To cite package seas in publications use:

Toews, M. W., Whitfield, P. H., and Allen, D. M., 2007. Seasonal statistics: The `seas' package for R. Computers & Geosciences, 33(7): 944-951.<doi:10.1016/j.cageo.2006.11.011>

Corresponding BibTeX entry:

  @Article{,
    title = {Seasonal statistics: the 'seas' package for R},
    author = {Michael W. Toews and Paul H. Whitfield and Diana M.
      Allen},
    journal = {Computers \& Geosciences},
    year = {2007},
    volume = {33},
    pages = {1895},
    number = {7},
    doi = {10.1016/j.cageo.2006.11.011},
  }