Michael A. Quigley

Pontoporeia hoyi—a Direct Trophic Link between Spring Diatoms and Fish in Lake Michigan

Several lines of evidence suggest that the Lake Michigan benthic amphipod, Pontoporeia hoyi (an important fish prey in large, temperate, low-nutrient lakes), may obtain a large portion of its annual energy directly from the spring diatom bloom: 1) Energetic considerations suggest that P. hoyi must assimilate a large fraction of energy from incoming organic material, but that summer input rates are not sufficient to support observed annual production of P. hoyi. 2) The weight-specific lipid content of P. hoyi at some locations in Lake Michigan doubles within a few weeks after the spring diatom bloom. 3) Lipids accumulate in P. hoyi primarily as the storage products, triglycerides. 4) P. hoyi feeds intermittently and can survive for months without food. 5) The dominant spring diatom in Lake Michigan, Melosira, is not significantly cropped by zooplankton and settles rapidly through the water column in the spring and early summer. 6) After the spring diatom bloom, the phytoplankton changes to a dominance of flagellates that are mostly eaten by pelagic zooplankton and therefore largely unavailable to benthic organisms.

Abundance and Biomass of the Meiobenthos in Nearshore Lake Michigan with Comparisons to the Macrobenthos

The meiobenthos of nearshore southeastern Lake Michigan was quantified by taking cores from three depths (11,17, and 23 m) at monthly intervals from May to November 1976–79. Total meiobenthic abundance ranged from 69,700/m2 to 1,300,000/m2 and total biomass ranged from 0.03 to 0.87 g/m2. Nematodes accounted for 80% of all individuals and 66% of the biomass. Most of the major groups peaked in late spring/summer, but some peaked in early spring and fall. With the exception of nematodes, mean annual densities varied from two-fold to twelve-fold at a given station. Harpacticoids, tardigrades, and ostracods tended to be more abundant at the deeper depths, while cyclopoids and cladocerans were less abundant. There was no consistent relationship between sampling depth and the abundance of nematodes and rotifers. Temporal and spatial variation in many of the groups appeared related to changes in the amount of bottom detritus. The overall macrobenthos:meiobenthos biomass ratio was 15:1 and ranged from 5:1 to 45:1 on an annual basis.

Comparative Ecology of Pontoporeia Hoyi Populations in Southern Lake Michigan: The Profundal Region Versus the Slope and Shelf Regions

Abstract This study investigates Pontoporeia hoyi ecology in the profundal region (approximated by a 97-m station) of southern Lake Michigan and compares these results with previous investigations conducted in the slope and shelf regions of the lake. Pontoporeia typically attains its maximum abundance in the slope region, suggesting that this region of the lake is the most favorable for amphipod growth and survival. Profundal and slope P. hoyi exhibited little seasonal variation in mean size while shelf-region populations exhibited strong seasonal variation in mean size. Deepwater sculpins, a major predator on profundal populations of P. hoyi , selectively consumed the largest amphipods: mean size consumed was 6–7 mm. The relative sparsity of larger (> 5 mm) P. hoyi in the profundal, slope, and shelf-regions of the lake may arise from intense size-selective fish predation on this size class of amphipods. Gut content studies revealed that profundal P. hoyi populations feed intermittently. Such feeding behavior was subtly different from that previously observed for slope P. hoyi populations: profundal populations feed more continuously but less intensively than slope populations. Regional differences in feeding behavior may be related to differences in food regime and to predation avoidance strategies. Pontoporeia hoyi apparently is capable of inhabiting a broad range of depth regimes by modifying its physiology (reproductive cycles, generation time) and behavior (feeding, motility) to adjust to spatial variations in temperature, food level, and predation.

Gut Fullness of the Deposit-Feeding Amphipod, Pontoporeia hoyi, in Southeastern Lake Michigan

Pontoporeia hoyi Smith, a deposit-feeding amphipod and a prominent member of the Great Lakes macrobenthic community, was collected at a 45-m-deep site in southeast Lake Michigan during October 1983 and March, May, June, August, and September 1984. Substantial numbers of animals with empty or partially-full guts indicated that P. hoyi feeds intermittently, unlike other continuous-feeding amphipods which routinely maintain uniformly full guts among all members of a population. The incidence of animals with empty, partially-full, and full guts also varied significantly with animal size and sampling date and implied that aspects of the amphipods life history, and seasonality, were primarily responsible for changes in feeding rates. Overall, P. hoyi gut fullness was greatest in spring and, secondarily, in autumn, reflecting an opportunistic feeding strategy that exploited newly-deposited detritus generated from spring and autumn phytoplankton blooms. This strategy, in association with P. hoyis known capacity to store energy as lipids may largely explain P. hoyis exclusive and widespread distribution in profundal areas of the upper Great Lakes, compared to other amphipods.

Ingestion of live filamentous diatoms by the Great Lakes amphipod, Diporeia sp.: a case study of the limited value of gut contents analysis

Individuals of the Great Lakes amphipod, Diporeia sp. (formerly named Pontoporeia hoyi) were collected from a 45-m deep station in southeastern Lake Michigan and isolated in small laboratory feeding vessels at 4 °C, after the animals had voided their guts over a 24-hour period. Over a 20-day period, following introduction of a single ration of live cells of the filamentous diatom, Melosira varians, 9 of 10 animals had ingested this material, and 7 of these 9 individuals had deposited fecal pellets. Subsequent examination of gut contents and fecal pellets showed that although animals had ingested whole algal cells/filaments, little of the material in gut contents or fecal pellets bore any identifiable structural similarity to cells/filaments prior to ingestion. The results suggest that earlier studies of pontoporeiid gut contents may seriously underestimate the importance of algal components in the amphipods diet and imply that Diporeia sp. growth and production may be more closely linked to primary production than previously thought.

