Evidence Of Nutrient Limitation Research Articles

Stream bryophytes promote “cryptic” productivity in highly oligotrophic headwaters

AbstractRecent observations document increased abundance of algae in the headwater streams of Hubbard Brook Experimental Forest (HBEF). It is possible that this “greening up” of HBEF streams may be due to climate change, with rising temperatures, altering terrestrial phenology, and shifting hydrologic regimes. Alternatively, stream “greening” could be from the slow recovery of stream chemistry after decades of acid rain, which has led to rising pH, declining concentrations of toxic Al3+, and low solute concentrations. Four years of weekly algal measurements on artificial moss and ceramic tiles, along with six nutrient enrichment experiments, revealed new insights about the interactions between these two autotrophs. We found that in protected weir ponds and in stream channels, algal biomass was higher on artificial moss substrates than on tiles—with this effect amplified in the stream channels. These results suggest that bryophytes can provide physical protection from flood scour or may trap nutrients to support algal growth. In stream channels, algal biomass was higher in well‐lit habitats and time periods indicating strong light limitation. We only measured nitrogen and phosphorus limitation of algal biomass in nutrient enrichment experiments conducted within weir ponds, with higher light availability and lower flow. By comparison, results from the remaining four instream experiments provided little evidence for nutrient limitation, with only one trial showing increased algal growth in response to nutrient addition. The most striking implication of our study is the role of bryophytes in providing refugia, and potentially nutrients, to algae in shaded and oligotrophic headwater streams.

Temporal variability of the physical and chemical environment, chlorophyll and diatom biomass in the euphotic zone of the Beagle Channel (Argentina): Evidence of nutrient limitation

Coastal, semi-enclosed aquatic ecosystems of sub-polar regions surrounded by continental glaciers are among the most sensitive environments to climate change. The Beagle Channel (Argentina-Chile, ~55°S, 71°-66°W), which is part of the Patagonian Cold Estuarine System of austral South America, receives fresh water input from rivers, glaciers and mountain ice sheets. Like in most fjord systems, its productivity relies on the input of nutrients from marine and terrestrial sources. Bathymetry and circulation patterns in the western (inner) sector of the Beagle Channel are known to differ from those of the eastern (outer) sector. Physical and chemical conditions of the water column and climatic trends in the area have been understudied, as all previous surveys were strongly limited in time, spatial coverage and water depth (especially in wintertime). For the first time and during a 2.5-year period (2012–2014) this study covers the spatial and temporal variation of hydrographic and meteorological conditions, total chlorophyll concentration and diatom biomass at three different sites within the Argentine sector of the Beagle Channel. An overall vertical homogeneity and silicate limitation for diatom growth were observed in the euphotic zone of both the inner and outer sectors. These two sectors, however, differed in other ways: salinity was always higher in the outer sector occasionally showing a west-east gradient. Conversely, total chlorophyll and diatom biomass were higher in the inner sector at every sampling date. In both sectors, biotic and abiotic variables showed a seasonal pattern with clear differences between summer and winter. Nevertheless, interannual differences may cast shadow over seasonal patterns: The warmer summer and milder winter of year 2013, together with a delay in spring stratification, led to higher nutrient concentrations, lower total chlorophyll and lower diatom biomass during that year as compared to years 2012 and 2014. Inorganic nutrients and chlorophyll levels found in this study were compared with all available literature on the Beagle Channel. The present results represent a base line to encourage further time-sustained investigation relating biogeochemical, oceanographic and phytoplanktonic processes involving mainly silicate-dependent organisms. In turn, they will allow tracking changes and responses of the Beagle Channel ecosystem to climatic-oceanographic phenomena (e.g., ENSO, SAM) and climate change.

Future Changes in Oceanography and Biogeochemistry Along the Canadian Pacific Continental Margin

