Robert S. Stelzer

Can't See the Forest for the Stream? In-stream Processing and Terrestrial Nitrogen Exports

Abstract There has been a long-term decline in nitrate (NO3−) concentration and export from several long-term monitoring watersheds in New England that cannot be explained by current terrestrial ecosystem models. A number of potential causes for this nitrogen (N) decline have been suggested, including changes in atmospheric chemistry, insect outbreaks, soil frost, and interannual climate fluctuations. In-stream removal of NO3− has not been included in current attempts to explain this regional decline in watershed NO3− export, yet streams may have high removal rates of NO3−. We make use of 40 years of data on watershed N export and stream N biogeochemistry from the Hubbard Brook Experimental Forest (HBEF) to determine (a) whether there have been changes in HBEF stream N cycling over the last four decades and (b) whether these changes are of sufficient magnitude to help explain a substantial proportion of the unexplained regional decline in NO3− export. Examining how the tempos and modes of change are distinct for upland forest and stream ecosystems is a necessary step for improving predictions of watershed exports.

ROSEMARY MACKAY FUND ARTICLE: Ecological stoichiometry of trophic interactions in the benthos: understanding the role of C:N:P ratios in lentic and lotic habitats

This paper considers how the theory of ecological stoichiometry may be applied to issues of importance to benthic ecologists. Ecological stoichiometry considers both the causes of elemental (C:N:P) imbalances between trophic levels and their consequences on foodweb dynamics (e.g., predator–prey interactions) and ecosystem processes (e.g., nutrient cycling). Elemental imbalances are created between consumers and their food, in part, by the accumulation of C relative to other nutrients (N and P) in benthic organic matter as a result of the deposition of detritus and/or unbalanced growth in aquatic producers. High C:N and C:P ratios in food material can reduce growth and reproduction and alter related processes such as nutrient release in benthic consumers. By affecting consumer metabolism, elemental imbalances may affect population dynamics, trophic interactions, and gross transfer efficiencies in benthic systems. Future work is needed to quantify the frequency and magnitude of elemental imbalances, to determine why elemental ratios differ within and among trophic levels, and to examine how stoichiometric imbalances affect fundamental ecosystem processes (e.g., nutrient cycling and spiraling, consumer growth dynamics, and responses to environmental disturbance) in benthic systems.

Ecological stoichiometry in running waters: Periphyton chemical composition and snail growth

Mismatches between the elemental composition of primary consumers and their food suggest that the production of primary consumers may be limited by food mineral content. Although the consequences of these mismatches for consumers have been studied extensively for zooplankton and terrestrial herbivores, the relationship between periphyton elemental composition and primary consumer performance has been rarely explored. We manipulated periphyton elemental composition by growing stream periphyton under all combinations (2 × 2 factorial) of low and high inorganic N and P and offered low and high quantities to Elimia livescens snails. Periphyton quantity, as measured by ash-free dry mass and total carbon, was kept the same among nutrient treatments within a level of food quantity. We measured Elimia growth as change in blotted wet mass and shell-free ash-free dry mass. Periphyton chemical composition was described by measurements of percentages of C, N, P, ash and chlorophyll a. At low periphyton quantity, Elim...

Particulate organic matter quality influences nitrate retention and denitrification in stream sediments: evidence from a carbon burial experiment

Organic carbon supply is linked to nitrogen transformation in ecosystems. However, the role of organic carbon quality in nitrogen processing is not as well understood. We determined how the quality of particulate organic carbon (POC) influenced nitrogen transformation in stream sediments by burying identical quantities of varying quality POC (northern red oak (Quercus rubra) leaves, red maple (Acer rubrum) leaves, red maple wood) in stream mesocosms and measuring the effects on nitrogen retention and denitrification compared to a control of combusted sand. We also determined how POC quality affected the quantity and quality of dissolved organic carbon (DOC) and dissolved oxygen concentration in groundwater. Nitrate and total dissolved nitrogen (TDN) retention were assessed by comparing solute concentrations and fluxes along groundwater flow paths in the mesocosms. Denitrification was measured by in situ changes in N2 concentrations (using MIMS) and by acetylene block incubations. POC quality was measured by C:N and lignin:N ratios and DOC quality was assessed by fluorescence excitation emission matrix spectroscopy. POC quality had strong effects on nitrogen processing. Leaf treatments had much higher nitrate retention, TDN retention and denitrification rates than the wood and control treatments and red maple leaf burial resulted in higher nitrate and TDN retention rates than burial of red oak leaves. Leaf, but not wood, burial drove pore water to severe hypoxia and leaf treatments had higher DOC production and different DOC chemical composition than the wood and control treatments. We think that POC quality affected nitrogen processing in the sediments by influencing the quantity and quality of DOC and redox conditions. Our results suggest that the type of organic carbon inputs can affect the rates of nitrogen transformation in stream ecosystems.

