David R. Christensen

Newman Lake restoration: A case study. Part I. Chemical and biological responses to phosphorus control

Abstract In the late 1960s and early 1970s, summer and fall blooms of cyanobacteria began to occur in Newman Lake, Washington (zavg: 5.6 m, zmax: 9.0 m); through the next decade, these blooms intensified and became an annual occurrence. Community efforts in the mid-1980s sparked a Restoration Feasibility assessment of the lake and watershed that indicated total annual gross phosphorus loading of at least 3000 kg, with a major portion (∼83%) attributable to internal recycling associated with summer hypolimnetic oxygen depletion. Implementation activities began September 1989, with watershed controls and a whole-lake alum treatment, followed in 1992 by installation of a Speece cone for hypolimnetic oxygenation and in 1997 by addition of a dual-port, microfloc alum injection system. Average summer volume-weighted total phosphorus has declined from prerestoration levels exceeding 50 μg-P/L to an average of 21 μg-P/L over the past 7 years (15–28 μg-P/L). Most notably, peak annual biovolumes of cyanobacteria and their representation within the phytoplankton community have decreased substantially, with increased prevalence of diatoms, green and golden-brown algae. A clearwater phase following spring blooms of diatom and/or golden-brown algae has occurred during those last 7 years, although this phenomenon was observed in the prior three decades. Overall, the restoration has been a success, and lake response to nutrient reduction at Newman Lake is consistent with worldwide observations that emphasize the need for long-term perspectives and commitment in lake restoration and management. Continuation of internal load controls and increased emphasis on external nutrient abatement will be necessary to continue the positive water quality trends despite future development increases and land use changes.

Using stable isotopes and a multiple-source mixing model to evaluate fish dietary niches in a mesotrophic lake

Abstract Christensen, D.R. and B.C. Moore. 2009. Using stable isotopes and a multiple-source mixing model to evaluate fish dietary niches in a mesotrophic lake. Lake Reserv. Manage. 25:167–175. We used stable isotope analysis (SIA) of δ13C and δ15N and a multiple-source mixing model to evaluate dietary niches within the Twin Lakes, Washington, fish community to identify potential for exploitative competition, resource partitioning, and predation. The SIA revealed distinct spatial feeding niches; pelagic species were δ13C depleted, while littoral organisms were δ13C enriched. Trophic feeding niches were identified from δ15N enrichment with each successive trophic level. We found δ15N progression with increasing largemouth bass size, suggesting ontogenetic diet variability. Model results suggested that the smaller bass (i.e., ⩽299 mm length) fed principally on golden shiner, crayfish, and macroinvertebrates while larger bass (⩾300 mm) primarily consumed brook trout and golden shiner. Isotopic signatures for golden shiner were intermediary, indicating both pelagic and littoral feeding sources, including phantom midges (nocturnal and pelagic) and damselflies (littoral) as principal diet sources, suggesting possible horizontal diel migration in golden shiner. Pelagic zooplankton was indicated as the most important food source for rainbow trout. Food resources appeared to be partitioned among the Twin Lakes fish community, possibly limiting competitive interactions. Piscivory in largemouth bass was focused on golden shiner and brook trout, suggesting possible top-down regulation of lower trophic levels. Stable isotope analysis and multiple-source mixing models are useful tools that can improve lake and fishery management decisions by providing data on food web dynamics such as competition, resource partitioning, and predation in lakes.

Diet Composition and Overlap in a Mixed Warm- and Coldwater Fish Community

ABSTRACT In Twin Lakes, Washington, illegal introductions of largemouth bass (Micropterus salmoides) and golden shiner (Notemigonus crysoleucas) are feared to be impacting economically important rainbow trout (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis) populations. We evaluated the stomach contents of 69 golden shiner, 146 rainbow trout, 83 brook trout, and 561 largemouth bass during summer stratification in 2004 and 2005, to determine community diet composition and overlap when food resources were partitioned thermally and spatially. Gut content data revealed some diet overlap but also illustrated distinct resource partitioning among all species but not between salmonids. Rainbow and brook trout had similar pelagic based diets of zooplankton and chaoborids, with high estimated diet overlap. Largemouth bass ≥300 mm was piscivorous and consumed principally golden shiner, with some consumption of rainbow and brook trout during late spring and early fall. Largemouth bass ≤299 mm primarily consumed benthic invertebrates in littoral macrophyte beds. Golden shiner diet contained both littoral and pelagic items, consisting of algae, benthic invertebrates, and zooplankton. Preferential differences in temperature, dissolved oxygen, and habitat as well as species size and ontogeny may all contribute to resource partitioning in the Twin Lakes.

