kanas Lake in the Altai Mountains, China, to understand the responses of aquatic ecosystems to meltwater influxes. Core intervals with high sand fractions and enriched silicon dioxide and zirconium (~11.5, 11.7, 11.8, 12.2 and 13.0 cal kyrs B.P.) reflect intense meltwater pulses from alpine glaciers. In the core, these pulses generally coincide with low abundance of littoral cladocerans, high abundance of planktonic Pediastrum, and high phosphorus concentrations. Our results suggest that intense meltwater influxes not only brought nutrients (nitrogen and phosphorus) into the lake and promoted phytoplankton growth, they also generated cold, turbid water masses and restricted the growth of benthic algae and invertebrates. Therefore, we infer that future anthropogenic warming may lead to significant changes in the structure and function of aquatic ecosystems in glacier-fed lakes worldwide.

Nutrient limitation of primary production in rivers along a land use gradient in the Lake Biwa Basin, Shiga Prefecture, Japan

Benthic microalgae (BMA) biomass and community structure in freshwater lotic systems are often limited by inorganic nutrient availability. We examined nutrient limitation of benthic algae biomass and community structure using nutrient diffusing substrates in four rivers along a land use gradient in the Lake Biwa basin, Japan. Ambient in-stream nutrient concentrations were correlated to catchment land use, with the highest nitrogen and phosphorus concentrations being found in rivers draining more forested catchments. Nutrient limitation of primary producer biomass and nutrient-driven changes in community structure were evident in all four rivers, regardless of in-stream nutrient concentrations and surrounding land use. BMA biomass (measured as chlorophyll log-response ratio) exhibited the greatest nutrient limitation in rivers with higher in-stream nutrient concentrations. The relationship to catchment land use was less clear, with the highest nutrient limitation being observed in the catchment with moderate amounts of agricultural and forested land use. Nutrient additions resulted in a shift from dominance by Bacillariophyceae (diatoms) to a mixed Bacillariophyceae-Chlorophyceae (chlorophyte) community in all rivers and this shift was most pronounced in the forested catchments. Changes within the diatom community with nutrient additions were also observed, although the shifts in diatom community within a river in response to nutrient additions were much smaller than the differences in diatom community composition among rivers. Diatom taxa classified as highly motile increased with nutrient additions in all rivers. Our results suggest that primary producer community in rivers may be sensitive to nutrient inputs even in areas with elevated nutrient concentrations and catchments dominated by agricultural land use. There is likely widespread nutrient limitation in rivers of Japan, across both in-stream nutrient and land use gradients and any increases in nutrient loading will likely stimulate benthic algal growth. Our findings highlight the importance of looking at both biomass and species composition to assess ecosystem-level impacts of elevated nutrient levels.

Impacts of phosphorus loading temporal pattern on benthic algae growth in Lake Ontario

Nuisance growth of the alga Cladophora has been a critical issue in the Laurentian Great Lakes for decades. Although loading of total phosphorus (TP) has been below target values since the 1990s, the nearshore of the lakes still experience Cladophora blooms. This study explores the effects of TP loading temporal pattern on Cladophora growth in Rochester Embayment in Lake Ontario, where Cladophora blooms are frequently observed. Long-term TP loadings from the Genesee River, which is the primary tributary to the embayment, are calculated with a hydrological model, and Cladophora indicators based on beach closures and satellite data are collected for 2008–2017 to analyze the TP loading temporal patterns and their relation with Cladophora abundance. Using time series clustering for both TP loading and Cladophora indicators, it is found that besides total annual TP loading, the timing of high TP loading also plays an important role for Cladophora growth. High TP loading in mid-March to mid-June, compared to that in January to mid-March, is found to produce higher Cladophora abundance in summer. This finding is supported and explained using an integrated hydrodynamic and ecological modeling framework that includes a state-of-the-art Cladophora module. This study suggests that future Cladophora control should consider not only the total annual TP loading mass, but also the temporal loading patterns.

