Copepod grazing during spring blooms: Does Calanus pacificus avoid harmful diatoms?

During late winter and spring of 2002 and 2003, 24, 2–3 day cruises were conducted to Dabob Bay, Washington State, USA, to examine the grazing, egg production, and hatching success rates of adult female Calanus pacificus and Pseudocalanus newmani. The results of the copepod grazing experiments for C. pacificus are discussed here. Each week, copepod grazing incubation experiments from two different depth layers were conducted. Grazing was measured by both changes in chlorophyll concentration and cell counts. In 2002, there was one moderate bloom consisting mainly of Thalassiosira spp. in early February, and a larger bloom in April comprised of two Chaetoceros species and Phaeocystis sp. Similarly, in 2003, there were two blooms, an early one dominated by Thalassiosira spp., and a later one consisting of Chaetoceros spp. and Thalassiosira spp. Clearance rates on individual prey species, as calculated by cell counts, showed that C. pacificus are highly selective in their feeding, and may have much higher clearance rates on individual taxa than rates calculated from bulk chlorophyll disappearance. During weeks of high phytoplankton concentration, the copepods generally ate phytoplankton. However, they often rejected the most abundant phytoplankton species, particularly certain Thalassiosira spp., even though the rejected prey were often of the same genus and similar size to the preferred prey. It is speculated that this avoidance may be related to the possible deleterious effects that certain of these diatom species have on the reproductive success of these copepods. During weeks of medium to low phytoplankton concentration, the copepods selectively ate certain species of phytoplankton, and often had high electivity for microzooplankton. The selection mechanism must consist of active particle rejection most likely based on detection of surface chemical properties, since the diatoms that were selected were of the same genus, nearly the same size, and at lower numerical abundance than those cells that were avoided. The grazing choices made by these copepods may have important consequences for the overall ecosystem function within coastal and estuarine systems through changes in the transfer efficiency of energy to higher trophic levels.     

The Alleret maar (Massif Central, France): A new lacustrine sequence of the early Middle Pleistocene in western Europe

The Alleret maar (Massif Central, France) provides a long lacustrine sequence (40.6 m) attributed to the early Middle Pleistocene. Sediment, pollen and diatoms analysis of its upper part (AL2 core, 14.6 m) indicates two temperate phases marked by high lake levels, forest development and vegetation expansion. They are separated by a cold period during which lake level drops, coarse sediment input increases and steppic and xerophilous plants develop. Pollen data suggests that this sequence belongs to the upper part of the Cromerian complex. These results are in agreement with the 557 ± 3 ka (±12 ka, including all errors) ⁴⁰Ar/³⁹Ar age obtained from an interbedded tephra layer emitted by the Mont-Dore/Sancy strato-volcano and establish that this sequence probably covers the MIS 15 substages.     

Diatom-inferred lake level from near-shore cores in a drainage lake from the Experimental Lakes Area,northwestern Ontario,Canada

We inferred late Holocene lake-level changes from a suite of near-shore gravity cores collected in Lake 239 (Rawson Lake), a headwater lake in the Experimental Lakes Area, northwestern Ontario. Results were reproduced across all cores. A gravity core from the deep central basin was very similar to the near-shore cores with respect to trends in the percent abundance of the dominant diatom taxon, Cylcotella stelligera. The central basin, however, does not provide a sensitive site for reconstruction of lake-level changes because of the insensitivity of the diatom model at very high percentages of C. stelligera and other planktonic taxa. Quantitative estimates of lake level are based on a diatom-inferred depth model that was developed from surficial sediments collected along several depth transects in Lake 239. The lake-level reconstructions during the past ~3,000 years indicate that lake depth varied on average by ±2 m from present-day conditions, with maximum rises of ~3–4 m and maximum declines of ~3.5–5 m. The diatom-inferred depth record indicates several periods of persistent low levels during the nineteenth century, from ~900 to 1100 AD, and for extended periods prior to ~1,500 years ago. Periods of inferred high lake levels occurred from ~500 to 900 AD and ~1100 to 1650 AD. Our findings suggest that near-shore sediments from small drainage lakes in humid climates can be used to assess long-term fluctuations in lake level and water availability.     

