Abundance Of Primary Producers Research Articles

Linking prescribed fire, nutrient deposition and cyanobacteria dominance through pyroeutrophication in a subtropical lake ecosystem from the mid Holocene to present

Prescribed fire (Rx-fire) is a common management tool for many forested ecosystems and promotes tree and forest soil health. Although burned materials from Rx-fire areas can enter adjacent aquatic environments, very few studies have focused on the water quality impacts of increased nutrients on aquatic primary producer communities. Here, we applied paleolimnological techniques on a 170-cm sediment core collected from Ditch Pond, AL, USA, a subtropical lake system located in the Conecuh National Forest where Rx-fire has been the primary management tool for ∼90 years. Macroscopic charcoal, nutrients (C, N, P) and photosynthetic pigments were measured throughout the core which spans from the middle Holocene until modern day. Our research questions were: 1) What were the sedimentary nutrient and stoichiometric changes associated with the Rx-Fire period beginning in 1937 CE? and 2) Did these nutrient changes alter historic algae/cyanobacteria groups? Following the onset of Rx-fire, nutrients (C, N, P) increased in deposition in the lake with P showing the greatest proportional increase at over 300%, suggesting that P inputs from Rx-fire are a primary artifact of burning. Photosynthetic pigments showed that increases in nutrients from Rx-fire caused extensive increases in total primary producer abundance and cyanobacteria dominance, called pyroeutrophication. These data suggest a greater need to understand the implications of fire-associated nutrients on aquatic primary producers wherever fire (but especially Rx-fire) is occurring, as well as an increase in collaboration between forest and aquatic ecosystem managers.

Facing up to climate change: Community composition varies with aspect and surface temperature in the rocky intertidal

Marine rocky intertidal organisms are amongst those most affected by climate change with regional distributional changes observed for many species. Although often ascribed to increased sea surface temperatures, precise assessment of the local habitat conditions underpinning observed and predicted changes in community assembly is lacking. Here we examine how aspect (i.e. north-south orientation) affects intertidal community composition and how rock surface temperatures and stress responses of two dominant grazer species (Patella spp.) elucidate emergent differences. We quantified year-round temperature variation and surveyed intertidal community composition on paired natural rock gullies with Equator- (EF) and Pole-facing (PF) surfaces. We also investigated variation in limpet (Patella spp.) reproductive phenology and osmotic stress. Average annual temperatures were 0.8 °C (1.6 °C at low tide) higher, with six-fold more frequent extremes (i.e. > 30 °C) on EF than PF surfaces. Intertidal community composition varied with aspect across trophic levels with greater overall species richness, abundance of primary producers and grazers on PF-surfaces, and greater barnacle abundance on EF-surfaces. Although species richness of organisms from different biogeographical origins (‘Boreal’ or ‘Lusitanian’) did not vary, the Lusitanian limpet Patella depressa exhibited earlier reproductive development on EF-surfaces and both limpet species exhibited greater thermal stress on EF-surfaces. We argue that our study system provides a good model for understanding how temperature variation at local scales can affect community composition, as well as ecophysiological and ecological responses to climate change and so better inform and predict regional range shifts over coming decades.

Sasso Pisano Geothermal Field Environment Harbours Diverse Ktedonobacteria Representatives and Illustrates Habitat-Specific Adaptations.