Interactive effects of light and nutrients on stream algal growth modified by forest management in boreal landscapes

Boreal forests account for 30% of the world’s total forest cover and in many places are subject to intensive forest management, which often involves complete removal of overstory vegetation by clear-cutting. However, we still do not fully understand how forest management affects aquatic ecosystems in these landscapes. Here we asked how forest management-induced changes in environmental conditions, such as incident light and nutrient availability, affect benthic algal growth and nutrient limitation in boreal headwater streams of northern Sweden. We answered this question using a combination of correlative and experimental approaches across a range of forested streams spanning a gradient of site (e.g. canopy openness and water chemistry) and catchment-level (e.g. age of forest regrowth) parameters, with variation among the study streams influenced by different forest management histories and underlying natural variation. We found that benthic algal growth in these forested streams was largely driven by local interactions between dissolved inorganic nitrogen (N) availability and incident light reaching benthic surfaces. Greater water temperature and shallower depths were also associated with greater algal growth. Although high dissolved organic carbon (DOC) concentrations often play a role in reducing light availability to autotrophs in boreal aquatic systems, it was not an important predictor of algal growth in small forested streams despite a large DOC concentration gradient (5 – 32 mg/L). Results from experimental nutrient additions supported the role of N as a key limiting nutrient, but also revealed both spatial and seasonal factors that modulate the effects of altered nutrient availability. Overall, our results suggest that differences in how light regimes and nutrient loading respond to forest management generate small-scale variation in the controls over stream primary productivity, which likely shift in relative importance at the time scale of a forest rotation (60 to 100 years).

Climatic and environmental change in the western Tibetan Plateau during the Holocene, recorded by lake sediments from Aweng Co

Understanding the strength and extent of the Asian summer monsoon (including the East Asian summer monsoon and the Indian summer monsoon) in the Tibetan Plateau (TP) region is crucial for predicting possible changes in the regional eco-environment and water resources under global warming. Due to the lack of well-dated and high-resolution paleoclimate records, long-term monsoon dynamics are still not well understood in the western TP, which is currently influenced by both the Indian summer monsoon (ISM) and the westerlies. Here we present a multi-proxy lacustrine record covering the past 10,500 years from Aweng Co, an alpine lake at the northern limit of the modern ASM in western Tibet. Our results show that the western TP was mainly controlled by the ISM during the Holocene and the regional ecosystem/environment was sensitive to climate change. The climate was the wettest between 10.5 and 7.3 cal kyr BP, when terrestrial plants in the catchment were productive and the biomass of benthic algae was low possibly due to limited sunlight at the lake bottom due to high lake level. From 7.3 to 5.0 cal kyr BP the climate shifted towards drier conditions, resulting in a decline in terrestrial plant cover. Between 5.0 and 3.1 cal kyr BP, the climate became even drier, resulting in a further decline in vegetation cover in the catchment. Between 4.6 and 3.1 cal kyr BP, 100% endogenic dolomite precipitated from the lake water, possibly induced by high Mg/Ca ratios. After 3.1 cal kyr BP, the climate was the driest and frequent centennial-scale droughts occurred. The lake level was low and would have resulted in more light reaching the lake bottom, favoring the growth of benthic algae. The reconstructed lake level change of Aweng Co agrees well with the paleo-shoreline records in the southern TP, demonstrating that the ISM evolution played a key role in lake hydrological processes in this region. A comparison of paleoclimate records shows the ISM reached 34.5° N in the western TP during the Holocene.

Use of sucralose and caffeine as tracers of human waste in a coral reef ecosystem

Over the past ten years, divers have noted a decrease in healthy coral cover and an increase in benthic algae in Vatia Bay, on the island of Tutuila, American Samoa. The cause for this is unknown, but one hypothesis is that nutrient pollution from the local village may be driving the coral decline. Excess nutrients (especially nitrogen and phosphorus) can impact corals directly by lowering fertilization success, and reducing both photosynthesis and calcification rates, or indirectly such as through stimulation of the growth of benthic algae. Declining coral health adversely affects the biodiversity of the Bay and likely decreases ecosystem services. The objectives of this study were to determine the nutrient status of Vatia Bay (i.e. are levels elevated) and attempt to use caffeine and sucralose as tracers to assess the potential importance of human waste to the nutrient budget of the system.Water samples were collected monthly at sixteen sites, selected using a stratified random design, for analysis of nitrate, nitrite, ammonium, urea, total nitrogen, orthophosphorus, total phosphorus, silica and salinity. These data confirm that nutrient concentrations are elevated in the Bay, when compared to territorial water quality standards (total nitrogen < 0.15 mg N/L; total phosphorus < 0.02 mg P/L). Land based contributions of phosphorus and reactive nitrogen can enter the environment from a variety of sources, but in Vatia the most likely sources are piggeries and septic systems. Analysis of water samples for tracers of human waste (caffeine and sucralose) confirmed that human derived nutrients are contributing to the nutrient budget of the Bay. Caffeine was detected in 82% of samples and sucralose was detected in 51% of samples, definitively confirming that human waste is reaching the Bay. Additionally, sucralose concentrations are significantly correlated with some nutrient constituents, including urea, a major component of waste. These data are useful not only to enhance the understanding of the role that anthropogenic nutrients play in the biodiversity and ecosystem health of the Bay, but also serve as an important “baseline” against which to measure future change.