Response of a Lake Michigan coastal lake to anthropogenic catchment disturbance

A paleolimnological investigation of post-European sediments in a Lake Michigan coastal lake was used to examine the response of Lower Herring Lake to anthropogenic impacts and its role as a processor of watershed inputs. We also compare the timing of this response with that of Lake Michigan to examine the role of marginal lakes as early warning indicators of potential changes in the larger connected system and their role in buffering Lake Michigan against anthropogenic changes through biotic interactions and material trapping. Sediment geochemistry, siliceous microfossils and nutrient-related morphological changes in diatoms, identified three major trophic periods in the recent history of the lake. During deforestation and early settlement (pre-1845–1920), lake response to catchment disturbances results in localized increases in diatom abundances with minor changes in existing communities. In this early phase of disturbance, Lower Herring Lake acts as a sediment sink and a biological processor of nutrient inputs. During low-lake levels of the 1930s, the lake goes through a transitional period characterized by increased primary productivity and a major shift in diatom communities. Post-World War II (late 1940s–1989) anthropogenic disturbances push Lower Herring Lake to a new state and a permanent change in diatom community structure dominated by Cyclotella comensis. The dominance of planktonic summer diatom species associated with the deep chlorophyll maximum (DCM) is attributed to epilimnetic nutrient depletion. Declining Si:P ratios are inferred from increased sediment storage of biogenic silica and morphological changes in the silica content of Aulacoseira ambigua and Stephanodiscus niagarae. Beginning in the late 1940s, Lower Herring Lake functions as a biogeochemical processor of catchment inputs and a carbon, nutrient and silica sink. Microfossil response to increased nutrients and increased storage of biogenic silica in Lower Herring Lake and other regional embayments occur approximately 20–25 years earlier than in a nearby Lake Michigan site. Results from this study provide evidence for the role of marginal lakes and bays as nutrient buffering systems, delaying the impact of anthropogenic activities on the larger Lake Michigan system.     

The use of sedimentary algal pigments to infer historic algal communities in Lake Apopka,Florida

The primary producer community of Lake Apopka, a large (125 km²), shallow (mean depth, 1.7 m), polymictic Florida lake, shifted from macrophyte dominance to phytoplankton dominance in the 1940s. Today, frequent wind resuspension of highly organic, unconsolidated sediments supports a meroplanktonic community that is predominantly diatoms, but during calm periods the algal community is dominated by planktonic cyanobacteria. Sedimentary algal pigments (chlorophyll derivatives and carotenoids) and chemical proxies for nutrient enrichment (polyphosphate, total phosphorus and biogenic silica) in three sediment cores were used to investigate historic changes in primary producers. Sediments were separated into three stratigraphic zones using multivariate statistical techniques. Stratigraphic zonation was established in each core although sediment deposition at one site was insufficient to adequately resolve temporal changes. These results show the importance of selecting suitable sites for paleolimnological studies. The oldest zone represents macrophyte-derived sediments, and the two overlying zones represent phytoplankton-derived sediments deposited since the 1940s. Algal pigments in the most recent sediment zone show little degradation, which might be due to the presence of viable meroplankton in the sediment. After the initial primary producer shift from macrophytes to phytoplankton, the lake experienced a short period of cyanobacterial dominance followed by a period of benthic diatom abundance before being replaced by the present algal community consisting of cyanobacteria and meroplanktonic diatoms. Chlorophyll derivatives and carotenoids were highly correlated with total phosphorus. Historic trends inferred from the data include algal and cyanobacterial productivity that increased with increased phosphorus loading. The study demonstrates that valid paleolimnological proxies for historic eutrophication are available in loosely consolidated sediments of shallow, subtropical lakes.     