The hydrothermal steam environment of Sasso Pisano (Italy) was selected to investigate the associated microbial community and its metabolic potential. In this context, 16S and 18S rRNA gene partial sequences of thermophilic prokaryotes and eukaryotes inhabiting hot springs and fumaroles as well as mesophilic microbes colonising soil and water were analysed by high-throughput amplicon sequencing. The eukaryotic and prokaryotic communities from hot environments clearly differ from reference microbial communities of colder soil sites, though Ktedonobacteria showed high abundances in various hot spring samples and a few soil samples. This indicates that the hydrothermal steam environments of Sasso Pisano represent not only a vast reservoir of thermophilic but also mesophilic members of this Chloroflexi class. Metabolic functional profiling revealed that the hot spring microbiome exhibits a higher capability to utilise methane and aromatic compounds and is more diverse in its sulphur and nitrogen metabolism than the mesophilic soil microbial consortium. In addition, heavy metal resistance-conferring genes were significantly more abundant in the hot spring microbiome. The eukaryotic diversity at a fumarole indicated high abundances of primary producers (unicellular red algae: Cyanidiales), consumers (Arthropoda: Collembola sp.), and endoparasite Apicomplexa (Gregarina sp.), which helps to hypothesise a simplified food web at this hot and extremely nutrient-deprived acidic environment.

Seasonal and spatial variations of mesozooplankton energy transfer efficiency determined using remotely sensed SST and Chl-a in the Bay of Bengal

Mesozooplankton (MSP) are the major group of secondary producers in the ocean. They play an important role in both the carbon flux and energy transfer from primary producers to higher trophic levels. The present work explores the significance of how MSP biomass relates to the energy transfer between primary and secondary trophic levels by applying the MSP biomass model developed and validated by Mahesh et al. (2018) in the Bay of Bengal (BoB). The remote sensing products, mean sea surface temperatures (SST) and chlorophyll-a (Chl-a), were acquired from the Aqua (EOS PM) satellite’s Moderate Resolution Imaging Spectroradiometer (MODIS) sensor for the pre-monsoon period in 2013 through to summer 2015. These data were incorporated into the model to estimate MSP biomass which ranged from 0.12 to 33.52 mg C m−3 in the summer and post-monsoon periods in 2014, respectively, and was statistically significant between the seasons (F=19. 43; P=0. 008). The MSP productivity rate varied from 0.07 (summer 2014) to 15.35 mg C m−3 d−1(post-monsoon 2015). The rate was low (1 mg C m−3 d−1) across the central bay during the pre-monsoon period, whilst it was slightly higher (1-3 mg C m−3 d−1) along the neritic waters of the Palk Bay and Kakinada coastlines. The mean energy transfer efficiency from primary producers to MSP production was between 8.81 and 62.36 % during the post-monsoon and summer periods, respectively. This varied significantly between seasons (F=16.61; P=0.005) and was higher in the central bay than coastal waters. The complex interactions between water temperature, primary producer abundance, MSP grazing potential, seasonal variability, and the influential role of monsoons were the major controlling factors governing MSP biomass across the BoB.

Benthic Filtering Reduces the Abundance of Primary Producers in the Bottom Water of an Open Sandy Beach System (Kashimanada Coast, Japan)

AbstractThe effect of benthic filtering by the upogebiid shrimp Austinogebia narutensis on the abundance of primary producers in the water column was investigated on a wave‐dominated nearshore zone in central Japan facing the Pacific Ocean. Chlorophyll a concentration in the bottom layer of the water column decreased significantly with increasing population density of A. narutensis, suggesting that shrimp feeding reduces the abundance of phototrophic microorganisms in the bottom layer. Because the upogebiid shrimp commonly occur in the nearshore zone of sandy beaches, similar effects of their benthic filtering as reported in this study can be expected in areas where the shrimp is as densely present.

Hydrodynamic Controls on Primary Producer Communities in Spring‐Fed Rivers

AbstractIn many spring‐fed rivers, benthic macroalgae and periphytic algae are increasing and, in some cases, replacing rooted vascular plants, which are critical to ecosystem function. While most research has focused on the role of nutrients in driving this change, in‐channel hydrodynamics also control vascular plant and algal abundances and their interactions. Understanding relationships between hydrology and primary producers is essential for developing ecologically relevant flow regulations. We investigated the relationship between flow velocity and primary producer abundance in spring‐fed rivers using observational data from 16 springs to determine critical velocity thresholds for periphyton, macroalgae, and vascular plants. We also used flow suppression experiments to quantify periphyton growth rates and test for hysteretic behavior. Results suggest a critical velocity of 0.22 m/s (95% CI: 0.13–0.28 m/s) for periphyton but no specific thresholds for macroalgae or vascular plants. Experimental and theoretical results supported these findings and suggest periphyton establishment is not hysteretic.