Using Freshwater Bivalves (Corbicula Fluminea) to Alleviate Harmful Effects of Small-Sized Crucian Carp (Carassius Carassius) on Growth of Submerged Macrophytes during Lake Restoration by Biomanipulation

Increased recruitment of small-sized fish following biomanipulation by reducing the biomass of plankti-benthivorous fish, not least in (sub)tropical lakes, may deteriorate water quality and thereby potentially hamper the recovery of submerged macrophytes. Filter-feeding bivalves remove suspended particles from the water and may, thereby, somewhat or fully counteract this negative effect of the increasing abundance of small-sized fish. So far, only few studies have investigated the interactive effects of fish and bivalves on water clarity and macrophyte growth. We conducted a 2 × 2 factorial designed outdoor mesocosm experiment with two densities of small crucian carp Carassius carassius (low 10 g m−2 and high 40 g m−2) and two densities of bivalves Corbicula fluminea (low 204 g m−2 and high 816 g m−2). We found significant interactive effect of fish and bivalves on the growth of the macrophyte Vallisneria natans. In the low density bivalve regime, the relative growth rates, root mass, root:shoot ratio and number of tubers were 30.3%, 30.8%, 21.6% and 27.8% lower in the high than in the low density fish treatments, while the decrease was less pronounced in the high density bivalve regime: 1.2%, 8.7%, 2.1% and 13.3%, respectively. Thus, bivalves reduced the negative effects of fish, not least when bivalve density was high. The weaker effects of small fish on plants in the high- than in the low-density C. fluminea regime can be attributed to lower total suspended solids (TSS) and Chl a in the first week of the experiment. Better light conditions further stimulated the growth of benthic algae, potentially increasing the removal of nutrients from the water and reducing fish-driven resuspension of the sediment. In addition, high densities of C. fluminea also enriched the sediment total nitrogen (TN) and total phosphorus (TP) content, favouring plant growth as indicated by an increase in leaf tissue TN and TP contents. Our results demonstrate that filter-feeding bivalves can alleviate harmful effects of small fish by prolonging a clear-water state that facilitates submerged macrophyte growth. Addition of the bivalve C. fluminea can be a promising tool for the restoration of submerged macrophytes in shallow eutrophic lakes, in particular lakes containing small, rapidly reproducing fish that due to their small sizes are not capable of controlling the bivalves.

Dissolved organic matter regulates nutrient limitation and growth of benthic algae in northern lakes through interacting effects on nutrient and light availability

AbstractWidespread increases in dissolved organic matter (DOM) concentration across northern lakes can alter rates of primary production by increasing nutrient availability and decreasing light availability. These dual effects of DOM generate a unimodal relationship in pelagic primary production and primary producer biomass among lakes over a gradient of DOM concentration. However, the responses of benthic algae to variation in DOM loading are less clear because of their potential to access sediment nutrients. We tested algal production and nutrient limitation along a DOM gradient in northern Sweden. Without added nutrients, benthic algal production showed a unimodal relationship with DOM, similar to reported pelagic responses. Nutrient addition revealed widespread nitrogen limitation, with decreasing severity in lakes with higher DOM. Because the majority of northern Swedish lakes currently fall below the inflection point in this unimodal relationship, moderate increases in DOM have the potential to increase benthic primary production, particularly for epilithic algae.

Transport of N and P in U.S. streams and rivers differs with land use and between dissolved and particulate forms.