Sedimentary diatoms along a temporal and spatial gradient of metal contamination

We examined changes at the community and population level of sedimentary diatoms over a wide temporal and spatial gradient of metal pollution encountered in cores from three lakes in the Abitibi mining region (Québec, Canada). Diatom communities on the whole appeared to be very tolerant of metal contamination, as shown by diatom cell accumulation rates decreasing only under the most severe conditions of contamination, which were found from the 1930s to the 1980s in Lac Dufault (cadmium, up to 94 μg/g dry sediment; Cu, up to 8600 μg/g; Zn, up to 9000 μg/g). Under the moderate conditions of contamination observed in the other two lakes and in the most recent sediment of Lac Dufault, diatom cell accumulation rates tended to increase over values typical of the pre-mining period. However, there were increasing rearrangements of the community composition along the contamination gradient. Under moderate metal enrichment, the diatom community of Lac Vaudray experienced only subtle changes, with Cyclotella stelligera, albeit decreasing, remaining the dominant taxon. In the intermediately contaminated Lac Caron, several benthic taxa, noticeably Cymbella silesiaca and several Fragilaria species, rose in taxonomic importance. The most extreme contamination observed in Lac Dufault led to a severely impoverished community almost entirely represented by Achnanthes minutissima and Brachysira vitrea. With increasing levels of contamination, there was a shift from planktonic to benthic taxa and morphotypes and a consistent decrease in the siliceous stomatocysts/diatom frustules ratio. These trends suggest that littoral zones may represent an important refugium under conditions of high contamination. Responses to metal stress were in general more evident at the population than at the community level. Cyclotella stelligera and B. vitrea had a consistent negative and positive response, respectively, along the gradient and are the most promising indicators of metal pollution for this region.     

Multi-proxy evidence of postglacial climate and environmental change at Two Frog Lake,central mainland coast of British Columbia,Canada

Pollen and diatoms preserved in the radiocarbon dated sediments of Two Frog Lake in the Seymour-Belize Inlet Complex of the central mainland coast of British Columbia document postglacial climate change. Two Frog Lake was isolated from the sea prior to 11,040 ± 50 yr BP (13,030 cal. yr BP) when the climate was cool and dry, and open Pinus contorta woodlands covered the landscape. These woodlands were replaced by a mixed conifer forest ca. 10,200 yr BP (ca. 12,300 cal. yr BP) when the climate became moister. A relatively dry and warm early Holocene climate allowed Pseudotsuga menziesii to migrate northward to this site where it grew with Picea, Tsuga heterophylla and Alnus. The climate became cooler and moister at ca. 8,000 yr BP (ca. 9,200 cal. yr BP), approximately 500–1,000 years prior to sites located south of Two Frog Lake and on the Queen Charlotte Islands, but contemporary with sites on the northern mainland coast of British Columbia and south coastal Alaska. Climate heterogeneity in central coastal British Columbia appears to have occurred on a synoptic scale, suggesting that atmospheric dynamics linked to a variable Aleutian Low pressure system may have had an important influence on early Holocene climate change in the Seymour-Belize Inlet Complex. The transition to cooler and moister conditions facilitated the expansion of Cupressaceae and the establishment of a modern-type coastal temperate rainforest dominated by Cupressaceae and T. heterophylla. This was associated with progressive lake acidification. Diatom changes independent of vegetation change during the late Holocene are correlative with the mid-Neoglacial period, when cooler temperatures altered diatom communities.     