Model projections of ocean circulation and biogeochemistry are used to investigate large scale climate changes under moderate mitigation (RCP 4.5) and high emissions (RCP 8.5) scenarios along the continental shelf of the Canadian Pacific Coast. To reduce computational cost, an approach for dynamical downscaling of climate projections was developed that uses atmospheric climatologies with augmented winds to simulate historical (1986–2005) and future (2046–2065) periods separately. The two simulations differ in initial and lateral open boundary conditions. For each simulation, the daily climatology of surface winds in the driving model was augmented with high-frequency variability from an atmospheric reanalysis product. The “time-slice” approach was able to reproduce the observed climate state for the historical period. Sensitivity tests confirmed that the high frequency wind variability plays an essential role in freshwater distribution in this region. Projections suggest that sea surface temperature will increase by 1.8–2.4°C and surface salinity will decrease between −0.08 and −0.23 depending on whether a moderate or high emissions scenario is used. Stratification increases throughout the region and there is some evidence of nutrient limitation near the surface. Primary production and phytoplankton productivity (chlorophyll) also increase. Density surfaces are relocated deeper in the water column and this change is mainly driven by surface heating and freshening. Changes in saturation state are mainly due to anthropogenic CO2with minor contributions from solubility, remineralization and advection. There is little difference between RCP 4.5 and RCP 8.5 with regard to projections of deoxygenation and acidification. The depths of the aragonite saturation state and the oxygen minimum zone are projected to become shallower by ≃ 100 and ≃ 75 m respectively. Extreme states of temperature, oxygen and acidification are projected to become more frequent and more extreme, with the frequency of occurrence of[O2]<60 mmolm-3expected to approximately double under either scenario.

Nutrient limitation of plant productivity in scrubby flatwoods: does fire shift nitrogen versus phosphorus limitation?

Differences in the biogeochemistry of nitrogen (N) and phosphorus (P) lead to differential losses and inputs during and over time after fire such that fire may affect nutrient limitation of primary productivity. We conducted a nutrient addition experiment in scrubby flatwoods, a Florida scrub community type, to test the hypothesis that nutrient limitation of primary productivity shifts from N limitation in recently burned sites to P limitation in longer unburned sites. We added three levels of N, P, and N and P together to sites 6 weeks, 8 years, and 20 years postfire and assessed the effects of nutrient addition on above- and belowground productivity and nutrient concentrations. At the community level, nutrient addition did not affect aboveground biomass, but root productivity increased with high N + P addition in sites 8 and 20 years after fire. At the species level, N addition increased leaf biomass of saw palmetto (Serenoa repens) in sites 6 weeks and 20 years postfire, while P addition increased foliar %P and apical shoot growth of scrub oak (Quercus inopina) in sites 8 and 20 years postfire, respectively. Contrary to our hypothesis, nutrient limitation does not appear to shift with time after fire; recently burned sites show little evidence of nutrient limitation, while increased belowground productivity indicates that scrubby flatwoods are co-limited by N and P at intermediate and longer times after fire.

Periphyton response to simulated nonpoint source pollution: local over regional control

We investigated functional and structural responses of periphyton communities to simulated nonpoint source (NPS) pollution over a 2-year period. Periphyton communities were examined in three unique but hydrologically connected habitats in the lower Muskegon River: river, wetland, and lake. Our study addressed several major questions: (1) what are the effects of nonpoint source pollutants on periphyton structure and function, (2) to what degree do environmental conditions, including the pollutant delivery system and hydrologic conditions, affect periphyton response, and (3) do periphyton communities show synchronous responses to nonpoint source pollution? Nutrients were added over a 21-day period in each of the four seasons over 2 years. The effect of road salt on metabolism was evaluated in the winter and spring seasons in the river habitat only. In general, the periphyton community showed complex responses to the NPS amendments, but there was little evidence of nutrient limitation in this system based on community structure, biomass accrual, metabolism, or alkaline phosphatase activity. This result was unexpected given the relatively low ambient concentrations of phosphorus. We discuss the possibilities that nutrient delivery was ineffective or that herbivory and/or irradiance might have constrained periphyton growth, but ultimately conclude that our results suggest that periphyton communities in the lower Muskegon River watershed were (1) nutrient-replete, (2) tolerant of short-term salt additions, and (3) influenced more by local conditions (habitat) than by regional (watershed) factors. In systems where nonpoint source pollutant delivery is diffuse and hydrologic residence time is short, the influence of nonpoint source pollutants on periphyton may be either modest or too difficult to detect using traditional endpoints.

Nutrient addition extends flowering display, which gets tracked by seed predators, but not by their parasitoids

Although phenological matching between two and three trophic interactions has received some attention, it has largely been disregarded in explaining the lack of strong cascade dynamics in terrestrial systems. We studied the response of the specialist seed predator, Paroxyna plantaginis (Tephritidae) and associated generalist parasitoids (Chalcidoidea) to controlled fertilisation of individuals of naturally growing Tripolium vulgare (Asteraceae) on four island populations (Skeppsvik Archipelago, Sweden). We consistently found evidence of nutrient limitation: fertilised plants increased their biomass, produced more capitula (the oviposition units for tephritid flies), were more at risk of attack by the tephritids, and puparia were heavier in fertilised plants. During some parts of the season tephritids became more heavily parasitized, supporting the presence of cascade dynamics, however net parasitism over season decreased in response to nutrient addition. We found no evidence that capitulum size complicated parasitoid access to the tephritids, however the extended bud production prolonged the flowering season. Thus, tephritids utilized the surplus production of capitula throughout the entire season, while parasitoids did not expand their oviposition time window accordingly. Implications for top down regulation and cascade dynamics in the system are discussed.