Carbon balance of a sewage-fed aquaculture wetland

Wetlands are considered as one of the most effective carbon (C) sink among various ecosystems. To substantiate this view, the present investigation was conducted in one of the units of sewage fed East Kolkata wetlands (EKW), a Ramsar site, over a period of 3 years. The result revealed that because of high primary productivity due to abundance of primary producers and other sources of C input primarily through sewage influx, there is net positive balance of C in the system. The rate of C accumulation estimated through C budget was 1245.27–2490.55 kg C ha−1 year−1 while the rate estimated through sediment core analysis came out to be 1584 kg C ha−1 year−1. The amount of total C deposited up to 15 cm depth in sediment was 29.37 Mg ha−1, quite higher than the C deposited up to 15 cm depth of reference upland soil which was measured at 23.79 Mg ha−1. If C burial from this experiment in 55 ha is up-scaled for entire EKW of 12,500 ha area, then it empirically corresponds to 56.97–113.94 Gg CO2 captured from atmosphere per annum. EKW is not only used for aquaculture production thereby harvesting blue carbon but also has great potential to accumulate C in its sediments and thus helping mitigate climate change.

Effects of Bridge Construction and Wastewater Effluent on Phytoplankton Abundance and Sediment Geochemistry in an Atlantic Temperate Coastal Bay Since 1930

Despite a recent proliferation of bridges in coastal zones, little is known of the long-term impacts of bridge construction on phytoplankton abundance and community composition in nearshore waters. Here, we used high performance liquid chromatographic (HPLC) analysis to quantify historical changes in phytoplankton using fossils from cores on either side of a 57-year-old bridge that reduced fetch by 76% and created a discrete enclosed inner embayment. We hypothesized that phytoplankton abundance should be greater in the inner bay after bridge construction due to reduced hydrological flow and increased nutrient influx associated with terrestrial development and more profound anoxia in the enclosed basin. As expected, primary producer abundance, especially cryptophytes, increased in the inner bay during the 1950–1960s, although the effect was transient despite continuous fourfold elevation in sediment organic matter content (%C, %N) after ca. 1960 in response to wastewater influx. The pulse in cryptophytes appeared to correspond to influx of humic-rich waters derived from locally exploited peatlands, whereas the phytoplankton community of the outer basin largely reflected historical variations in climate, with warmer April months associated with increased abundance of diatoms and cryptophytes. Overall, sediment stable isotope values in the inner basin varied in response to a complex combination of changes in wastewater treatment (δ15N), release of untreated fish plant effluents (δ15N), and connectivity with the open ocean (δ13C). Taken together, these findings suggest that the effects of bridge construction on coastal phytoplankton production and community composition operated through multiple pathways, rather than just via modifications of basin hydrology.

Predicting rates of consumer-mediated nutrient cycling by a diverse herbivore assemblage

Herbivores mediate the abundances of primary producers both from the top-down, by consuming them, and from the bottom-up, by recycling nutrients. Whereas the top-down effects of herbivores on algae in marine ecosystems are well-documented, less is known about their roles as mediators of local-scale nutrient availability. We conducted a series of surveys and measurements of tide pools and the grazers in those pools between October of 2016 and June of 2017 at an intertidal site on the coast of Southern California, USA (33°35′16.3″N, 117°52′1.5″W). We surveyed grazer abundances in the field, measured biomass of representatives from four different grazer groups (littorine snails, limpets, chitons, and turban snails), measured ammonium excretion rates, and quantified ammonium accumulation rates in tide pools at our study site. We found that different grazer groups were characterized by different per-biomass ammonium excretion rates. Some grazer groups—turban snails and chitons—contributed more ammonium than predicted by their biomass, whereas other grazer groups—littorine snails and limpets—contributed less ammonium than predicted by biomass. Because of these differences between grazer groups, ammonium accumulation rates in tide pools at our study site were effectively predicted based on the ammonium excretion rates of the different grazer groups. However, ammonium accumulation rates were not related to total herbivore biomass. Our results highlight the importance of grazer identity—and particularly the role of species such as turban snails that contribute disproportionately to nutrient recycling—in understanding the contributions of grazers as mediators of bottom-up processes in marine systems.

