Aim  To examine native‐exotic species richness relationships across spatial scales and corresponding biotic homogenization in wetland plant communities. Location  Illinois, USA. Methods  We analysed the native‐exotic species richness relationship for vascular plants at three spatial scales (small, 0.25 m2 of sample area; medium, 1 m2 of sample area; large, 5 m2 of sample area) in 103 wetlands across Illinois. At each scale, Spearman’s correlation coefficient between native and exotic richness was calculated. We also investigated the potential for biotic homogenization by comparing all species surveyed in a wetland community (from the large sample area) with the species composition in all other wetlands using paired comparisons of their Jaccard’s and Simpson’s similarity indices. Results  At large and medium scales, native richness was positively correlated with exotic richness, with the strength of the correlation decreasing from the large to the medium scale; at the smallest scale, the native‐exotic richness correlation was negative. The average value for homogenization indices was 0.096 and 0.168, using Jaccard’s and Simpson’s indices, respectively, indicating that these wetland plant communities have been homogenized because of invasion by exotic species. Main Conclusions  Our study demonstrated a clear shift from a positive to a negative native‐exotic species richness relationship from larger to smaller spatial scales. The negative native‐exotic richness relationship that we found is suggested to result from direct biotic interactions (competitive exclusion) between native and exotic species, whereas positive correlations likely reflect the more prominent influence of habitat heterogeneity on richness at larger scales. Our finding of homogenization at the community level extends conclusions from previous studies having found this pattern at much larger spatial scales. Furthermore, these results suggest that even while exhibiting a positive native‐exotic richness relationship, community level biotas can/are still being homogenized because of exotic species invasion.

The fugacity of CO2 in surface water (fCO2w) was measured in the eastern equatorial Pacific (EEP) during the boreal spring and fall of 1992 and in the spring of 1993. A prolonged E1 Nifio occurred during this period with anomalously warm sea surface temperatures (SST) during the spring of 1992 and 1993. Correspondingly, the fCO2w values were lower than historical non-E1 Nifio values at the equator. However, the fCO2w in the spring of 1993 was up to 50 gatm higher than in the spring of 1992, despite similar SSTs. The trend is attributed to the slower response times of factors causing fCO2w decrease compared to rapid increase of fCO2w by upwelling of cold water with high carbon content and subsequent heating. During the fall of 1992, SSTs south of the equator were 5 øC cooler than in the spring, which is indicative of vigorous upwelling of water with high CO2 content from below the thermocline. Decreases in fCO2w due to net biological productivity and gas exchange take of the order of months, causing the fCO2w levels during the spring of 1993 to be elevated compared to the spring of the previous year. Our data and data obtained in 1986 and 1989 along 110øW suggest that fCO2 maxima in the equatorial Pacific can be either associated with temperature minima or temperature maxima. Despite the multitude of factors which influence fCO2w, most of the variance can be accounted for with changes in nitrate and SST. A multilinear regression of fCO2w with SST and nitrate for the 1992 data has a standard error in predicted fCO2w of 10 gatm. Air-sea fluxes of CO2 in the EEP were estimated to be 30% higher in the spring of 1993 and 10% higher in the fall of 1992 than in the spring of 1992. decreased upwelling is partly counteracted by increases due to warming (which increases fCO2w by about 4.2% øC-i). The carbon cycle in the upper water column of the EEP was investigated in detail during the U.S. Joint Global Ocean Flux Study (JGOFS) Equatorial Pacific (EqPac) study in the boreal spring (henceforth called S-92) and boreal fall of 1992 (F-92) [Murray et al., 1994]. The S-92 cruises occurred during a moderate E1 Nifio. The Ocean-Atmosphere Carbon Exchange Study (OACES) of the National Oceanic and Atmospheric Administration (NOAA) performed a regional survey that included upper water column (down to 1000 m) measurements of temperature (T), salinity (S), DIC, fCO2w, total alkalinity (TAlk), and major nutrients in S-92 and F-92 from nominally 10øN to 10øS along 170øW, 140øW, 125øW, 110øW, and 95øW (95øW was occupied in the fall only). Surface water fCO2w measurements were made on hourly intervals on all cruises. Surface water fCO2w measurements in S-93 were made by the survey departments on NOAA ships Malcolm BaMrige and Discoverer, servicing the Tropical Ocean-Global Atmosphere/Equatorial Pacific Ocean Climate Study (TOGA/ EPOCS) mooring array. DIC and nitrate (NO3) samples in S-93 were taken at 1 o to 2 ø latitude spacing along the 140øW, 125 øW, 110øW transects and analyzed on shore.The surface water data in 1992 and 1993, along ...

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