We used a recently published, open‐access data set of U.S. streamwater nitrogen (N) and phosphorus (P) concentrations to test whether watershed land use differentially influences N and P concentrations, including the relative availability of dissolved and particulate nutrient fractions. We tested the hypothesis that N and P concentrations and molar ratios in streams and rivers of the United States reflect differing nutrient inputs from three dominant land‐use types (agricultural, urban and forested). We also tested for differences between dissolved inorganic nutrients and suspended particulate nutrient fractions to infer sources and potential processing mechanisms across spatial and temporal scales. Observed total N and P concentrations often exceeded reported thresholds for structural changes to benthic algae (58, 57% of reported values, respectively), macroinvertebrates (39% for TN and TP), and fish (41, 37%, respectively). The majority of dissolved N and P concentrations exceeded threshold concentrations known to stimulate benthic algal growth (85, 87%, respectively), and organic matter breakdown rates (94, 58%, respectively). Concentrations of both N and P, and total and dissolved N:P ratios, were higher in streams and rivers with more agricultural and urban than forested land cover. The pattern of elevated nutrient concentrations with agricultural and urban land use was weaker for particulate fractions. The % N contained in particles decreased slightly with higher agriculture and urbanization, whereas % P in particles was unrelated to land use. Particulate N:P was relatively constant (interquartile range = 2–7) and independent of variation in DIN:DIP (interquartile range = 22–152). Dissolved, but not particulate, N:P ratios were temporally variable. Constant particulate N:P across steep DIN:DIP gradients in both space and time suggests that the stoichiometry of particulates across U.S. watersheds is most likely controlled either by external or by physicochemical instream factors, rather than by biological processing within streams. Our findings suggest that most U.S. streams and rivers have concentrations of N and P exceeding those considered protective of ecological integrity, retain dissolved N less efficiently than P, which is retained proportionally more in particles, and thus transport and export high N:P streamwater to downstream ecosystems on a continental scale.

Benthic algal (periphyton) growth rates in response to nitrogen and phosphorus: Parameter estimation for water quality models.

Nitrogen (N) and phosphorus (P) are significant pollutants that can stimulate nuisance blooms of algae. Water quality models (e.g., WASP, CE-QUAL-R1, CE-QUAL-ICM, QUAL2k) are valuable and widely used management tools for algal accrual due to excess nutrients in the presence of other limiting factors. These models utilize the Monod and Droop equations to associate algal growth rate with dissolved nutrient concentration and intra-cellular nutrient content. Having accurate parameter values is essential to model performance, however published values for model parameterization are limited, particularly for benthic (periphyton) algae. We conducted a 10-day mesocosm experiment and measured diatom-dominated periphyton biomass accrual through time as chlorophyll a (chl a) and ash-free dry mass (AFDM) in response to additions of N (range 5-11,995 μg NO3-N/L) and P (range 0.89-59.51 μg SRP/L). Resulting half saturation coefficients and growth rates are similar to other published values, but minimum nutrient quotas are higher than those previously reported. Saturation concentration for N ranged from 150 to 2450 μg NO3-N/L based on chl a and from 8.5 to 60 μg NO3-N/L when based on AFDM. Similarly, the saturation concentration for P ranged from 12 to 29 μg-P/L based on chl a, and from 2.5 to 6.1 μg-P/L based on AFDM. These saturation concentrations provide an upper limit for streams where diatom growth can be expected to respond to nutrient levels and a benchmark for reducing nutrient concentrations to a point where benthic algal growth will be limited.

Long‐term assessment of nutrient flow pathway dynamics and in‐stream fate in a temperate karst agroecosystem watershed

AbstractNutrient dynamics in karst agroecosystems remain poorly understood, in part due to limited long‐term nested datasets that can discriminate upland and in‐stream processes. We present a 10‐year dataset from a karst watershed in the Inner‐Bluegrass Region of central Kentucky, consisting of nitrate (nitrate‐N [NO3−]), dissolved reactive phosphorus (DRP), total organic carbon (TOC), and total ammoniacal‐N (TAN) measurements at nested spring and stream sites as well as flowrate at the watershed outlet. Hydrograph separation techniques were coupled with multiple linear regression and Empirical Mode Decomposition time‐series analysis to determine significance of seasonal processes and to generate continuous estimates of nutrient pathway loadings. Further, we used model results of benthic algae growth and decomposition dynamics from a nearby watershed to assess if transient storage in algal biomass could explain differences in spring and downstream watershed nutrient loading. Results highlight statistically significant seasonality for all nutrients at stream sites, but only for NO3− at springs with longitudinal variability showing significant decreases occurring from spring to stream sites for NO3− and DRP, and significant increases for TOC and TAN. Pathway loading analysis highlighted the importance of slow flow pathways to source approximately 70% of DRP and 80% of NO3−. Results for in‐stream dynamics suggest that benthic autotroph dynamics can explain summer deviations for TOC, TAN, and DRP but not NO3−. Regarding upland dynamics, our findings agree well with existing perceptions in karst for N pathways and upland source seasonality but deviate from perceptions that karst conduits are retentive of P, reflecting the limited buffering capacity of the soil profile and conduit sediments in the Inner‐Bluegrass. Regarding in‐stream fate, our findings highlighted the significance of seasonally driven nutrient processing in the bedrock‐controlled streambed to influence nutrient fluxes at the watershed outlet. Contrary to existing perceptions, we found high N attenuation and an unexplained NO3− sink in the bedrock stream, leading us to postulate that floating macrophytes facilitate high rates of denitrification.