Spatial and temporal dynamics of primary producers in shallow lakes as seen from space: Intra-annual observations from Sentinel-2A

Under the current high anthropic pressure and climate change scenarios, a trend towards increasing changes in the trophic status of shallow lakes, and the development of opportunistic floating species is to be expected. This raises the need for monitoring and management actions to prevent widespread environmentally negative effects (e.g., anoxia). An efficient approach to monitoring water quality and primary producers in inland waters is to integrate in situ with remote sensing data. In this work, satellite multispectral data acquired by Sentinel-2 A are used to assess the intra-annual spatial and temporal dynamics of phytoplankton abundance, in terms of chlorophyll-a (Chl-a) concentration and macrophyte Leaf Area Index (LAI) in a shallow eutrophic fluvial lake system (Mantua Lakes, Italy). Chl-a concentrations and LAI were derived from Sentinel-2 A data by applying a semi-empirical band ratio algorithm combined with a bio-optical model (BOMBER) for the former (Chl-a), and a semi-empirical model for the latter (LAI). These products were validated against in situ data (rRMSE = 20% for both products; R2 = 0.93 for Chl-a; R2 = 0.83 for LAI). Phytoplankton maps showed a marked intra-annual spatial and temporal variability, generally revealing a Chl-a concentration gradient from lotic to lentic waters. Air temperature was the main driver of Chl-a concentration, followed by water discharge and precipitation. The macrophyte LAI followed aquatic plant growth seasonality, and was independent of the hydro-meteorological data. Allochthonous and invasive macrophyte species (such as Nelumbo nucifera and Ludwigia hexapetala) had higher LAI compared than the Mantua Lakes’ autochthonous floating-leaved species (e.g., Trapa natans and Nuphar lutea). Maps of the abundance of primary producers can be used to follow the temporal and spatial evolution of different communities and support management actions, e.g., by identifying potential algal bloom hotspots, or the optimal timing for measures to control invasive species overgrowth.

Chronic prescribed burning alters nutrient deposition and sediment stoichiometry in a lake ecosystem.

Prescribed fire is a common management practice for forests and other terrestrial environments. Following a prescribed burn, ash erodes into aquatic environments potentially altering terrestrial-aquatic connectivity and water quality. In this study, we collected a sediment core from Ocean Pond, FL, USA, a lake that has received fire ash from decades of prescribed burning events. Paleolimnological measurements of macrocharcoal, nutrients, stable isotopes (δ13C, δ15N), and photosynthetic pigments were used to reconstruct fire regimes, material inputs, and lake primary producer responses for periods of prescribed burns and other lake periods throughout the last 8000years [corrected]. Results show that the period of repeated modern-prescribed fires coincided with decreased C and N depositions in the lake, while P deposition increased causing alterations to nutrient storage and stoichiometry. However, photosynthetic pigments indicated low primary producer abundance during the prescribed fire period. These changes in nutrient dynamics could provide new insights into biogeochemical pathways in land-water connected systems where burning has not been considered.

Energy Landscapes in Hydrothermal Chimneys Shape Distributions of Primary Producers.