Cladoceran stratigraphy in two shallow brackish lakes with special reference to changes in salinity, macrophyte abundance and fish predation

Sub-fossils of Cladocera and Foraminifera were used to reconstruct changes since 1870 in the trophic dynamics of two brackish lakes, Glombak and Han Vejle, located in the Vejlerne nature reserve, Denmark, a site of international conservation importance. After creation of the lakes in the mid-1870s following land reclamation, the two lakes have developed quite differently; today Glombak is turbid, while Han Vejle is clear. In both lakes, stratigraphic changes in the assemblages of foraminifers and cladocerans indicate an abrupt shift from marine to brackish conditions at the end of the 19th century, coinciding with land reclamation. However, the composition of the fossil invertebrate assemblages in the 20th century implies differences in the exposure to salinity, in fish predation and in habitat diversity. In Glombak, the cladoceran record suggests relatively saline conditions in the first quarter of the last century and high macrophyte abundance followed by lower salinities and subsequently a major decrease in macrophyte abundance and an increase in fish predation during the past ca. 40 years. By contrast, in Han Vejle low salinity, high abundance of macrophytes and only minor changes in fish predation seem to have prevailed throughout most of the 20th century. The results are consistent with recent contemporary data, the few historical records, as well as with trends in the records of diatoms and macrofossils. This study highlights the potential of using crustacean remains as indicators of long-term changes in the trophic dynamics of brackish lakes.     

Palaeolimnological evidence for submerged plant loss in a floodplain lake associated with accelerated catchment soil erosion (Murray River,Australia)

Shallow lakes have been described as existing in two alternative equilibrium states, dominated by either submerged plants or phytoplankton. Causes of, often catastrophic, shifts between these states have been widely debated but may often result from displacement of the dominant community by stochastic influence. In Australian cut-off river meanders (also known as ‘billabongs’), anecdotal and palaeolimnological evidence suggests widespread loss of aquatic macrophytes since European occupation of the region c. post-1800. Our detailed and high-resolution stratigraphic study of a sediment core from Hogan’s Billabong (Murray River, Australia) seeks to identify the causes of the loss of aquatic macrophytes. Little direct evidence of the past extent and composition of submerged macrophyte communities was recovered. Nevertheless, results derived from other sediment proxies, including declines in the abundance of epiphytic diatoms and in plant-associated invertebrates, provide further indirect evidence of macrophyte disappearance. Despite limitations with radiometric dating, the sequence of events in the derived record suggests that a period of high abiotic turbidity, leading to a critical reduction in water transparency and caused by widespread erosion during the late 19th century, is the most likely factor contributing to loss of submerged vegetation from this billabong.     

Twentieth century eutrophication of the St. Croix River (Minnesota–Wisconsin,USA) reconstructed from the sediments of its natural impoundment

Evaluation of land-use effects on coastal and marine ecosystems requires better understanding of the role of rivers in regulating mass transport from terrestrial to oceanic environments. Here we take advantage of the presence of a riverine lake to use paleoecological techniques to quantify impacts of logging, European-style agriculture, urbanization and continued terrestrial disturbance on mass transport and water quality in the northern drainage of the Mississippi River. Two 2-m sediment-cores recovered in 1999 from Lake St. Croix, a natural impoundment of the St. Croix River, were dated using ²¹⁰Pb and ¹³⁷Cs, and analyzed for historical changes (c. 1840–present) in sediment magnetic susceptibility, inorganic and organic matter content, biogenic silica, fossil pigments, and diatom microfossils. Inorganic sediment accumulation increased threefold between the mid-1800s and present, whereas clear signs of eutrophication were only evident after the mid-twentieth century when biogenic silica accumulation increased sixfold, diatom accumulation rates increased 20- to 50- fold, and the diatom community shifted from predominantly benthic species to assemblages composed mainly of planktonic taxa. Similarly, fossil pigment concentrations increased during the 1960s, and diatom-inferred total phosphorus (DI-TP) increased from ~30 μg TP l⁻¹ c. 1910 to ~60 μg l⁻¹ since 1990, similar to historical records since 1980. Together, these patterns demonstrate that initial land clearance did not result in substantive declines in water quality or nutrient mass transport, instead, substantial degradation of downstream environments was restricted to the latter half of the twentieth century. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.