RESUMO
The amphipod Diporeia spp. has historically been an important component of the benthic food web of the Laurentian Great Lakes. The Great Lakes Water Quality Agreement included its population density as an indicator of ecological condition for Lake Superior, with target values of 220-320 m-2 in nearshore areas (≤100 m depth) and 30-160 m-2 in offshore areas (>100 m). To assess the status of Diporeia in Lake Superior, we used a probability-based lake-wide survey design to obtain estimates of Diporeia density and biomass in 2006, 2011 and 2016. A PONAR grab sampler was used to collect Diporeia at 50-53 sites each year, with approximately half in the nearshore (<100 m depth) region of the lake and half in the offshore. The mean area-weighted lake-wide density was 395 ± 56 (SE) m-2 in 2006, 756 ± 129 m-2 in 2011, and 502 ± 60 m-2 in 2016. For all years, both density and biomass were greater in the nearshore than in the offshore stratum. The densities for 2006-2016 were 3-5 times higher than those reported from a lake-wide survey conducted in 1973 by the Canada Centre for Inland Waters. The severe declines in Diporeia populations observed in the other Great Lakes during recent decades have apparently not occurred in Lake Superior. Further research is needed to understand spatial and temporal variability of Diporeia populations in Lake Superior to enhance the utility of Diporeia density as an indicator of benthic condition.
RESUMO
Over 400 sites were sampled in the nearshore of the U.S. Great Lakes during the U.S. National Coastal Condition Assessment (NCCA) field survey in summer 2010. Underwater video images were recorded in addition to routine NCCA benthic assessment measures. This paper has two objectives: (1) to develop a process to evaluate video performance with acceptance criteria, exploring reasons for poor images, and (2) to use acceptable videos in an example application with invasive mussels, evaluating the enhancement potential of video to supplement traditional grab sampling. A standard hierarchical protocol was developed to rank video performance based on quality and clarity. We determined controllable and uncontrollable factors affecting video performance. Moreover, specific thresholds limiting video were identified: >0.5/m for light extinction and >3.5 µg/L for chlorophyll a concentration. To demonstrate the utility and enhancement potential of video sampling, observed dreissenid presence from excellent (221 of 362 videos) videos was compared with NCCA benthic taxonomy, in the context of the statistically based NCCA survey. Including video increased the overall area estimate of the U.S. Great Lakes nearshore with invasive mussels by about 15% compared to PONAR alone; 44% (7570 km(2)) of the surveyed region had mussels. The proportion of the nearshore area having mussels varied from low (3.5%) in Lake Superior to >50% in the lower lakes. PONAR and video have unique strengths and weaknesses as sampling tools in the Great Lakes nearshore environment, but when paired were complimentary and thus provided a more thorough benthic condition assessment at lake and regional scales.