Hydrothermal systems are excellent natural laboratories for the study of how chemical energy landscapes shape microbial communities. Yet, only a few attempts have been made to quantify relationships between energy availability and microbial community structure in these systems. Here, we have investigated how microbial communities and chemical energy availabilities vary along cross-sections of two hydrothermal chimneys from the Soria Moria Vent Field and the Bruse Vent Field. Both vent fields are located on the Arctic Mid-Ocean Ridge, north of the Jan Mayen Island and the investigated chimneys were venting fluids with markedly different H2S:CH4 ratios. Energy landscapes were inferred from a stepwise in silico mixing of hydrothermal fluids (HFs) with seawater, where Gibbs energies of relevant redox-reactions were calculated at each step. These calculations formed the basis for simulations of relative abundances of primary producers in microbial communities. The simulations were compared with an analysis of 24 samples from chimney wall transects by sequencing of 16S rRNA gene amplicons using 454 sequencing. Patterns in relative abundances of sulfide oxidizing Epsilonproteobacteria and methane oxidizing Methylococcales and ANME-1, were consistent with simulations. However, even though H2 was present in HFs from both chimneys, the observed abundances of putative hydrogen oxidizing anaerobic sulfate reducers (Archaeoglobales) and methanogens (Methanococcales) in the inner parts of the Soria Moria Chimney were considerably higher than predicted by simulations. This indicates biogenic production of H2 in the chimney wall by fermentation, and suggests that biological activity inside the chimneys may modulate energy landscapes significantly. Our results are consistent with the notion that energy landscapes largely shape the distribution of primary producers in hydrothermal systems. Our study demonstrates how a combination of modeling and field observations can be useful in deciphering connections between chemical energy landscapes and metabolic networks within microbial communities.

Persistent differences between coastal and offshore kelp forest communities in a warming Gulf of Maine.

Kelp forests provide important ecosystem services, yet coastal kelp communities are increasingly altered by anthropogenic impacts. Kelp forests in remote, offshore locations may provide an informative contrast due to reduced impacts from local stressors. We tested the hypothesis that shallow kelp assemblages (12–15 m depth) and associated fish and benthic communities in the coastal southwest Gulf of Maine (GOM) differed significantly from sites on Cashes Ledge, 145 km offshore by sampling five coastal and three offshore sites at 43.0 +/- 0.07° N latitude. Offshore sites on Cashes Ledge supported the greatest density (47.8 plants m2) and standing crop biomass (5.5 kg m2 fresh weight) of the foundation species Saccharina latissima kelp at this depth in the Western North Atlantic. Offshore densities of S. latissima were over 150 times greater than at coastal sites, with similar but lower magnitude trends for congeneric S. digitata. Despite these differences, S. latissima underwent a significant 36.2% decrease between 1987 and 2015 on Cashes Ledge, concurrent with a rapid warming of the GOM and invasion by the kelp-encrusting bryozoan Membranipora membranacea. In contrast to kelp, the invasive red alga Dasysiphonia japonica was significantly more abundant at coastal sites, suggesting light or dispersal limitation offshore. Spatial differences in fish abundance mirrored those of kelp, as the average biomass of all fish on Cashes Ledge was 305 times greater than at the coastal sites. Remote video censuses of cod (Gadus morhua), cunner (Tautaogolabrus adspersus), and pollock (Pollachius virens) corroborated these findings. Understory benthic communities also differed between regions, with greater abundance of sessile invertebrates offshore. Populations of kelp-consuming sea urchins Stronglyocentrotus droebachiensis, were virtually absent from Cashes Ledge while small urchins were abundant onshore, suggesting recruitment limitation offshore. Despite widespread warming of the GOM since 1987, extraordinary spatial differences in the abundance of primary producers (kelp), consumers (cod) and benthic communities between coastal and offshore sites have persisted. The shallow kelp forest communities offshore on Cashes Ledge represent an oasis of unusually high kelp and fish abundance in the region, and as such, comprise a persistent abundance hotspot that is functionally significant for sustained biological productivity of offshore regions of the Gulf of Maine.