Influence of Stream Characteristics on Diversity and Distribution of Benthic Algae in Gongawwa Ela in Kottawa Rain forest, Galle, Sri Lanka

Influence of stream characteristics on diversity and distribution of benthic algae were studied at Gongawwa Ela during the period of September to November in 2016. Four sampling sites were selected and the distance between two sampling sites was 1.5km. Substrate types were categorized by sieve analysis and grain size distribution curves. Principle Component Analysis (PCA) was performed to identify the most important stream characteristics that could potentially influence on the growth of benthic algae. Physicochemical characteristics of water (nitrate, Total Dissolved Solids, temperature, pH, and Dissolved Oxygen) and substrates (pH, phosphate and nitrate) showed significant temporal variations. Conductivity and water velocity levels of the substrate varied significantly among the sampling sites. Species richness of benthic algae (29 genera) showed significantly negative correlations with the level of phosphate content (R2=-0.9592, P&lt;0.05) and pH levels (R2=-0.999, P&lt;0.05) of the substrate. Well graded substrate was observed only at the most downstream site (site 3) while other sites showed poorly graded substratum consisting over 90% of sand. The most abundant algal division was Bacillariophyta and the abundance of Flagilaria sp., Pinnularia sp., Stigeoclonium sp., and Pediastrum sp. showed significant temporal variation. Pinnularia sp., Synedra sp., Anabaena sp. and Navicula sp. were possible indicators of the upstream characteristics. Meanwhile, Cosmarium sp., Pinnularia sp., Surirella sp., Tabellaria sp., Synedra sp., Closterium sp. and Navicula sp. were possible indicators of the downstream characteristics. The PCA showed that the most important stream characteristics were DO, conductivity, Total Suspended Solids, water pH, nitrate in water and water discharge, while conductivity, nitrate, phosphate, % organic matter, pH, and % canopy cover were the types of substrate characteristics.

The influence of turbidity and water temperature on black fly species in the middle and lower Orange River, South Africa

Water clarity in the Orange River of South Africa alternates between highly turbid and clearer conditions. Multiple ecological effects result from this that affect aquatic community structure and species composition, including switching of dominant black fly species and growth of benthic algae. Fourteen sites in the middle and lower Orange River were sampled for black fly species and abundances, presence of benthic algae, and water quality and turbidity seasonally between November 2015 and December 2016. Logistic regression models, time series analyses, and multivariate ordinations showed that clearer water favours benthic algae and the minor pest black fly species Simulium adersi Pomeroy, 1922 and Simulium nigritarse Coquillett, 1902, while high turbidity favours the major pest black fly species Simulium chutteri Lewis, 1965 and Simulium damnosum Theobald, 1903. Moreover, these switches appear to be less frequent under post-impoundment conditions. We conclude that black fly species composition is dynamic and reflects changes in flow and turbidity levels. Flow regulation has resulted in a trade-off between improved water supply and ecological costs resulting from reduced ecosystem variability.

Phytoplankton and benthic algal response to ecosystem engineers and multiple stressors in the nearshore of Lake Huron

Abstract Resurgence of nuisance benthic algae in the Great Lakes, despite substantial efforts to reduce phosphorus loading, has stimulated renewed interest in exploring the diverse drivers of near-shore water quality. Interestingly, broad similarity in the underlying causes of shore fouling by benthic algae in Lakes Ontario, Erie, and Michigan appear to contrast with Lake Huron where system productivity and dreissenid abundance are lower. While total phosphorus was the primary predictor of chlorophyll concentrations (70–90% of variation) in the water column, we identified a series of spatial patterns that underpin this relationship (up to 28% of variation) and which integrate catchment processes, tributary influences, shoreline complexity, and distance from shore. Dreissenid mussels were the most important predictors of benthic algae cover and biomass in our models, explaining between 20 and 52% of variation. Spatial patterns explained an additional 21–48% of the variation in benthic algae cover and biomass and highlight the importance of site-specific spatial heterogeneity in benthic algae growth. Our results are consistent with the nearshore shunt hypothesis, wherein higher algal cover and biomass coincided with higher mussel density and biomass, although correlative effects with lake depth and loss of algal and mussel biomass due to physical disturbance must also be considered. These results underscore the difficulty associated with identifying the potential drivers of nearshore water quality as the diverse processes of nutrient loading, changes in catchment land use, and ecosystem change associated with invasion by dreissenids all vary in relative influence over a range of spatial scales.