Independent and interactive effects of crayfish and darters on a stream benthic community

A growing body of research suggests that direct and indirect interactions among consumers can have strong effects on the abundance of their prey. We examined the independent and interactive effects of 2 common freshwater consumers, crayfish (Orconectes propinquus) and rainbow darters (Etheostoma caeruleum), on benthic macroinvertebrates in a southern Michigan, USA, stream. A secondary goal was to assess the effects of crayfish and darters on the basal trophic level (periphyton) in the system. Presence and absence of crayfish and darters were manipulated in situ in wire mesh cages (0.2 m2, 6-mm mesh) using a 2 × 2 factorial, randomized block design. Invertebrates and periphyton (for chlorophyll a analysis) were sampled biweekly for 6 wk from artificial rock substrates within these cages. Neither consumer significantly affected chlorophyll a accumulation. Darters alone had no significant effects on total invertebrate abundance. Crayfish alone caused, on average, a 58% reduction in total invertebrate abundance relative to exclosures, and most common invertebrate taxa were negatively affected. In the presence of darters, however, crayfish reduced invertebrate numbers by only 29% relative to exclosures. Daters apparently modified the ability of crayfish to suppress invertebrate abundance, although the mechanism is unclear. Evaluations of community structure based on manipulations of individual consumer species may be misleading when interaction modifications such as these occur.

Carbon sources for lake sturgeon in Lake Winnebago, Wisconsin.

Abstract Because lake sturgeon Acipenser fulvescens, like other sturgeon species, are threatened or endangered in many aquatic ecosystems, it is imperative that we increase our understanding of their role in food webs. Our main objective was to determine the carbon sources for lake sturgeon in Lake Winnebago, Wisconsin, which contains one of the largest populations of lake sturgeon in North America. Gut content analysis revealed that gizzard shad Dorosoma cepedianum and Chironomus plumosus larvae (56% and 33% by gut content mass, respectively) were the primary prey items for lake sturgeon in the winter. Larger lake sturgeon were more piscivorous than smaller individuals. A mixing model using δ13C and δ15N suggests that Chironomus contributes 49% and gizzard shad 37% to the carbon assimilated by lake sturgeon. We estimated the carbon half-life in lake sturgeon to be about 0.6-3.0 years based on a model incorporating metabolism and growth. Thus, the stable isotope results integrate over a considerably longe...

Effects of deposited fine sediment on life history traits of Physa integra snails

Fine sediment loading is one of the leading causes of ecological impairment to rivers and streams in the United States. However, few studies have addressed how deposited silt affects the life history traits of benthic macroinvertebrates. We determined how deposited silt affects somatic growth, behavior, and egestion rates of Physa integra snails from Emmons Creek, WI by exposing snails to 3 levels of silt (ambient, low, and high) in stream-side once-through flumes and recirculating chambers. Deposited silt did not affect somatic growth rates of snails. Snails that received added silt spent less time on algal-covered tiles than those not receiving silt. Elemental analysis of periphyton revealed higher %C, C:N, and C:P for the silt addition treatments. Per capita egestion rates were higher for the treatments with added silt. Snails exposed to increased deposited silt may have increased ingestion rates to compensate for the reduced food quality after silt addition. Results such as these about how fine sediment affects life history traits of primary consumers can aid in the interpretation of how fine sediment affects aquatic ecosystems.

