Eric P. McElravy

Year-to-Year Variation in the Aquatic Macroinvertebrate Fauna of a Northern California Stream

The benthic macroinvertebrate community of a third-order coastal stream in northern California was examined in mid May (end of the wet season) and late August (near the end of the dry season in the prevailing Mediterranean climate of the region) over a 7-yr period in which there was substantial year-to-year variability in precipitation and, consequently, stream discharge. Ephemeroptera, Trichoptera, and Diptera were the dominant components of the macrobenthic community, accounting for 93% of total individuals and 62% of the 81 taxa collected in mid May, and 96% of total individuals and 64% of the 69 taxa collected in late August. In mid May, significant reductions in species richness and Simpsons diversity were observed during a year of extreme drought and in years with above-average wet season rainfall. Macroinvertebrate density decreased and relative abundance of Chironomidae increased as wet season rainfall increased. In late August, year-to-year variability in community parameters measured was substantially reduced. Drought conditions favored proliferation of a few tolerant taxa (e.g., the caddisfly Gumaga nigricula). An understanding of the influence of abiotic conditions on biotic patterns can be useful in separating the effects of perturbation from natural variability.

Long-term macroinvertebrate responses to climate change: implications for biological assessment in mediterranean-climate streams

Abstract Climate change is expected to have strong effects on mediterranean-climate regions worldwide. In some areas, these effects will include increases in temperature and decreases in rainfall, which could have important implications for biological assessment programs of aquatic ecosystems. We used a consistently collected, 20-y benthic macroinvertebrate data set from 4 sites along 2 small northern California streams to examine potential climate-change effects on aquatic communities. The sites represented unique combinations of stream order and flow intermittency. The North Coast benthic macroinvertebrate index of biotic integrity (B-IBI) developed for northern California streams was not influenced by temperature extremes (cool and warm) or precipitation extremes (wet and dry). Other common indices and metrics used in biological monitoring studies, such as the ratio of observed to expected taxa (O/E), % Ephemeroptera, Plecoptera, and Trichoptera (EPT) individuals, and total richness were unaffected by temperature and precipitation variability. For future monitoring of climate-change effects on small streams, we developed a local climate-change indicator that is composed of the presence/absence of 9 macroinvertebrate taxa, identified to genus level. This indicator detected significant differences between years that were grouped based on temperature, precipitation, and a combination of temperature and precipitation. It also detected significant differences between groups in an external data set including 40 reference sites throughout the San Francisco Bay area, a result that suggests this indicator could be used at larger spatial scales in this region. Two biological trait categories found in large, long-lived organisms decreased with increasing temperature and decreasing precipitation at the most intermittent site. This result indicates that climate change might selectively affect taxa with certain traits. The robustness of the North Coast B-IBI and other common indices and metrics to temperature and precipitation variability demonstrates their continued applicability for examining water quality under future climate-change scenarios, but suggests that they probably will not be good indicators for detecting climate-change effects. The effects of climate change in mediterranean-climate streams can be monitored effectively within the framework of existing biological assessment programs by using regional indicators based on specific taxa identified to the generic level and information on their species traits.

How common are rare taxa in long-term benthic macroinvertebrate surveys?

Abstract The term rare has spatial and temporal, as well as conservation and management, connotations when applied to ecological surveys. We examined the frequency of temporal occurrence of benthic macroinvertebrates in 19- to 20-y survey collections from 4 sites and 7- to 8-y survey collections from 6 sites in northern California streams. We found that a large proportion of taxa (17–33%) were rare (occurred in only 1 y) in all of the sites examined, regardless of season, and that density increased with increasing temporal commonness. Taxa that could be identified only to higher taxonomic levels, i.e., unresolved taxa, were an additional component (0–7%) of the rare species encountered. The biological traits of the rare and common taxa generally provided explanations for the cause of rarity or commonness, e.g., common taxa tended to have short life cycles with multiple generations per year and rare taxa had low dispersal capabilities. The results of our study clearly demonstrate that temporally rare taxa are common in long-term benthic macroinvertebrate surveys; this prevalence may require re-examination of how long-term surveys are interpreted in both ecological studies and biomonitoring.

Distribution and seasonal occurrence of the hyporheic fauna in a northern California stream

The surface and hyporheic fauna of a second-order reach of a northern California Coast Range stream (Big Canyon Creek, Lake Co., CA, USA) was examined using substrate colonization samplers (i.e. hyporheic pots) during both the wet and dry seasons in the prevailing Mediterranean climate of the region. In terms of total number of macroinvertebrate taxa, the surface (0 cm to −5.0 cm within the stream substrate) level had higher richness than any level within the hyporheic (−5.1 cm to −15.0 cm, −15.1 cm to −25.0 cm, or −25.1 cm to −35.0 cm) from the beginning of the dry season (May) to the beginning of the wet season (October); during this period the surface was not subject to disturbance from wet-season storms. During the wet season, richness at the surface was similar to that observed at any of the three hyporheic levels examined.Macroinvertebrate density at the surface was substantially reduced during the wet season; however, mean surface densities always exceeded those found within the hyporheic zone (from 25% to 78% of total macroinvertebrate numbers were found at the surface). Seasonal fluctuations in abundance of total macroinvertebrates and density of many constituent populations were less within the hyporheic zone than at the surface. Apparently, early instars of abundant surface taxa do not penetrate the interstices, and substrate disturbance due to spates is less in deeper levels. Compared with many hyporheic faunas described from other temperate-zone sites, this hyporheic community shows reduced numerical dominance by the Chironomidae.

Long-term population and community patterns of benthic macroinvertebrates and fishes in Northern California Mediterranean-climate streams

Long-term studies can document temporal patterns in freshwater ecosystems, and this is particularly important in mediterranean-climate (med-climate) regions because of strong interannual variation in precipitation amounts and consequently stream flow. We review long-term studies of populations and communities of benthic macroinvertebrate and fishes from sites throughout the med-climate region of California and develop generalities that may apply broadly to med-climate streams worldwide. Severe drought may result in community shifts, and alter age-structure in both macroinvertebrates and fishes. Within-year seasonal patterns in macroinvertebrate communities can be influenced by annual variability in flow regimes. Macroinvertebrate biological-monitoring metrics with consistently low intra-annual variability may be especially applicable in med-climate streams, as is the use of different temporal windows to describe reference periods to reduce influence of interannual variability on impact detection. Long-term data can be used to develop macroinvertebrate-based metrics that can either show or be independent of climate-change effects. Most macroinvertebrate species are temporally rare in their annual occurrence. Multiple components of natural flow regimes can favor native over invasive fishes. Long-term, quantitative information from med-climate streams is generally lacking, which is a hindrance to both management practices and development of appropriate ecological constructs.