Marine ecosystems are subject to global and local impacts, both contributing to dramatic changes in coastal communities. Assessing such changes requires time series or the revisitation of sites first surveyed in the past. In both cases, data are not necessarily collected by the same observers, which could lead to a bias in the results. 

This study revisited rocky reef communities that were first assessed in 2000 in the Marine Protected Area (MPA) of Capo Carbonara (Sardinia, Italy). Twenty years later, the same rocky reefs were resurveyed using the same method by two other diving scientists. 

The results show how the reef communities of Capo Carbonara MPA have distinctly changed over time, mostly under the effect of seawater warming, as highlighted by the occurrence of thermophilic species and by other climate-related indicators. The authors also highlight that species vulnerable to local human pressures have increased over time, demonstrating the effectiveness of the protection measures undertaken by the MPA. 

The authors conclude that, by comparing data collected by different observers in two different periods, this study demonstrates that change over time was significantly greater than variability between the observers. 

https://www.sciencedirect.com/science/article/abs/pii/S0141113622000629

Azzola et al. (2022) Variability between observers does not hamper detecting change over time in a temperate reef. Marine Environmental Research 177:105617.