Eurasian tench (Tinca tinca): the next Great Lakes invader

A globally invasive fish, Eurasian tench (Tinca tinca) is spreading through the St. Lawrence River and poses an imminent invasion threat to the Great Lakes. Following its illegal release into a tributary of the St. Lawrence River in 1991, tench has spread throughout the river’s main stem over the past decade, and its abundance in commercial fishing bycatch in the river has grown exponentially. The tench is a generalist benthic consumer with largely undocumented ecological impacts in North America. Reports from other invaded regions indicate that it can compete with other benthic fishes, host a diverse assemblage of parasites and pathogens, degrade water clarity in shallow lakes, limit submerged macrophyte growth, reduce gastropod populations, and promote benthic algal growth through top-down effects. Risk assessments and climate-match models indicate that the Great Lakes are vulnerable to tench invasion, and they signal the need for timely comprehensive actions, including development and implementation of monitoring and rapid-response protocols, including prevention or slowing of natural dispersal through canals.

Herbivorous snails can increase water clarity by stimulating growth of benthic algae

Eutrophication in shallow lakes is characterized by a switch from benthic to pelagic dominance of primary productivity that leads to turbid water, while benthification is characterized by a shift in primary production from the pelagic zone to the benthos associated with clear water. A 12-week mesocosm experiment tested the hypothesis that the herbivorous snail Bellamya aeruginosa stimulates the growth of pelagic algae through grazing on benthic algae and through accelerating nutrient release from sediment. A tube-microcosm experiment using 32P-PO4 as a tracer tested the effects of the snails on the release of sediment phosphorus (P). The mesocosm experiment recorded greater total nitrogen (TN) concentrations and a higher ratio of TN:TP in the overlying water, and a higher light intensity and biomass of benthic algae as measured by chlorophyll a (Chl a) in the snail treatment than in the control. Concentrations of total phosphorus (TP), total suspended solids (TSSs), and inorganic suspended solids (ISSs) in the overlying water were lower in the snail treatment than in the control, though no significant difference in Chl a of pelagic algae between the snail treatment and control was observed. In the microcosm experiment, 32P activity in the overlying water was higher in the snail treatment than in the control, indicating that snails accelerated P release from the sediment. Our interpretation of these results is that snails enhanced growth of benthic algae and thereby improved water clarity despite grazing on the benthic algae and enhancing P release from the sediment. The rehabilitation of native snail populations may therefore enhance the recovery of eutrophic shallow lakes to a clear water state by stimulating growth of benthic algae.

Positive feedback between chironomids and algae creates net mutualism between benthic primary consumers and producers.

The chironomids of Lake Mývatn show extreme population fluctuations that affect most aspects of the lake ecosystem. During periods of high chironomid densities, chironomid larvae comprise over 90% of aquatic secondary production. Here, we show that chironomid larvae substantially stimulate benthic gross primary production (GPP) and net primary production (NPP), despite consuming benthic algae. Benthic GPP in experimental mesocosms with 140,000 larvae/m2 was 71% higher than in mesocosms with no larvae. Similarly, chlorophyll a concentrations in mesocosms increased significantly over the range of larval densities. Furthermore, larvae showed increased growth rates at higher densities, possibly due to greater benthic algal availability in these treatments. We investigated the hypothesis that larvae promote benthic algal growth by alleviating nutrient limitation, and found that (1)larvae have the potential to cycle the entire yearly external loadings of nitrogen and phosphorus during the growing season, and (2) chlorophyll a concentrations were significantly greater in close proximity to larvae (on larval tubes). The positive feedback between chironomid larvae and benthic algae generated a net mutualism between the primary consumer and primary producer trophic levels in the benthic ecosystem. Thus, our results give an example in which unexpected positive feedbacks can lead to both high primary and high secondary production.