Differential Prey Selectivity of Largemouth Bass Functional Feeding Groups in Twin Lakes, Washington

Abstract Largemouth bass (Micropterus salmoides) can be top-down regulators in a fish community. It is important for fisheries biologists who manage predator-prey populations to understand when bass become piscivorous. We examined the stomach contents of 622 largemouth bass in watershield (Brasenia schreberi) beds in North and South Twin Lakes, Washington. Bass displayed temporal and ontogenetic diet variation within and between lakes. Bass <100 mm fed principally on zooplankton and scuds in June and September and on benthic invertebrates, especially midge and damselfly larvae, during July and August. Damselflies, midges and scuds were major diet constituents for bass 100–199 mm. Bass 200–299 mm consumed large numbers of macroinvertebrates but also fed on crawfish and golden shiner (Notemigonus crysoleucus). Overall, the importance of large prey items like golden shiners and crawfish increased while macroinvertebrate importance to bass diets decreased as the fish grew larger. Bass piscivory was focused on golden shiners and was only observed in larger fish >300 mm. Golden shiner consumption was the lowest during June and September when cannibalism, crawfish and trout consumption increased. Temporal and ontogenetic variability in bass diets is most likely due to habitat variability, fish size, and prey availability and size. Manipulation of bass piscivory through slot-length-limits and/or macrophyte removal could be examined as a potential method for controlling nuisance forage fish such as golden shiners.

Largemouth bass consumption demand on hatchery rainbow trout in two Washington lakes

High mortality (84–89%) of stocked rainbow trout (Oncorhynchus mykiss) in Twin Lakes, Washington, has been partially blamed on predation from non-native largemouth bass (Micropterus salmoides). We examined the gut-content of 434 largemouth bass and applied a bioenergetics model to determine the consumption demand on hatchery-released rainbow trout and other prey species in the Twin Lakes. Largemouth bass consumed approximately 4915 (95% CI; 2393–13,452) fall stocked rainbow trout in South Twin. No rainbow trout consumption was observed in North Twin, despite a similar stocking regime. Approximately 6.3% (95% CI; 3–17%) of total fall stocked rainbow trout in South Twin were consumed by largemouth bass. Rainbow trout stocked in the fall ranged from 100 to 160 mm total length and were all subject to predation by large largemouth bass ⩾300 mm. Large largemouth bass consumed rainbow trout that reached up to 210 mm in length and 58% of bass body length. No predation was observed on larger rainbow trout (215–370 mm) stocked during the spring. Smaller largemouth bass ⩽299 mm consumed primarily invertebrates, including crayfish, and did not consume rainbow trout in either lake. During spring and summer in South Twin Lake, large largemouth bass consumed principally golden shiner (Notemigonus crysoleucas) and crayfish. In North Twin Lake, golden shiner constituted most of the larger bass diet for the entire year. Differences in macrophyte distribution, bathymetry, temperature and/or predator-prey demographics likely influenced variability in largemouth bass predation between lakes. Largemouth bass predation did not account for overall rainbow trout mortality.

Newman Lake restoration: A case study. Part II. Microfloc alum injection

Abstract Microfloc alum injection, in contrast to whole-lake alum treatments, is a relatively new technique for alum delivery that has been implemented in a relatively small number of lakes and reservoirs. Microfloc alum injection is primarily designed to precipitate phosphorus in the water column using low doses applied on a continuous or intermittent basis. The technique employs diffusers to create ultra-small aluminum hydroxide particles with lower settling velocities and longer residence times, compared to whole lake treatments, to strip dissolved phosphorus from the water column. Because relatively few microfloc systems have been deployed, a brief review of their characteristics and performance is provided. In Newman Lake, Washington, a microfloc alum injection system has been in use since spring 1997. We tested microfloc residence times using in situ enclosures; residence time was determined to be at least 10 days, supporting the underlying concept. In the lake itself, turnover events prior to alum injection consistently produced higher algae growth with accompanying lower Secchi transparency. Since alum injection, post-turnover Secchi depths have all improved. In 18 years with 32 monitored turnover events, this relatively robust dataset consistently indicates the short-term ability of microfloc to improve transparency through algae reduction. Volume-weighted total phosphorus concentrations also have generally declined following alum injection. Adverse biological impacts were not observed in either enclosures or the lake. In the enclosure studies, total zooplankton and cladoceran densities and biomass in treatments were not significantly different from controls. Further applications and study of this promising technology are warranted.

Hypolimnetic oxygenation in Twin Lakes, WA. Part I: Distribution and movement of trout

Abstract Summer temperature/dissolved oxygen squeeze conditions have negatively impacted coldwater fisheries, including rainbow (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis), in North and South Twin lakes of northeastern Washington. In 2009, line diffuser hypolimnetic oxygenation (HO) was initiated in North Twin to prevent anoxia and increase summer trout habitat. Suitable trout habitat volume increased from a low of 0% preoxygenation to 100% of the North Twin hypolimnion following HO. To assess if trout utilized the expanded habitat following oxygenation, summer trout movements and habitat utilization were assessed from 2006 through 2012 using various combinations of ultrasonic telemetry, archival tags, gillnetting, and hydroacoustics. Trout utilized North Twin hypolimnetic habitat more frequently compared to pre-HO years and to unoxygenated South Twin, and swim speeds significantly decreased with oxygenation. Chaoborus zooplankton distributions have also changed following HO; water column densities were significantly lower with oxygenation compared to preoxygenation and to unoxygenated South Twin.