Lipid Composition Related to Size and Maturity of the Amphipod Pontoporeia Hoyi

Micro-gravimetric determination of the lipid content of amphipods (Pontoporeia hoyi) obtained from a 45-m-deep Lake Michigan sampling site indicated that the mean lipid content of adult females was 30% on a non-lipid dry weight (NLDW) basis, and that juveniles and adult males contained 21 and 10% lipid (NLDW basis), respectively. Thin layer chromatography-flame ionization detection (TLC-FID) analyses revealed that lipids of females were composed primarily of triacylglycerols (81%), the principal energy storage lipid of amphipods. Lipids of juveniles were composed largely of triacylglycerols (41%) and phospholipids (44%). Adult male P. hoyi lipids consisted mostly of phospholipids (64%) and, secondarily, of triacylglycerols (12%). The relatively low triacylglycerol concentrations in males may be associated with the minimal requirements for energy stores to support metabolic needs during the males brief (10 day) life span. By contrast, the high lipid content and marked abundance of triacylglycerols in adult females represents an important energy store supporting subsequent egg development, particularly since females appear to halt all feeding upon maturation. In juvenile P. hoyi, increased individual size (NLDW) was accompanied by increased lipid dry weight, implying that juveniles accumulate lipids during growth. Overall, the results demonstrated the importance of considering P. hoyi size, life stage, and sex when describing a populations lipid content or composition. This consideration is particularly critical when evaluating the role of P. hoyi in the transfer of energy and/or organic contaminants within the Great Lakes food web.

Measurement of amphipod body length using a digitizer

A digitizer/camera lucida method was developed to measure total body length and gut contents of an amphipod Pontoporeia hoyi. The method was more accurate and precise than two conventional methods (the forceps/ocular micrometer method, and the map wheel/camera lucida method). The digitizer/camera lucida method also provided rapid and direct transfer of body and gut length values to a computer file for subsequent analysis.

Sampling Efficiency of the Ponar Grab in Two Different Benthic Environments

Numbers of benthic organisms collected with the Ponar grab were compared to numbers in diver-collected cores in a nearshore, sandy habitat in Lake Michigan, and to numbers in box cores taken with a manned submersible in a deep, silty habitat in Lake Superior. The Ponar underestimated benthic abundances at both sampling sites. Ordered from most to least efficiently sampled were sphaeriids, Pontoporeia, oligochaetes, and chironomids; overall mean abundances in core samples were 1.5, 1.7, 3.4, and 11.3 times greater than abundances in Ponar samples for the four groups. The extent by which abundances were underestimated was remarkably similar at the two sampling locations. This would indicate that underestimates are consistent, thus allowing appropriate correction factors to be applied if absolute abundances are required for a particular study. Total biomass in the box core samples was 1.7 times greater than in the Ponar samples.

Silica Regeneration Processes in Nearshore Southern Lake Michigan

The seasonal depletion of dissolved silica to levels that limit diatom production is particularly critical in Lake Michigans nearshore zone where diatom biomass is greatest, and where silica regeneration from sediments is not well-understood. In our study, intact, medium-fine sand cores, collected from an 11 m deep site in nearshore Lake Michigan during July-August 1980, released soluble reactive silica (SRS) at a mean rate of 2,707 ± 122 (SE) μg Si cm−2 yr−1 when incubated in darkness and at 12° C. This measured SRS release was greater than a diffusive flux (270 ± 49 (90% C.I.) μg Si cm−2 yr−1) estimated from SRS pore water profiles and physical sediment properties. SRS release from individual cores was not correlated with abundance of most macroinvertebrates (chironomids, pisidiid clams, or oligochaetes). However, a significant (P < 0.05) and inverse relationship between SRS release and Pontoporeia hoyi densities implied that amphipods suppressed SRS release through mixing and burial of a surficial floe layer, where most dissolution of biogenic silica occurs. Moreover, SRS release rates measured from our coarse-grained nearshore sediments were comparable to rates reported for fine-grained offshore material and further implicate dissolution of surficial biogenic silica as the source of remineralized SRS. Because nearshore areas of Lake Michigan undergo strong seasonal variations in temperature and diatom production, and because significant riverine silica inputs exist, we cannot extrapolate our results on a lakewide, or season-long basis. The data, however, strongly imply that nearshore sediments are an important participant in the Lake Michigan silica cycle.

Distribution of Photosynthetic Pigments in Nearshore Sediments of Lake Michigan

Abstract To characterize nearshore detrital deposits and to identify potential input sources, photosynthetic pigments were examined in sediment cores taken from three stations in nearshore Lake Michigan between May and October 1979. Two stations were located at 11 m and one was at 23-m depth. At the 11-m stations, total pigment concentrations in the upper 0–1 cm layer were greatest in May and then declined, while seasonal changes at the 23-m station were not apparent. A high proportion of the spring influx of pigment at the 11-m depth was undegraded, indicating freshly settled material from the water column as a likely source. Sediment chlorophyll concentrations in the spring were as high as 212 μg/g, but typical summer values were 1–7 μ/g. Although these pelagic inputs are temporary, strong links exist between this material and benthic invertebrate distributions.