Linking biogeochemical processes and historic primary producer communities in a SE USA sinkhole lake from the mid-Holocene to present

Many freshwater resources receive materials from human development causing a decrease in ecological services when compared to pre-disturbance periods. As a result, the understanding of eutrophication and limnological change has increased, but less attention has been given to systems under intense human impact that have not eutrophied so that drivers precluding eutrophication can be documented. The primary objective of this research was to reconstruct allochthonous inputs and in-lake processes for Long Pond, Georgia, USA from the mid Holocene to present and link them to primary producer community changes. Long Pond is a mesotrophic lake located in a highly altered watershed from agricultural and municipal land use and housing developments. A 5 m sediment core was collected from Long Pond, and organic matter, nutrients (C, N, P), metals (Al, Fe, Cu), and photosynthetic pigments were measured. Long Pond existed in three limnological states spanning the past ~6000 years. Prior to modern lacustrine conditions, Long Pond was a wetland/peat system that experienced the highest primary producer abundance recorded in the core. The modern lacustrine state began in the late Holocene and was characterized by increased connectivity with the surrounding watershed and low productivity. Human impacts began around 1900 AD and included high levels of phosphorus and metal deposition but moderate levels of primary producer abundance. As a result, in-lake dynamics are believed to be regulating the trophic status of Long Pond. Low concentrations of available phosphorus in the water column combined with high concentrations of sedimentary phosphorus may imply the binding of phosphorus to the sediments by certain materials such as aluminum and iron. Long Pond serves as an example of the complex in-lake processes that can occur from allochthonous inputs and autochthonous responses in lake systems thus complicating management decisions.

Assessment of eutrophication in a subtropical lagoon in the Gulf of California

Understanding the susceptibility of coastal ecosystems to nutrient loads from anthropogenic sources and the extent of eutrophication in subtropical and tropical estuaries is still a challenge. This study assessed eutrophication in Guaymas Bay, a subtropical lagoon that receives nutrient-rich urban wastewater discharges. Eutrophication was evaluated by means of the Assessment of Estuarine Trophic Status model. Our results showed that the trophic status of Guaymas Bay was “good.” It is important to consider that: (1) the dissolved oxygen and chlorophyll a levels observed are indicative of a healthy ecosystem, and (2) the key factors explaining this finding are the high susceptibility to eutrophication of the lagoon and the dominance of macroalgae species that respond to nutrient enrichment. This implies that the Overall Eutrophic Condition Index of the Assessment of Estuarine Trophic Status model is a valuable tool for use as an eutrophication indicator based on the abundance of primary producers (macroalgae and phytoplankton). One source of uncertainty in the application of the model to this semi-arid coastal lagoon deals with estimating its susceptibility to eutrophication. The model showed that the lagoon is a highly sensitive ecosystem; however, significant factors are in place, such as an adequate water exchange, that reduce its susceptibility to eutrophication. In conclusion, the trophic status observed at Guaymas Bay is acceptable; however, an environmental management plan is necessary to prevent any adverse effects of eutrophication.

Trophic interactions of proteolytic bacteria Proteinivorax tanatarense in an alkaliphilic microbial community

Lythic action of an anaerobic proteolytic bacterium Proteinivorax tanatarense on organisms with different cell wall types was studied. In the absence of photosynthetic oxygen release, this proteolytic was able to grow on intact biomass of cyanobacteria belonging to various systematic groups. Itis probably their usual saprotrophic-satellite responsible for the regulation of abundance of primary producers during the dark phase. Growth also occurred on the biomass of a nonphototrophic gram-negative microorganism-Halomonas campisalis, a common component of alkaliphilic. microbial communities: Comparative analysis of the interaction of the proteolytic with.H. campisalis cells at different physiological states revealed the lytic action to be re- stricted to dead and/or weakened cells, rather than the actively dividing ones. Strict specificity of the action of the proteolytic bacterium on gram-negative microorganisms with no effect on gram-positive ones was shown.

