Small changes in environmental conditions can unexpectedly tip an ecosystem from one community type to another, and these often irreversible shifts have been observed in semi-arid grasslands, freshwater lakes and ponds, coral reefs, and kelp forests. A commonly accepted explanation is that these ecosystems contain multiple stable points, but experimental tests confirming multiple stable states have proven elusive. Here we present a novel approach and show that mussel beds and rockweed stands are multiple stable states on intertidal shores in the Gulf of Maine, USA. Using broad-scale observational data and long-term data from experimental clearings, we show that the removal of rockweed by winter ice scour can tip persistent rockweed stands to mussel beds. The observational data were analyzed with Anderson's discriminant analysis of principal coordinates, which provided an objective function to separate mussel beds from rockweed stands. The function was then applied to 55 experimental plots, which had been established in rockweed stands in 1996. Based on 2005 data, all uncleared controls and all but one of the small clearings were classified as rockweed stands; 37% of the large clearings were classified as mussel beds. Our results address the establishment of mussels versus rockweeds and complement rather than refute the current paradigm that mussel beds and rockweed stands, once established, are maintained by site-specific differences in strong consumer control.

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Offshore wind farms often co-occur with biodiverse marine ecosystems with high ecological, economic, and cultural value. Yet there are many uncertainties about how wind farms affect marine organisms and their environment. The before–after–control–impact (BACI) design, an approach that compares an impact location with an unaffected control both before and after the intervention, is the most common method used to study how offshore wind farms affect finfish. Unfortunately, this design has several methodological limitations that undermine its ability to detect effects in these studies. An alternative approach, the before–after-gradient (BAG) design, would sample along a gradient with increasing distance from the turbines both before and after the intervention, and could overcome many of the limitations of BACI. The BAG design would eliminate the difficult task of finding a suitable control, allow for the assessment of the spatial scale and extent of wind farm effects, and improve statistical power by incorporating distance as an independent variable in analytical models rather than relegating it to the error term. This article explores the strengths and weaknesses of the BACI and BAG designs in the context of offshore wind development and suggests an approach to incorporating the BAG design into existing fisheries surveys and a regional monitoring framework.

Fish distributions are related to several habitat factors. We explored how the distribution of a 24 species assemblage is related to depth, temperature, substrate, season, and time-block using a 35 yr time series in the Gulf of Maine-Georges Bank region. We examined the relative importance of each factor, how it changes with season, and how individual species shift their relative distribution along environmental gradients on a seasonal basis. Distribution patterns were more strongly related to depth and temperature than to substrate type in both fall and spring. We observed 4 major patterns: (1) some species remained in relatively deep waters in both fall and spring; (2) some remained in relatively shallow habitats in both seasons and experienced wide temperature fluctuations as a result; (3) some moved from warmer shallow areas in the fall to warmer deep areas in the spring; (4) some traveled from the cool deep portion of the region in the fall to the cool shallow portion of the region in the spring. Of the 24 species examined, 19 declined in biomass over the study period in response to exploitation. The relationships between abundance and substrate type previously established for some species at local scales were weak at more synoptic spatial scales, although some trends in substrate associations were observed. Defining habitat at broad spatial scales remains a unique challenge. Compared to temperate systems, more refined habitat delineations for demersal marine fish have been established in tropical coral reef systems. Accordingly, much of our theory and the methodologies for applied spatial management have been derived from tropical systems. Differences between temperate and tropical systems necessitate modified approaches for temperate systems.

Multivariate ordination techniques were used to examine how size classes of four flatfish species, American plaice Hippoglossoides platessoides, winter flounder Pseudopleuronectes americanus, yellowtail flounder Limanda ferruginea and fourspot flounder Paralichthys oblongus, are related to bottom depth, bottom temperature, substratum grain size and temporal factors using a 35 year time series from autumn and spring in the Gulf of Maine-Georges Bank region. Depth had the most explanatory value during both seasons in most cases, with fish size generally increasing with depth. One exception was yellowtail flounder in the spring for which a temporal factor explained the most variance, reflecting an increase in size over the time series due to changes in fishing pressure. Temperature was secondarily important for yellowtail flounder in the autumn and for fourspot flounder in both seasons. Substratum type was secondarily important for winter and yellowtail flounders in the spring with smaller fish associated with larger substratum types. Seasonal associations with depth, temperature and substratum are related to seasonal spawning migrations, thermal preferences and other ecological factors. Journal compilation # 2007 The Fisheries Society of the British IslesNo claim to original US government works