Spring isothermal mixing in the Great Lakes: evidence of nutrient limitation and nutrient-light interactions in a suboptimal light environment

During the spring isothermal mixing period (April-May) in 1993-1995, photosynthesis-irradiance and growth-irradiance experiments were conducted in Lakes Erie, Huron, Michigan, and Ontario to assess light limitation. Additionally, nutrient enrichment experiments were conducted in Lake Ontario. Results from the photosynthesis-irradiance experiments suggested that phytoplankton communities in all the lakes can be either light limited or light saturated, as the threshold parameter (Ik) was similar to mean water column irradiances (mean Iwc, ratio = 1.0). Growth-irradiance experiments also suggested the potential for light saturation; mean daily irradiance exceeded the threshold growth irradiance (Ik,g) in 95% of cases. Growth rates became light saturated at lower irradiances than photosynthetic rates. Evidence for a nutrient-light interaction in controlling in situ growth rates was also found in the nutrient enrichment experiments at incubation irradiances [Formula: see text] mean Iwc. Our results suggest that an interaction between nutrients and light is often controlling phytoplankton growth during spring mixing in the Great Lakes. The role of these nutrient-light interactions has increased in the past decade due to increased light availability in the lower lakes caused by phosphorus load reductions and the filtering activities of nonindigenous mussels.

Spring isothermal mixing in the Great Lakes: evidence of nutrient limitation and nutrient-light interactions in a suboptimal light environment

Spring isothermal mixing in the Great Lakes: evidence of nutrient limitation and nutrient-light interactions in a suboptimal light environment

The relationship between suspended particulate material, phytoplankton and zooplankton during the retreat of the marginal ice zone in the Bellingshausen Sea

The distribution, abundance and composition of suspended particulates, phytoplankton and zooplankton biomass were investigated for the marginal ice zone in the Bellingshausen Sea during the Austral spring of 1992. Marked changes were observed between the amount and composition of particulates under the sea-ice and those in open waters. Measures of phytoplankton abundance (chlorophyll) ranged from ∼0.05 μg l −1 under the ice to 3 μg l −1 in the open waters to the north. The high nutrient concentrations and low level of phytoplankton under the ice suggest that this region is typical of over-wintering conditions and provides a suitable background comparison to the development of more productive, recently ice-free waters, further north. Nearly all basic measurements of particulate material showed a gradient from south to north. However, the biochemistry of particulates (lipid in particular), a more sensitive measure of environmental growth conditions, showed the area as a whole to be broadly split into two; under ice (light limited) and open water (no light limitation). Total particulate carbon was almost entirely composed of inorganic carbon under the ice; waters away from the ice edge also contained significant levels of inorganic carbon. Hence, C:chlorophyll was estimated from POC or phytoplankton carbon. There was no evidence of nutrient limitation at any of the stations investigated. Integrated phytoplankton carbon for the upper 100 m ranged from 0.1 gC m −2 under the ice to 11.5 gC m 2 in the diatom dominated bloom in open waters. The mesozooplankton biomass in the same depth interval increased from 0.02 gC m −2 under the ice to 0.21 gC m −2 in open waters. However, zooplankton biomass in the upper 600 m changed little from under the ice out to open waters north of the ice edge; although the northernmost station, in the region of highest chlorophyll, had approximately three times more zooplankton carbon (0.67 gC m −2 than the other stations. There is some evidence, however, that the northern station may be associated with a frontal feature, and that conditions observed may not be solely related to previous ice edge processes.

Bioassay of lower Narragansett Bay waters during the 1972‐1973 winter‐spring bloom using the diatom Skeletonema costatum1

Samples collected biweekly from lower Narragansett Bay from December 1972 through March 1973 were bioassayed with Skeletonema costatum at nine different enrichments with nutrients equivalent to f/25‐Tris medium. In early December growth rates were low in all treatments relative to complete enrichment; thereafter there was no evidence of nutrient limitation until late February. During this period of the winter–spring bloom growth was limited by the omission of NO3, SiO3, and PO4; deletion of vitamins and trace metals was less important. The addition of EDTA alone increasingly suppressed growth as the bloom developed; lysis was observed during late March.