Buried particulate organic carbon stimulates denitrification and nitrate retention in stream sediments at the groundwater–surface water interface

The interface between ground water and surface water in streams is a hotspot for N processing. However, the role of buried organic C in N transformation at this interface is not well understood, and inferences have been based largely on descriptive studies. Our main objective was to determine how buried particulate organic C (POC) affected denitrification and NO3− retention in the sediments of an upwelling reach in a sand-plains stream in Wisconsin. We manipulated POC in mesocosms inserted in the sediments. Treatments included low and high quantities of conditioned red maple leaves (buried beneath combusted sand), ambient sediment (sand containing background levels of POC), and a control (combusted sand). We measured denitrification rates in sediments by acetylene-block assays in the laboratory and by changes in N2 concentrations in the field using membrane inlet mass spectrometry. We measured NO3−, NH4+, and dissolved organic N (DON) retention as changes in concentrations and fluxes along groundwater flow paths in the mesocosms. POC addition drove oxic ground water to severe hypoxia, led to large increases in dissolved organic C (DOC), and strongly increased denitrification rates and N (NO3− and total dissolved N) retention relative to the control. In situ denitrification accounted for 30 to 60% of NO3− retention. Our results suggest that buried POC stimulated denitrification and NO3− retention by producing DOC and by creating favorable redox conditions for denitrification.

Secondary production of Chironomidae in a large eutrophic lake: implications for lake sturgeon production

Abstract.  Most foodweb research in lentic systems has focused on pelagic primary (phytoplankton) and secondary (zooplankton) production as the primary energy sources for higher trophic-level production. Recent research has demonstrated that secondary production of benthic primary consumers can affect pelagic fish production and foodweb structure in lakes. We used the instantaneous growth method to calculate secondary production of chironomids in Lake Winnebago, Wisconsin (USA), where previous research has shown that lake sturgeon (Acipenser fulvescens) rely heavily on the benthos (chironomids) as a food source. We also used literature-derived data and the instantaneous growth method to calculate annual production of lake sturgeon to test whether chironomid production is sufficient to support the current lake sturgeon population in Lake Winnebago. We collected benthic samples with an Ekman grab at 4 profundal sites during ice-free conditions on 11 dates from spring 2008 through spring 2009. We measured instantaneous growth rates for 7 chironomid length classes at 5 thermal regimes in the laboratory. Mean annual density of Chironomidae was 2714 individuals/m2, mean biomass was 2.75 g dry mass (DM)/m2, and mean annual production of Chironomidae was 7.59 g DM m−2 y−1. Estimated annual production of lake sturgeon was 0.02 g DM m−2 y−1 in 2007. We concluded that in 2008–2009 chironomid secondary production was sufficient to support the lake sturgeon population in Lake Winnebago. The annual production rates for chironomids in Lake Winnebago are higher than rates in many other lakes in North America, presumably because of the eutrophic condition of Lake Winnebago.

Seasonal variation of phosphorus in precipitation at Hubbard Brook Experimental Forest

Nitrogen and sulfur are important components of atmospheric deposition and have received considerable attention in the last three decades (e.g. LIKENS & BORMANN 1995). Other elements, such as phosphorus (P), generally comprise a smaller fraction of atmospheric deposition and have been measured less frequendy. Atmospheric deposition of P can be an important source of nutrients to undisturbed aquatic ecosystems (MIGON & SANORONI 2000), and in ecosystems where P availability is low, seasonal variation in P inputs could potentially comribute to temporal variation in populations of biota or ecosystem processes that are P limited. Atmospheric deposition of P to ecosystems can occur in wet deposition and dry fallout. Forms of phosphorus in atmospheric deposition indude soi! partides, aerosols from sea spray, fine leaf fragments, pollen, fungal spores and microorganisms (GRAHAM & DucE 1979, NEWMAN 1995). Phosphorus deposition to a particular ecosystem can originate from local, regional and global sources. Heavy partides in atmospheric deposition (e.g. large soi! partides or leaf fragments) tend to originate locally, as they are not transported far in the atmosphere. Lighter partides (e.g. fine dust) can travel hundreds or thousands ofkilometers in air masses (e.g. AVILA et al. 1998). Phosphorus in deposition is usually measured in collectors that are placed at various locations across the ecosysrem being studied. In forest ecosystems, collectors are best placed in a large dearing so that inputs from local sources of phosphorus (e.g. pollen from local trees) can be minimized. Bulk deposition collectors (LIKENS et al. 1967) measure P in wet and dry deposition. Wet-only collectors are designed to measure P in wet deposition only (GALLOWAY & LIKENS 1976). The objectives of this study were to: (l) describe seasonal variation in the orthophosphate concentration in bulk deposition in the Hubbard Brook Experimental Forest (HBEF) during a 13-year period, (2) compare the P concentration between bulk and wet-only collectors to gain some insight into the relative contributions of dry and wet-only inputs, and (3) compare seasonal variation of P in bulk deposition to that of stream water from an experimemal watershed at HBER