High chlorophyll a concentration in a low nutrient context: discussions in a subtropical lake dominated by Cyanobacteria

<p>Temporal variability in some water quality parameters can play an important role in determining the presence and abundance of primary producers, and consequently in the trophic state and other characteristics and uses of lake ecosystems. In this sense, the present study aimed at understanding temporal dynamics of some trophic relevant water quality parameters in different time scales and their correlation and influence in phytoplankton biomass (chlorophyll <em>a</em>) in a shallow subtropical coastal lake. Peri Lake is located in Florianópolis island in Southern Brazil and samples were taken monthly between March 2007 and February 2013. The lake showed low dissolved nutrients concentration, especially phosphorus (P) (median dissolved P: 2.0 µg.l<sup>-1</sup>) and high chlorophyll <em>a</em> (median: 20.8 µg.l<sup>-1</sup>) concentration. Total nitrogen (TN) concentration varied broadly, with a median of 672.8 µg L<sup>-1</sup>, and total P (TP) concentration was low (median: 13.5 µg L<sup>-1</sup>). A seasonal pattern of variation concerning dissolved and total P and chlorophyll a concentration was observed, associated mainly with temperature and wind speeds, but no clear pattern was observed for nitrogen (N) fractions. Significant differences were observed in different years for some parameters, with higher chlorophyll a and lower N concentration in the last three years sampled. The lake was considered potentially P limited during the majority of the study period and a positive correlation was found between chlorophyll <em>a</em> and total and dissolved P concentration. Phytoplankton biomass (as chlorophyll <em>a</em>) was apparently controlled by water temperature and P availability (TN:TP ratio and dissolved P). Water transparency (as Secchi depth) was strongly and negatively influenced by chlorophyll <em>a</em> concentration. <em>Cylindrospermopsis raciborskii</em> abilities to compete for P and light seem to be important factors determining its success and dominance in this low P coastal ecosystem. The fluctuating P supply, probably associated to sediment resuspension by wind in this shallow waterbody, is an advantageous factor for cyanobacteria and has an important role in chlorophyll <em>a</em> dynamics. Thus, high chlorophyll <em>a</em> concentration in this subtropical lake seems to be related to the P-limited condition, shallowness and low water column transparency, which are probably favouring the dominance of <em>C. raciborskii</em>, especially in higher summer temperatures, and leading to high chlorophyll <em>a</em> concentration even in a low dissolved nutrient environment.</p>

Herbivore regulation of plant abundance in aquatic ecosystems.

Herbivory is a fundamental process that controls primary producer abundance and regulates energy and nutrient flows to higher trophic levels. Despite the recent proliferation of small-scale studies on herbivore effects on aquatic plants, there remains limited understanding of the factors that control consumer regulation of vascular plants in aquatic ecosystems. Our current knowledge of the regulation of primary producers has hindered efforts to understand the structure and functioning of aquatic ecosystems, and to manage such ecosystems effectively. We conducted a global meta-analysis of the outcomes of plant-herbivore interactions using a data set comprised of 326 values from 163 studies, in order to test two mechanistic hypotheses: first, that greater negative changes in plant abundance would be associated with higher herbivore biomass densities; second, that the magnitude of changes in plant abundance would vary with herbivore taxonomic identity. We found evidence that plant abundance declined with increased herbivore density, with plants eliminated at high densities. Significant between-taxa differences in impact were detected, with insects associated with smaller reductions in plant abundance than all other taxa. Similarly, birds caused smaller reductions in plant abundance than echinoderms, fish, or molluscs. Furthermore, larger reductions in plant abundance were detected for fish relative to crustaceans. We found a positive relationship between herbivore species richness and change in plant abundance, with the strongest reductions in plant abundance reported for low herbivore species richness, suggesting that greater herbivore diversity may protect against large reductions in plant abundance. Finally, we found that herbivore-plant nativeness was a key factor affecting the magnitude of herbivore impacts on plant abundance across a wide range of species assemblages. Assemblages comprised of invasive herbivores and native plant assemblages were associated with greater reductions in plant abundance compared with invasive herbivores and invasive plants, native herbivores and invasive plants, native herbivores and mixed-nativeness plants, and native herbivores and native plants. By contrast, assemblages comprised of native herbivores and invasive plants were associated with lower reductions in plant abundance compared with both mixed-nativeness herbivores and native plants, and native herbivores and native plants. However, the effects of herbivore-plant nativeness on changes in plant abundance were reduced at high herbivore densities. Our mean reductions in aquatic plant abundance are greater than those reported in the literature for terrestrial plants, but lower than aquatic algae. Our findings highlight the need for a substantial shift in how biologists incorporate plant-herbivore interactions into theories of aquatic ecosystem structure and functioning. Currently, the failure to incorporate top-down effects continues to hinder our capacity to understand and manage the ecological dynamics of habitats that contain aquatic plants.

Eutrophication effects on greenhouse gas fluxes from shallow-lake mesocosms override those of climate warming.

Fresh waters make a disproportionately large contribution to greenhouse gas (GHG) emissions, with shallow lakes being particular hot spots. Given their global prevalence, how GHG fluxes from shallow lakes are altered by climate change may have profound implications for the global carbon cycle. Empirical evidence for the temperature dependence of the processes controlling GHG production in natural systems is largely based on the correlation between seasonal temperature variation and seasonal change in GHG fluxes. However, ecosystem-level GHG fluxes could be influenced by factors, which while varying seasonally with temperature are actually either indirectly related (e.g. primary producer biomass) or largely unrelated to temperature, for instance nutrient loading. Here, we present results from the longest running shallow-lake mesocosm experiment which demonstrate that nutrient concentrations override temperature as a control of both the total and individual GHG flux. Furthermore, testing for temperature treatment effects at low and high nutrient levels separately showed only one, rather weak, positive effect of temperature (CH4 flux at high nutrients). In contrast, at low nutrients, the CO2 efflux was lower in the elevated temperature treatments, with no significant effect on CH4 or N2 O fluxes. Further analysis identified possible indirect effects of temperature treatment. For example, at low nutrient levels, increased macrophyte abundance was associated with significantly reduced fluxes of both CH4 and CO2 for both total annual flux and monthly observation data. As macrophyte abundance was positively related to temperature treatment, this suggests the possibility of indirect temperature effects, via macrophyte abundance, on CH4 and CO2 flux. These findings indicate that fluxes of GHGs from shallow lakes may be controlled more by factors indirectly related to temperature, in this case nutrient concentration and the abundance of primary producers. Thus, at ecosystem scale, response to climate change may not follow predictions based on the temperature dependence of metabolic processes.

Exploitation and recovery of a sea urchin predator has implications for the resilience of southern California kelp forests.

Size-structured predator-prey interactions can be altered by the history of exploitation, if that exploitation is itself size-selective. For example, selective harvesting of larger sized predators can release prey populations in cases where only large individuals are capable of consuming a particular prey species. In this study, we examined how the history of exploitation and recovery (inside marine reserves and due to fisheries management) of California sheephead (Semicossyphus pulcher) has affected size-structured interactions with sea urchin prey in southern California. We show that fishing changes size structure by reducing sizes and alters life histories of sheephead, while management measures that lessen or remove fishing impacts (e.g. marine reserves, effort restrictions) reverse these effects and result in increases in density, size and biomass. We show that predation on sea urchins is size-dependent, such that the diet of larger sheephead is composed of more and larger sized urchins than the diet of smaller fish. These results have implications for kelp forest resilience, because urchins can overgraze kelp in the absence of top-down control. From surveys in a network of marine reserves, we report negative relationships between the abundance of sheephead and urchins and the abundance of urchins and fleshy macroalgae (including giant kelp), indicating the potential for cascading indirect positive effects of top predators on the abundance of primary producers. Management measures such as increased minimum size limits and marine reserves may serve to restore historical trophic roles of key predators and thereby enhance the resilience of marine ecosystems.